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

[CalifOkie]

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.

[MT-2500]

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.

[777stickman]

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.

Originally Posted by MT-2500

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.

[CalifOkie]

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 ?

[CalifOkie]

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-2500]

Originally Posted by CalifOkie

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.

[jdmccright]

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.

[Jeremy-WI]

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.

[ronaldk]

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

[CalifOkie]

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.

[ronaldk]

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

[Jeremy-WI]

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

[CalifOkie]

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.

[MT-2500]

Originally Posted by CalifOkie

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

[jonnik]

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.