Does a GM oil pressure switch serve as a fuel cutoff ?

The vehicle in question here is a 94 C-1500 with a 5.7L TBI Vin code 'K' engine. I personally know that the OP switch does infact serve as a safety fuel shut off and to prove my point I made the following posting in another thread. Please review the information yourself and please let me know rather or not you find it as cut and dried obvious evidence supporting the fact that Oil Pressure switch does serve as a fuel cutoff... Thank you and here is my previous posting...I posted this in reply to an AF Advisor who insists that the OP switch is only along for the ride and only serves as a backup fuel pump power source in the event of a fp relay failure. He also claimed that the switch is not monitored by the ECM in anyway... He asked me for proof and I think I did a pretty fair job of providing it... What do you think after reading this ??

CalifOkie said:


I called a buddy of mine and we went down to the local dealership (yes, on Saturday night) and we researched the matter in the GM database. We found a wiring diagram of the Fuel Injection and Fuel Pump Control System that illustrated our point. We then went to my shop and got on my AllData Online computer to compare the data. To my amazement the diagrams are exactly the same. Which serves my purpose well so you can see it too. But first let me say that I did check in Alldata under description and operation of the Oil Pressure Switch and it does say that it serves as a backup incase of fuel relay failure... Now we can use their own wiring diagram to prove that to be total nonsense... Please open your allData program and look at this diagram located under Powertrain Management/ Electrical Diagrams/ Fuel Injection and Fuel Pump Control for a 1994 c-1500 w/ 5.7L... First look at the relationship of the fuel Pump Relay and the Oil Pressure switch. Just as you had stated they are wired in parrallel with eachother utilizing the same 12v power source as well does the ECM. Now notice circuit 465 going from the request terminal of the FP Relay to ECM pin# F6 Labeled FP Relay Control. That is the circuit the ECM uses to energize the relay to close the switch to power the fuel pump. Now lets look at the Oil Pressure switch. Notice that it is similair to a relay/crossed with a variable resistor. Notice the location of pin A that leads to the Inst Cluster. When the sensor detects oil pressure it changes the resistance which is monitored by the guage or light on one side of the switch. That half energizes the switching of the other half ( So that switch can fail and store a PO520 and the guage or light still work properly on later models utilizing the OBD2 diagnostics systems). Now at this point it seems that the ECM powers up the fp relay and then oil pressure opens the OP switch and both power sources run at the same time powering up circuit 120 through the fp fuse to the fuel pump. That is where the mistake is made... Now look at the ECM pin# B12. Notice it is labeled Oil Pressure Input and not FP Input. That is where the ECM monitors for adequate oil pressure. Now notice the wire attached to that pin is a gray wire of circuit 120 (the very same circuit that powers the fuel pump). Now if you trace that wire it leads to splice#127 where it monitors the voltage supplied to the fuel pump.Now use some common sense... How can oil pressure be monitored on a wire that is powered up by the fp relay and the OP switch at the sametime? It can't... This is how it works... The ecm powers up the fp relay providing power to the fuel pump to start the engine. Then when oil pressure comes up and the OP switch closes the additional 12V power source causes a spike in the current flowing to the fuel pump that is monitored at pin# B12 of the ECM. The ECM then cutsoff fp relay request at pin# F6 and disables the fp relay. At that point all power flows through the OP switch to fuel pump allowing pin# B12 to monitor oil pressure and not fp relay voltage. So you should be able to see now that if the OP switch failed the engine would die from the fuel pump being shut off. Also if the engine was to suddenly stop ( as in a crash) the fuel pump is immediately killed very similairly to Ford's fuel inertia switch system.. IT IS A SAFETY ISSUE TO PREVENT FIRE AFTER A COLLISION.
So right there is the proof on one of your own wiring diagrams. It further proves that Mitchell and AllData has the information but they only understand about half of it.
__________________

Now you please be the judge and tell me if I am just suffering from GM factory sydrome, or is there some true evidence here to prove my point.

CalifOkie
If you do not mind me asking.
Where did you get information that the oil pressure shuts off fuel pump?
Or the information that pcm shuts off fuel pump relay after engine is running?

The pcm does not turn off fuel pump relay while engine is running.

To prove it does not and that oil pressure switch is not a fuel pump cut off switch while engine is running unplug oil pressure switch.

Or just unplug if before starting engine.

Or check pcm fuel pump relay request while engine is running.

On pcm pin B-12 Mitchell on demand lables it electric fuel pump in and shows it running to fuel pump power between fuse and pump.

Oil pump pressure switch would no affect voltage on that wire or give a spike when switching on or off.

B-12 wire tells the pcm the pump has power and if power is lost sets code 54for fuel pump circuit.

This is a simple request from MT-2500. Maybe we will all learn something from what happens when you try this.

FYI. I did this on my '98 7.4 K2500 Sub and the motor and pump just kept on running.

To prove it does not and that oil pressure switch is not a fuel pump cut off switch while engine is running unplug oil pressure switch.

Or just unplug it before starting engine.

MT, I don't mind you asking me anything. This info comes from GM tech classes not a shop manual. Though honestly no one ever told me exactly that the ECM turns off the relay ( that is only my assumption) The exact term they used is that the current "bypasses" the relay through the oil pressure switch. One way or the other the current is supposed to go around the relay. I guess it could be as simple as path of least resistance . The point they made was that once the OP switch closes it becomes the fuel pumps primary source of power and that ECM used that power going to the fuel pump to monitor the presence of oil pressure. It was designed to shut down if it loses it's oil pressure.

Now let me ask you this... Have you ever seen a GM vehicle with EFI that had been hit so hard in the frontend that the fuel lines got broken at the firewall ? I'm sure you have.. What do you think turned the fuel pump off and kept it from bursting into flames ?

777stickman, I never once said that unplugging the switch would keep one from running I am not even sure if that would be a valid test. If you just unplugged it while it was running long enough to see if it would stay running and then plugged it back in you just introduced a glitch. If you scanned it afterwards it may of showed a code PO520 as a pending code. It would have to fail 2 diagnostic cycles before it would be stored as a current code and then at that point it should shut down. Then if you turn the key off and restart it the check engine light should be on and it will run just long enough to run another diag cycle and shut down again if it fails. Now that is what one will do if it has a failed OP switch. I have never just unplugged one to see if it does the same thing. Though I really don't see any reason why it wouldn't react the same way. If it does you may have to use a scanner and clear the code to get the check engine light to turn off again.
Now the 94 won't store the PO520 code because it is an OBD1 system however the OBD2 type circuitry was used in it and it should shutdown the same way.

MT, I don't mind you asking me anything. This info comes from GM tech classes not a shop manual. Though honestly no one ever told me exactly that the ECM turns off the relay ( that is only my assumption) The exact term they used is that the current "bypasses" the relay through the oil pressure switch. One way or the other the current is supposed to go around the relay. I guess it could be as simple as path of least resistance . The point they made was that once the OP switch closes it becomes the fuel pumps primary source of power and that ECM used that power going to the fuel pump to monitor the presence of oil pressure. It was designed to shut down if it loses it's oil pressure.

Now let me ask you this... Have you ever seen a GM vehicle with EFI that had been hit so hard in the frontend that the fuel lines got broken at the firewall ? I'm sure you have.. What do you think turned the fuel pump off and kept it from bursting into flames ?

I do not see many after being wrecked.
And would be hard to tell if fuel pump shut down after head on.
Not even sure a 94 PCM would read out crash data.
The last one I saw hit head on at 70 + MPH flipped around about 2 times and landed up on shhoulder across the road and burnt up with driver still in seat belt.
But hard telling what burnt it.

The oil pressure switch and fuel pump relay are wired parlell to fuel pump power.
Both have to shut down to kill fuel pump.
As mitchell and all data both tell us the oil pressure switch is not a cut off saftey switch but only there to run fuel pump in case of a fuel pump relay fail.

Fuel pump relay stays turned on as long as engine is running.
If you do not think the unplug oil pressure test is a good test then check the fuel pump relay while engine is running.
It stays on while engine is running and would have to fail for oil pressure to be able to turn off fuel pump power.

In a wreck or head on the engine has to stop running before the PCM shuts off fuel pump relay.

On GM instructers term.
The exact term they used is that the current "bypasses" the relay through the oil pressure switch.
The point they made was that once the OP switch closes The point they made was that once the OP switch closes it becomes the fuel pumps primary source of power and that ECM used that power going to the fuel pump to monitor the presence of oil pressure. It was designed to shut down if it loses it's oil pressure.
and that ECM used that power going to the fuel pump to monitor the presence of oil pressure. It was designed to shut down if it loses it's oil pressure.

This quote from gm instructers is wrong.
The point they made was that once the OP switch closes it becomes the fuel pumps primary source of power and that ECM used that power going to the fuel pump to monitor the presence of oil pressure. It was designed to shut down if it loses it's oil pressure.


Oil pressure switch power to fuel pump does jump around relay to fuel pump but is also wired parwell so that either on can run fuel pump.

I do not see Where they got the idea that it was designed to shut down engine in case of lose of oil pressure.

If I can add a bit of logic to this discussion, I would tend to think that it would not shut off the fuel pump for safety reasons if the truck is in motion. If the OP sensor fails or detects zero pressure and cuts off fuel, you are now driving a dead truck with no steering assistance and limited braking....quite a safety issue IMO.

While preserving the motor may be of interest, I'd say that maintaining driveability until you can pull over and shut the engine off is more important. There's my:2cents:.

I am certain that the OP switch can not shut off the fuel pump on the 96 and newer, as Califokie has said they are wired in parallel. But he says that there is a input to the pcm for oil pressure and simply disconnecting the OP sensor isn't a true test, so maybe someone should hook up another sensor that isn't in the motor to check the theory. My manuals say that the pcm will energize the fuel pump relay for two seconds after the ignition switch is on and whenever the pcm determines that the engine is trying to be started or running.

Ok what you have in simplicity is two switches in parallel with one power source. Now how the oil pressure switch becomes a single switch that can conduct B+ to the fuel pump and how it can under one and only one condition shut the fuel pump.

As an electronic technician I have ascertained from the posts that MT and Califokie are both correct although MT is only incorrect in thinking that the oil pressure switch can not shut fuel pump, he is correct in all his other statements.
The way the system is set up is very intelligent and I take my hat off to GM's design tech's.
The oil pressure switch is a variable resistor driven by pressure. When enough pressure exists the switch closes and in that state can pass B+ to the fuel pump.
This is only pertinent if the main switch in the parallel circuit that drives B+ to the fuel pump fails (fuel pump relay) while under driving conditions (high enough rpm's to keep ops as a closed circuit).
One switch in parallel with another switch both closed has no effect on the circuit.
But if the main source of B+ to the fuel pump (fuel pump relay) should open under that condition the ops would maintain B+ allowing veh to remain running.
Now comes the part about how the ops can and will shut fuel pump.
Under the above condition the ops is a closed switch conducting and maintaining fuel pump power,but if and when RPM's slow and oil pressure lowers the resistance will then increase until the resistance is to high to maintain fuel pumps requirements and pump shuts down.
So one can see that the oil pressure switch under that condition takes over as MT stated for safety but will when pressure is low enough shut pump.
I am not taking sides just clarifying how a simple circuit is used in a very intelligent way.
So it is a safety circuit (but not as shut off of fuel pump in the way Califokie thinks)and one that without your debate may not have ever been noticed since symptom would be == I was driving home or to store etc parked veh shut down came back and now only turns over but will not start. Changed fuel pump relay and all is good. The driver would never know that while on highway doing 65 mph fuel pump relay failed and oil pressure switch allowed to safely park. It's nice to know GM'S got our backs. Thanks for the informative debate. Ron

Well it seems this debate is picking up... Some key points to remember about this circuit is that is not power up by B+ (battery).. Powered up by Ignition switch in start or run position, otherwise it couldn't be turned off unless you unhooked the battery. Another is the fact that the OP switch is the only constant, unswitched, power source for the fuel pump. Once oil pressure is established the only thing that can cut it off is loss of oil pressure or being turned off by ignition switch. The fp relay on the otherhand is not. The ECM has control of it. The ECM tells it when to work and when not to work. Therefore, they only way to thoroughly check this system out to see if it actually works the way I was taught is to monitir power between the ECM and the request terminal of the fp relay. If the instructers are correct than that power would have to stop sometime during running. So that would have to be the only valid test.

I tried the unplugging of the OP switch to test myself... It stayed running for 30 minutes and didn't even turn the check engine light on. Now through channels and the matter of why it didn't shutdown was presented to an area GM rep. He said the Intrument cluster itself became instrumental there. He claims a " check guages" light should illuminate before the OP switch will shut the fuel pump off. When the OP switch was unplugged the guage should have pegged out instead of dropping to zero so the function was disabled... I am starting to question this training myself now.. I am wondering why this is the first I've heard of this part of the function.... I'm starting to wonder if there isn't a little confusion developing between actual functionality and sales pitch from GM... I still believe my training but I'm going to start researching this further in a different light. If the IP monitoring sytem comes into play then why wasn't a red flag sent up when an open circuit was detected ???
I would like to think that if a guage circuit is capable of disabling a safety feature then that circuit should be one that is checked during a diag cycle. I'm starting to wonder if anyone knows exactly how it is supposed to work?
I'm going to dig deeper by researching diagnostics for a OBD2 (95 and newer) code PO520 ( OP switch circuit malfunction) to see exactly how it is detected. I think the answer may be found there. Wouldn't it be strange to find out that the system isn't even checked until the fp relay fails ? That would be just plain wierd.
As for the OBD1 system on the 94 C-1500... The only test I can think of to verify whether or not the ECM does interupt the fp relay request circuit is to monitor that circuit by tapping into it with a wire ran to a test light hooked to ground. Take it on a road test and verify that the test light goes out. If it doesn't than all the training class teaching would be based on inaccurate sales pitch. Which would mean that GM spent alot of money on training for nothing.

B+ is B+ from ign switch or not. If you want a simple test start eng if oil pressure is high enough (even may be at idle) the oil pressure switch will become a secondary B+ source to fuel pump(you may need to rev eng to acheive enough pressure simulating driving, then remove fuel pump relay. The veh should continue to operate. Once thats proven shut eng (or may be drop to idle I am not sure how low oil pressure needs to drop to open oil pressure switch)you should not be able to restart without replaceing fuel pump relay. That would prove fuel pump relay is main B+ and oil pressure switch is safety back up. Ron

I have myself observed the result of what the oil pressure switch can do. I have had the fuel pump run for 20 seconds after cranking the engine with a no start condition, the oil gauge on the instrument panel was still reading pressure. I think the factory service manual on the 96 says that the oil pressure switch will power the fuel pump as a backup power supply when the oil pressure is about 5 psi or higher. It would be nice if GM would have put a relay in place that would have allowed shock sensors like that used with the air bag systems to cut power to the fuel pump

OP switches are suppose to go open ( shut power off) at 4 psi however some have opened up at pressures as high as 9 psi and was still considered to be functional. However by the book 4 psi or more is oil pressure required to power flowing through the switch.

Oh.. MT, I owe you an apology.... I airheaded and my brother caught me on it. I told you that the last time I ran accross a PO520 code and a starting and stalling problem was on a 98 Camaro.... Sorry, I was wrong. I had two cars going at the same time and somehow got them crossed up in my head. OP switch problem was a 05 Monte Carlo with a 3.8L K. The 98 Camaro was a shelled AC compressor. My brother had to pull the tickets to prove it to me... Sorry all, my bad. It seems the 98's aren't capable of throwing a code PO520 either and you were right on when you said there wasn't a code PO520 for the 98 Camaro. Maybe I'm working too hard. I am literally getting jobs I have done scrambled up in my head.. Guess it's time for a vacation.

OP switches are suppose to go open ( shut power off) at 4 psi however some have opened up at pressures as high as 9 psi and was still considered to be functional. However by the book 4 psi or more is oil pressure required to power flowing through the switch.

Oh.. MT, I owe you an apology.... I airheaded and my brother caught me on it. I told you that the last time I ran accross a PO520 code and a starting and stalling problem was on a 98 Camaro.... Sorry, I was wrong. I had two cars going at the same time and somehow got them crossed up in my head. OP switch problem was a 05 Monte Carlo with a 3.8L K. The 98 Camaro was a shelled AC compressor. My brother had to pull the tickets to prove it to me... Sorry all, my bad. It seems the 98's aren't capable of throwing a code PO520 either and you were right on when you said there wasn't a code PO520 for the 98 Camaro. Maybe I'm working too hard. I am literally getting jobs I have done scrambled up in my head.. Guess it's time for a vacation.

Yes sometimes we can overload our brain.

It looks to me like that checking the fuel pump relay control while engine is running or truck is being drove down the road will tell the story on the system.

If fuel pump relay does not shut down then that system can not shut off fuel pump while engine is running.

When you get time check it out.

A lot of newer systems over 2000 up have done away with oil pressure switch to power fuel pump.
I do not know if any of the newer 2000 up OBD 11 can shut down fuel pump from low oil pressure with engine running or not.
The newer PCM's can do a lot of things if it is programed into them.
Good Luck
MT

My sons '98 Cheyanne had the oil pressure sensor go bad and the fuel pump would not run (0 fuel pressure at the fuel line valve between the fire wall and intake manifold). After studying the wiring diagrams in my Haynes manual, I jumpered the orange and grey wire sockets in the oil pressure sensor plug and the pump came on. Replaced the oil pressure sensor and the truck is still going a year later.

Jonnik: Ask your son to check to see if fuel pump turns on for a few seconds when key is put into on possition without turning eng over. If fuel pump does not energize have him check his fuel pump relay circuit,you don't want to loss your safety back up given by the oil pressure switch by using it as primary B+ source for the fuel pump.
Ron

Jonnik: Ask your son to check to see if fuel pump turns on for a few seconds when key is put into on possition without turning eng over. If fuel pump does not energize have him check his fuel pump relay circuit,you don't want to loss your safety back up given by the oil pressure switch by using it as primary B+ source for the fuel pump.
Ron

I'll second that

The pump does pressure up in ign. on-engine off.

Good then everything is working correctly. Just wanted to make sure.LOL

My sons '98 Cheyanne had the oil pressure sensor go bad and the fuel pump would not run (0 fuel pressure at the fuel line valve between the fire wall and intake manifold). After studying the wiring diagrams in my Haynes manual, I jumpered the orange and grey wire sockets in the oil pressure sensor plug and the pump came on. Replaced the oil pressure sensor and the truck is still going a year later.


There you go... I know the OP switch can take out the fuel pump. I just can't understand why unplugging the switch doesn't have the same effect... The ecm has to be able to recognize the open circuit somehow and substitute it with fp relay request, but if that was true then in the event of an actual failed OP switch killing the pump, then unplugging the switch would allow it to run again, right ?? That sounds too wierd. Now I'm gonna have to get my hands on a bad switch.

Just as jonnik posted it does happen. I've seen it too. A failed OP switch mimics no oil pressure and the ECM will not send FP Relay request. There is more then just FP voltage monitored at ECM pin# B12 (which GM labels as Oil Pressure Input). There are now three eyewitnesses (including jonnik's son) to support that point. Otherwise simply replacing the OP Switch alone couldn't possibly make the fuel pump run again. Thank you jonnik, and I am sure there are more out there who have witnessed this.

I've always understood that the oil pressure switch is not a cut off saftey switch but only there to run fuel pump in case of a fuel pump relay fail. I have a 92 C-1500 5.7 that had the 3 spade oil pressure switch but when it stopped working I replaced it with a one spade rather than pay the high price for the 3 spade switch just using the tan wire and leaving the orange and grey open. the engine starts and runs fine. So tell me which is it?

I've always understood that the oil pressure switch is not a cut off saftey switch but only there to run fuel pump in case of a fuel pump relay fail. I have a 92 C-1500 5.7 that had the 3 spade oil pressure switch but when it stopped working I replaced it with a one spade rather than pay the high price for the 3 spade switch just using the tan wire and leaving the orange and grey open. the engine starts and runs fine. So tell me which is it?

As you said the oil presure switch is on there to run the fuel pump in case the relay fails.
Just as you have done to save money.
Later years truck have done away with the 3 prong oil presure switch to feed the fuel pump incase of relay fail.

But answer this for us.
Did you engine shut down when the switch went bad?
That should answer your question.

Jonnick's son's veh has or had a faulty (most likely intermittent)fuel pump relay circuit. A black dog with white spots is not a white dog with black spots because the primary color is black and Chris Angel can't really fly although my 27 year old son thinks he can no matter what I say. This circuit is very simple and because its simple I find it a thing of beauty, yet it seems like my son, califokee you want it to be more than it is. It is what it is SIMPLE. You are correct when you say the ECM controls the fuel pump relay what you seem to not see is oil pressure and only oil pressure turns on the section of the oil pressure sensor, and that is a simple switch hooked to B+ on one side and fuel pump on other. As long as fuel pump relay also feeds B+ there is no noticeable effect on circuit and should it fail B+ still runs through oil ps but once veh is shut it needs to be turned over long enough for pressure to engage switch so as to get B+ to fuel pump since it's NOT controlled by ECM. If fuel pump relay decides to work then the failure would not even be noticed. A black dog with white spots is not white with black spots no matter how you hold the camera. LOL

Another way to look at this whole oil pressure switch issue is the fact that after changing my oil it takes 5 to 10 seconds for oil pressure to build. During that initial 5 to ten seconds, the engine runs while the guage is at 0. If the OP switch was supposed to cut fuel until oil pressure then the truck would only crank until there were oil pressure and not run with 0.

The ultimate test would be to drain all engine oil and start the engine. This is a brutal but perfect test to prove 100% either way if the OP switch will cut power if there is indeed no oil pressure. Maybe someone killing an engine for the cash for clunkers program could try this?

Another way to look at this whole oil pressure switch issue is the fact that after changing my oil it takes 5 to 10 seconds for oil pressure to build. During that initial 5 to ten seconds, the engine runs while the guage is at 0. If the OP switch was supposed to cut fuel until oil pressure then the truck would only crank until there were oil pressure and not run with 0.

The ultimate test would be to drain all engine oil and start the engine. This is a brutal but perfect test to prove 100% either way if the OP switch will cut power if there is indeed no oil pressure. Maybe someone killing an engine for the cash for clunkers program could try this?

My 96 service manual says that the pcm will power the fuel pump for approx 2 seconds after the ignition switch is on and when it receives a signal from the crankshaft position sensor indicating that the engine is turning, it also says that the oil pressure switch is a secondary power source for the fuel pump

My 96 service manual says that the pcm will power the fuel pump for approx 2 seconds after the ignition switch is on and when it receives a signal from the crankshaft position sensor indicating that the engine is turning, it also says that the oil pressure switch is a secondary power source for the fuel pump

Right on there Jeremy

*** UPDATED BY TSB 893206E, DATED OCTOBER 90

When the key is first turned ON, without the engine running, the control module will turn the fuel pump relay ON for two seconds. This builds up the fuel pressure to normal operating pressure. If the engine is not started within two seconds, the control module will shut the fuel pump OFF and wait until ignition reference pulses are present. As soon as the engine is cranked, the control module turns the relay ON, which powers the fuel pump. The control module continues to power the fuel pump during engine operation. If the fuel pump relay fails, it is backed up by the oil pressure switch, which continues to operate the fuel pump as long as oil pressure remains above 28.0 kPa (4 psi).

RESULTS OF INCORRECT FUEL PUMP SYSTEM OPERATION

A faulty fuel pump relay can result in long cranking times, particularly if the engine is cold.
An inoperative fuel pump would cause a no start condition.
A fuel pump which does not provide enough pressure can result in poor performance.

Operation:
When the key is first turned "ON" without the engine running, the control module turns the fuel pump relay "ON" for two seconds. This builds up the fuel pressure quickly. If the engine is not started within two seconds, the control module shuts the fuel pump "OFF" and waits for ignition reference pulses. As soon as the engine is cranked, the control module turns the relay "ON" and runs the fuel pump.
As a backup system to the fuel pump relay, the fuel pump is also turned "ON" by an oil pressure switch. When the engine oil pressure reaches about 28 kPa (4 psi), through cranking the oil pressure switch will close to complete the circuit to the fuel pump.
An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold.

Circuit Description
The status of the fuel pump CKT 120 is monitored by the ECM at terminal "B2" and is used to compensate fuel delivery based on system voltage. This signal is also used to store a Diagnostic Trouble Code (DTC) if the fuel relay is defective or fuel pump voltage is lost while the engine is running. There should be at least 12 volts on CKT 120 for at least 2 seconds after the ignition is turned "ON", or any time the reference pulses are being received by the ECM.

The vehicle in question here is a 94 C-1500 with a 5.7L TBI Vin code 'K' engine. I personally know that the OP switch does infact serve as a safety fuel shut off and to prove my point I made the following posting in another thread. Please review the information yourself and please let me know rather or not you find it as cut and dried obvious evidence supporting the fact that Oil Pressure switch does serve as a fuel cutoff... Thank you and here is my previous posting...I posted this in reply to an AF Advisor who insists that the OP switch is only along for the ride and only serves as a backup fuel pump power source in the event of a fp relay failure. He also claimed that the switch is not monitored by the ECM in anyway... He asked me for proof and I think I did a pretty fair job of providing it... What do you think after reading this ??

CalifOkie said:



I called a buddy of mine and we went down to the local dealership (yes, on Saturday night) and we researched the matter in the GM database. We found a wiring diagram of the Fuel Injection and Fuel Pump Control System that illustrated our point. We then went to my shop and got on my AllData Online computer to compare the data. To my amazement the diagrams are exactly the same. Which serves my purpose well so you can see it too. But first let me say that I did check in Alldata under description and operation of the Oil Pressure Switch and it does say that it serves as a backup incase of fuel relay failure... Now we can use their own wiring diagram to prove that to be total nonsense... Please open your allData program and look at this diagram located under Powertrain Management/ Electrical Diagrams/ Fuel Injection and Fuel Pump Control for a 1994 c-1500 w/ 5.7L... First look at the relationship of the fuel Pump Relay and the Oil Pressure switch. Just as you had stated they are wired in parrallel with eachother utilizing the same 12v power source as well does the ECM. Now notice circuit 465 going from the request terminal of the FP Relay to ECM pin# F6 Labeled FP Relay Control. That is the circuit the ECM uses to energize the relay to close the switch to power the fuel pump. Now lets look at the Oil Pressure switch. Notice that it is similair to a relay/crossed with a variable resistor. Notice the location of pin A that leads to the Inst Cluster. When the sensor detects oil pressure it changes the resistance which is monitored by the guage or light on one side of the switch. That half energizes the switching of the other half ( So that switch can fail and store a PO520 and the guage or light still work properly on later models utilizing the OBD2 diagnostics systems). Now at this point it seems that the ECM powers up the fp relay and then oil pressure opens the OP switch and both power sources run at the same time powering up circuit 120 through the fp fuse to the fuel pump. That is where the mistake is made... Now look at the ECM pin# B12. Notice it is labeled Oil Pressure Input and not FP Input. That is where the ECM monitors for adequate oil pressure. Now notice the wire attached to that pin is a gray wire of circuit 120 (the very same circuit that powers the fuel pump). Now if you trace that wire it leads to splice#127 where it monitors the voltage supplied to the fuel pump.Now use some common sense... How can oil pressure be monitored on a wire that is powered up by the fp relay and the OP switch at the sametime? It can't... This is how it works... The ecm powers up the fp relay providing power to the fuel pump to start the engine. Then when oil pressure comes up and the OP switch closes the additional 12V power source causes a spike in the current flowing to the fuel pump that is monitored at pin# B12 of the ECM. The ECM then cutsoff fp relay request at pin# F6 and disables the fp relay. At that point all power flows through the OP switch to fuel pump allowing pin# B12 to monitor oil pressure and not fp relay voltage. So you should be able to see now that if the OP switch failed the engine would die from the fuel pump being shut off. Also if the engine was to suddenly stop ( as in a crash) the fuel pump is immediately killed very similairly to Ford's fuel inertia switch system.. IT IS A SAFETY ISSUE TO PREVENT FIRE AFTER A COLLISION.
So right there is the proof on one of your own wiring diagrams. It further proves that Mitchell and AllData has the information but they only understand about half of it.
__________________

Now you please be the judge and tell me if I am just suffering from GM factory sydrome, or is there some true evidence here to prove my point.

Have you got it figured out by now?

On some 350s, there are actually 2 Oil pressure sitches. One bell Shaped , one lead, that sends info to the dash light. I don't believe this one impacts the ec system. or the fuel pump. Then there is another more traditional shaped switch (2 or 3 prong) that does serve as a fuel pump shut off.

On the 305, the single 3 prong switch serves both these functions.

My wiring diagram that I purchased does not match reality.

Also, in the Haynes manual, I can't even find a referece to an oil pressure switch.

On some 350s, there are actually 2 Oil pressure sitches. One bell Shaped , one lead, that sends info to the dash light. I don't believe this one impacts the ec system. or the fuel pump. Then there is another more traditional shaped switch (2 or 3 prong) that does serve as a fuel pump shut off.

On the 305, the single 3 prong switch serves both these functions.

My wiring diagram that I purchased does not match reality.

Also, in the Haynes manual, I can't even find a referece to an oil pressure switch.
I have never run into that setup.
What year and truck ans size and engine code?

The 2 oil pressure switches (1 1 lead, 1 2 pin) are on a 350 G20 Van.
The 3 pin is from a 1989 Caprive 305 Vin E I believe.

The 2 oil pressure switches (1 1 lead, 1 2 pin) are on a 350 G20 Van.
The 3 pin is from a 1989 Caprive 305 Vin E I believe.

Both of them us a oil pressure switch tied to fuel pump.
But it is not a cut off or a fuel pump saftey kill switch.

It is wired in with the fuel pump relay to aid and supply power to the fuel pump in case the fuel pump relay quits.

If fuel pump relay is working right engine will run with or without the oil pressure switch working.

The only GM engines that used the oil pressure swith to power the fuel pump engine running was on early 80 model V6 engines.

It was a wide blade 3 prong oil pressure switch that supplied fuel pump power to fuel pump after engine started and oil pressure came up.

I don't know about a GM, I had to put two gallons of gas in a 2000 Ford Taurus for the electric fuel pump to turn on, a one gallon gas container wasn't enough.

see F.O.R.D. Taurus
http://www.automotiveforums.com/vbulletin/showthread.php?t=970835

will a intermittant hard start be a symptom of a po520 code? i pulled this code from the history on my scanner ,but i have on check engine light on. thanks

will a intermittant hard start be a symptom of a po520 code? i pulled this code from the history on my scanner ,but i have on check engine light on. thanks

Welcome to AF on your first post.

To get you started on the right foot.
With a problem on yours best to start a New/thread/post.
Go to proper forum and upper right of it look for and hit the New thread button.
Hit us with all info.
Like what they ask when you go to the parts place for parts.
Year make and model and engine/engine code and transmission and mileage.
Also what work has or has not been done on it.
If check engine light still on after code clear you still have a problem.
Recheck for present code and post back code no.
Hard to start always confirm good fuel pressure up to specs.

Summarizing all the information above:

There are 2 switches in parallel that control the fuel pump.
This results in exactly 4 combinations of switch states that lead
to 1 of 2 possible fuel pump states.

State FP Relay OP Switch Pump
1 off off off
2 off ON ON
3 ON off ON
4 ON ON ON

That’s it.
For a normally operating car, the states are:

State 1 – key off, or key has been on for more than 2 seconds but is not cranking
State 2 – this state only occurs if something is wrong
State 3 – key on for less than 2 seconds, or cranking but oil pressure < 5psi.
State 4 – engine running, or cranking and oil pressure > 5psi

Note: if either switch is on, the pump will be on. The other switch cannot turn it off. In
The event of an accident, the ECU will see that the engine is not running and shut off the relay. The oil pressure switch will also be off. Since both are off, the pump will shut off.

Now, what if something goes wrong?

A. Normally switches don’t fail by being shorted. If either switch is shorted, the fuel pump will keep running, even after an accident. But this is unlikely. If a switch is shorted,
the pump will keep running the battery down when the car is shut off and you would not be reading this thread. You would be reading “why does my car hum and run the battery down when it’s off?”

B. If the fuel pump relay won’t close, the oil pressure switch allows you to limp home. As stated in one of the comments above, this mode will cause long cranking times since the fuel pump won’t start until after the oil pressure gets high enough during cranking. In this situation, the oil pressure switch can shut off the fuel pump in the event of an accident. The ECU could recognize that the fuel pump relay didn’t turn on when the key was turned on, and set a code.

C. If the oil pressure switch won’t close, you may never know it until the fuel pump relay also fails.

My 77 Vega would stall as I rounded curves (on exit ramps) with low oil (Vega's were always low on oil) so to keep it running, just straighten up your curve..or add oil ....the FP relay was only activated during engine crank...same car would run with no FP if you jacked rear end up higher than carb....easy to do with a floor jack---lower car, engine stopped...

My '95 Chevy Dually would cut -out -then not start- found out it had a bad OP switch that was partially shorting to ground (really messes up FP circuit) internally from being melted from those extremely hot exhaust manifolds (454's do that) -finally saw erratic OP guage as my hint.. same truck had the plastic rotor inside distributor cap melted later on)

I've seen 3 astro vans seize their engines due to low or no oil pressure- proof that FP will run and lock up with no oil pressure. I wired my own to be like the Vega cicuit above...I would rather have the engine shut-off and coast to a stop, then to seize while driving......same end result (while driving) IMO.

With loud fuel pumps, you can often hear them run 2-3 seconds after engine shut down, while oil pressure diminishes down to point below 4psi and OP switch opens.

"You need to plug-in the oil pressure switch to make the Fuel Pump Relay to work properly under critical condition" - this is the conclusion that I made after a few weeks of headache for sorting out why my 1995 VW Golf had intermittant engine cut off problem.

About a month ago, my VW Golf engine would shut down when I slowed it down upon turning. You could hear "tick-tick" sound from the fuel pump relay immediately before cut-off. The engine couldn't start immediately but 5 to 10 minutes later, it could start again. My first thinking was a faulty fuel pump relay but it didn't fix the problem with a new relay.

I sorted out later that my oil pressure switch had not been plugged in properly; there is a rubber barrel covering the electric terminal of the pressure switch, it was hard to tell there was a loose contact upon visual inspection. I was lucky to find when I checked for all electric contacts. I figure my mechanic did a lousy job when he replaced some parts earlier; very likely he just plugged in the rubber barrel without making sure that the plug did sit properly too.

The intermittant problem didn't appear again once I re-plug the oil pressure switch. I think the pressure switch does serves as a backup in case of fuel relay failure.

I once thought the ECU cut off the engine when it senses low or no oil pressure, it was until I read this post that the engine was cut off due to lack of support from the oil pressure switch connection. If the engine was cut-off by the ECU in case of engine overheating etc, it has to be immediately and I would not hear the "tick-tick" sound generated by the fuel pump relay.

"You need to plug-in the oil pressure switch to make the Fuel Pump Relay to work properly under critical condition" - this is the conclusion that I made after a few weeks of headache for sorting out why my 1995 VW Golf had intermittant engine cut off problem.

About a month ago, my VW Golf engine would shut down when I slowed it down upon turning. You could hear "tick-tick" sound from the fuel pump relay immediately before cut-off. The engine couldn't start immediately but 5 to 10 minutes later, it could start again. My first thinking was a faulty fuel pump relay but it didn't fix the problem with a new relay.

I sorted out later that my oil pressure switch had not been plugged in properly; there is a rubber barrel covering the electric terminal of the pressure switch, it was hard to tell there was a loose contact upon visual inspection. I was lucky to find when I checked for all electric contacts. I figure my mechanic did a lousy job when he replaced some parts earlier; very likely he just plugged in the rubber barrel without making sure that the plug did sit properly too.

The intermittant problem didn't appear again once I re-plug the oil pressure switch. I think the pressure switch does serves as a backup in case of fuel relay failure.

I once thought the ECU cut off the engine when it senses low or no oil pressure, it was until I read this post that the engine was cut off due to lack of support from the oil pressure switch connection. If the engine was cut-off by the ECU in case of engine overheating etc, it has to be immediately and I would not hear the "tick-tick" sound generated by the fuel pump relay.
Welcome to AF on your first post.
But.
VW Golf is a long way from being a GM.:sarcasmsign:

jfishtaker
Hey folks, I have a 1995 RV with 7.4L engine, engine starts right up and then the power to the fp shuts off. Turn key on power to fuel pump for 2 to 3 seconds. Changed OP switch no change. Check volts to fp 9, after two seconds 0. Any suggestions. New fuel pump, relay,ECM, new sensor on motor except EGR. Rebuilt TB pressure. The only question I have on op switch purchase at Advance Auto BWD brand(Oil Pressure Switch), ACDelco actually call theirs fuel pressure/oil pressure switch, should I have paid more and got the ACDelco brand. I talk to Advance Tech, said they were same except brand name??

autozone pressure switch should work, 9 volts to the fuel pump is too low . why? make sure grounds are also good

jfishtaker
Hey folks, I have a 1995 RV with 7.4L engine, engine starts right up and then the power to the fp shuts off. Turn key on power to fuel pump for 2 to 3 seconds. Changed OP switch no change. Check volts to fp 9, after two seconds 0. Any suggestions. New fuel pump, relay,ECM, new sensor on motor except EGR. Rebuilt TB pressure. The only question I have on op switch purchase at Advance Auto BWD brand(Oil Pressure Switch), ACDelco actually call theirs fuel pressure/oil pressure switch, should I have paid more and got the ACDelco brand. I talk to Advance Tech, said they were same except brand name??

As said 9 volts is to low.
You should have battery voltage at fuel pump.

Oil pressure switch only feeds voltage to fuel pump after oil pressure come up and only if fuel pump relay fails.



2 second prime up key on and then shut off is normal.
Then when engine is in start or run rpm signal from VCM should turn on relay and should feed 12 volts to fuel pump.

Have you checked for voltage to fuel pump engine cranking or running?

jfishtaker

>Check volts while cranking, drop from 9 to 6 volts,????
>Can not find a bad ground so far. Check for weak connection in wire harness, wish this was a Outboard motor I could fix it!!
>Will try a hot wire(with fuse) to the purge side at relay and check voltage at fuel pump connection, if volts are will try cranking and see what happens, hope I do not smoke something.
>There is a red wire in wire harness at the fuel relay labeled purge, is location I will connect hot wire??

Will tie hot to wire ign when key is turned to power!!

Jfishtaker

Hey to Chevy experts,

I straight wired fuel pump(pink wire) to ign, got 12 volts at fuel pump, good grounds, started motor cranked right up, ran about 1 to 2 seconds cut back off. Starts up everytime, could it be anything else, maybe coil??
1995 Chevy, 7.4L gas v8 Georgie Boy RV. Where can I get a wire diagram?
What does the brown/tan wire do in operation of fuel pump(maybe sending to gage?).
vin #1GBKP37N2S3318036, 1995

Jfishtaker

Hey to Chevy experts,

I straight wired fuel pump(pink wire) to ign, got 12 volts at fuel pump, good grounds, started motor cranked right up, ran about 1 to 2 seconds cut back off. Starts up everytime, could it be anything else, maybe coil??
1995 Chevy, 7.4L gas v8 Georgie Boy RV. Where can I get a wire diagram?
What does the brown/tan wire do in operation of fuel pump(maybe sending to gage?).
vin #1GBKP37N2S3318036, 1995

I maybe wrong !

IMO the engine starts and runs because your in the 2 sec start time period. because I don't have this vehicles wiring diagram this maybe wrong about the oil pressure switch. on the s10 pick ups 1994/95 the oil pressure switch must be working to apply power to the pcm. no oil pressure on from this oil switch the PCM shutsdown injectors, and the feed to fuel pump is removed.

this is the old style of safety to prevent fuel pump run on in a accident.
no oil pressure after start no fuelpump or injectors opening. so if you jump out the fuel pump so it will continue to run the injectors will not be commanded to open and the engine will stop..if this switch or wiring/connectors are defective.

I just did a repair on the 1994 s10 which had random stall failures. the oil pressure switch next to the distributor was the problem. prior to that we had random missfires. this was a bad ICM had it tested first. with the ICM replaced engine started very fast but we still had the random engine stall at any speed.

after the oil pressure switch replacement the oil pressure reads correct now 40-55 psi.

jfishtaker

This may or not be a clue to what wrong, when I disconnect the Map Sensor (new map sensor)engine continues to run with plenty of fuel at injectors, check engine light comes on and give a voltage code.
When you press back petal and pull into reverse or forward the fuel shuts off??

Jfishtaker

What effect does the 10052973 ASM Module on fuel pump shutting off after you start motor? In my RV Chevy Chassis manual it list this module and defines it function as:
QUOTE
The 1990 fuel module is connected to the instument panel wiring harness. The module and instrument panel wiring harness are shipped loose and must be installed by the body builder. This fuel cycler override module is necessarey due to the lower cranking speed of the L19(454) engine. The module overrides the ECM for 20 seconds to provide fuel pressure to the injectors.
I have located this module on my RV is located exactly where manual says, my engine starts right up but fuel shuts off after 2 seconds not 20 seconds. Is there a way to check module other than replaces it??
7.4l chevy engine(1995 year)
Any thoughts or suggestions to solve problem

you stated that you hot wired the fuel pump and the engine still died.


how can the fuel pump stop pumping if you have it connected directly to the ignition feed circuit ?

my guess is that the engine is not getting a spark command because of a loss of engine run info. if the pcm does not get any engine running info the engine will not get spark or injector operation.

so what is happening exactly with this hot wire bypass ? I suggest you hook up fuel pressure guage to determine whats not working correctly. see if you get injector grounding signal . see if your loosing spark..most likely with everything hooked up correctly your loosing fuel pump,injector grounding and the spark ..........

The straight wire done the same thing, engine cranked ran about 2 seconds and the injector fuel flow stopped. I put everthing back to normal.
I found this in my Chevy Motor home manual, 10052973 Module Asm, which overrides the ECM and keeps the fuel flow for 20 seconds. I have found location on RV, exactly where manual says it is located. Based on the description in manual must be back cause engine only runs about 2 seconds instead of 20 seconds. I assume the 20 seconds would allow the engine to reach oil pressure and continue running fuel to injectors???

The was used 1990 to 1995 engine model with 454 engines per manual!! If you go ACDleco and punch in the module no. it give you models used. Question is how to check it or just buy new and hopoe it works.

Grounds are good and spark.

The straight wire done the same thing, engine cranked ran about 2 seconds and the injector fuel flow stopped. I put everthing back to normal.
I found this in my Chevy Motor home manual, 10052973 Module Asm, which overrides the ECM and keeps the fuel flow for 20 seconds. I have found location on RV, exactly where manual says it is located. Based on the description in manual must be back cause engine only runs about 2 seconds instead of 20 seconds. I assume the 20 seconds would allow the engine to reach oil pressure and continue running fuel to injectors???

The was used 1990 to 1995 engine model with 454 engines per manual!! If you go ACDleco and punch in the module no. it give you models used. Question is how to check it or just buy new and hopoe it works.

Grounds are good and spark.

this module is bypassing the oil pressure switch for 20 sec ? this is because of the time it would take to build up pressure ?

did you replace the oil pressure sender / switch ? could be it is the problem.

I looked up your engine and it does have an oil pressure switch. this is similar to the 1994 v6 s10 that was stalling intermittently on me. this part also gave false low oil pressure readings which added to this stalling..

DID YOU REPLACE THIS PART ? IT IS NEXT TO THE DISTRIBUTOR .

Yes I replace the oil pressure switch last week, after reading the forum information on the oil pressure switch I sure thought you guys had solved the problem. Maybe engine needs more time(which the 20 seconds ASM 10052973 gives by overriding the ECM) to build enough pressure for oil pressure switch to keep the fuel going to injectors?? I have ordered a new ecm module $38 bucks.

Yes I replace the oil pressure switch last week, after reading the forum information on the oil pressure switch I sure thought you guys had solved the problem. Maybe engine needs more time(which the 20 seconds ASM 10052973 gives by overriding the ECM) to build enough pressure for oil pressure switch to keep the fuel going to injectors?? I have ordered a new ecm module $38 bucks.

did you measure the oil pressure ? the distributor cAM/ROD GOES INTO THE oil pump.

Oil pressure is 40psi Plus depending on rpm's.

Oil pressure is 40psi Plus depending on rpm's.

40 psi at idle is good. when cranking can you get it to 40 psi. when it was starting did the oil pressure shoot up quick. sometimes if the lifters leak down it takes longer to build pressure on start.

could be that 20 sec delay component.

jump out the oil pressure switch see if that allows start. then that would eliminate that. the ECM is part of the circuit so it could also be that.

There is a orange wire,tan wire and grey wire, which would be used to jump switch??

There is a orange wire,tan wire and grey wire, which would be used to jump switch??

according to my 1994/5 s10 diagram the orange is the feed 12volts from the ecm fuse. this goes to the switch of this oil sender/pressure switch. the other side of the switch is the grey wire. that wire goes to the ecm input and the fuel pump motor. so I would jump the orange to the grey wires. they should also be a bigger sized wire to handle the current .

the tan wire would go to the oil pressure guage. the body of the unit is ground.