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

[ronaldk]

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

[Jeremy-WI]

Originally Posted by ronaldk

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

[jonnik]

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

[ronaldk]

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

[CalifOkie]

Originally Posted by 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.

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.

[CalifOkie]

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.

[dstrick32]

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?

[MT-2500]

Originally Posted by dstrick32

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.

[ronaldk]

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

[b1lk1]

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?

[Jeremy-WI]

Originally Posted by b1lk1

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

[MT-2500]

Originally Posted by Jeremy-WI

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.

[MT-2500]

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.

[MT-2500]

Originally Posted by 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.

Have you got it figured out by now?

[ImpalaSS1965]

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.