What determines a 1 or 3 pin oil pressure sensor?

[Schurkey]

ECM will Deliver power thru the FP relay for some time period and then remove its drive, depending on the power to be supplied through the oil pressure sender. This will protect against fuel pump-out in case of a rollover or other blown engine event.

This is the first time I've heard of the ECM turning the fuel pump relay off when the ECM is still getting a crank signal from the ignition or from the crank sensor.

I have to admit that it makes some sense as a safety precaution.

Gonna have to look in the service manual again to see if there's any verification.

 

[alpinecrick]

My Lisle socket won't fit in the limited space on my 96 with the 5.7.

Pizzes me off somethin' fierce..........

 

[alpinecrick]

This is the first time I've heard of the ECM turning the fuel pump relay off when the ECM is still getting a crank signal from the ignition or from the crank sensor.

I have to admit that it makes some sense as a safety precaution.

Gonna have to look in the service manual again to see if there's any verification.

I have read there is a roll over/fuel pump shut off feature on these trucks.

 

[Schurkey]

This is the first time I've heard of the ECM turning the fuel pump relay off when the ECM is still getting a crank signal from the ignition or from the crank sensor.

I have to admit that it makes some sense as a safety precaution.

Gonna have to look in the service manual again to see if there's any verification.

I have read there is a roll over/fuel pump shut off feature on these trucks.

Dug through my '97 service manual. The manual says--in multiple places--how the oil pressure switch will provide power to the fuel pump if the relay fails. There's zero mention of the oil pressure switch supplying power to the pump if the relay is TURNED OFF DELIBERATELY by the VCM.

I conclude that at least in '97, the VCM does not deliberately turn off the fuel pump relay as long as it's getting signal from the crank position sensor. (i.e., the VCM knows the crank is still turning, so the engine is running or cranking.) TBI trucks would have the reference pulses from the pickup coil serving the same purpose--telling the ECM that the engine is running or cranking.

There's similarly no mention of rollover protection on the fuel pump circuit.

If I missed something--possible, since my service manual is on DVD and it's a major project to scroll through the pages--I'd appreciate correction if you can cite a reputable and verifiable source.

 

[Supercharged111]

Dug through my '97 service manual. The manual says--in multiple places--how the oil pressure switch will provide power to the fuel pump if the relay fails. There's zero mention of the oil pressure switch supplying power to the pump if the relay is TURNED OFF DELIBERATELY by the VCM.

I conclude that at least in '97, the VCM does not deliberately turn off the fuel pump relay as long as it's getting signal from the crank position sensor. (i.e., the VCM knows the crank is still turning, so the engine is running or cranking.) TBI trucks would have the reference pulses from the pickup coil serving the same purpose--telling the ECM that the engine is running or cranking.

There's similarly no mention of rollover protection on the fuel pump circuit.

If I missed something--possible, since my service manual is on DVD and it's a major project to scroll through the pages--I'd appreciate correction if you can cite a reputable and verifiable source.

From what I recall, the PCM commands the relay on during cranking regardless of oil pressure. Once fired up, the oil pressure sensor has the ultimate say in addition to the PCM. I don't recall any PCM logic that supports anything contrary to what you have posted.

 

[alpinecrick]

Shurkey,
I only mentioned what I have read about the subject, I'll have to go with your research then!

 

[Schurkey]

From what I recall, the PCM commands the relay on during cranking regardless of oil pressure. Once fired up, the oil pressure sensor has the ultimate say in addition to the PCM. I don't recall any PCM logic that supports anything contrary to what you have posted.

ECM/PCM/VCM..."The Computer"...turns on the fuel pump relay when the ignition is turned "on", and holds it for two seconds (perhaps 20 seconds if the vehicle has a "hot fuel module") regardless of oil pressure (which would be zero) or crankshaft-rotation signal.

If The Computer gets a signal from the crank sensor or from the pickup coil, indicating the engine is cranking or running, it keeps the fuel pump relay turned on, or turns it back on.

IF there's a failure anywhere in The Computer, wire harness, or relay, so that the fuel pump relay doesn't supply power to the fuel pump, the oil pressure switch is wired in parallel. Once you get ~4psi of oil pressure, the switch provides a second source of power to the pump.

So the oil switch doesn't have "ultimate say", it's just a redundancy based on a second way of determining crankshaft rotation--because if the engine has oil pressure the oil pump must be turning, and that means the crank is turning.

If everything is working the way it's supposed to, with the engine running the power to the fuel pump would be split between the fuel pump relay and the oil pressure switch based on Ohm's Law and the various resistances in each part of the circuit. The short story is that the relay would provide "about" half the power, and the oil pressure switch would provide "about" half the power. But either one could supply full power if needed due to a failure in the other part of the circuit.

 

[Supercharged111]

ECM/PCM/VCM..."The Computer"...turns on the fuel pump relay when the ignition is turned "on", and holds it for two seconds (perhaps 20 seconds if the vehicle has a "hot fuel module") regardless of oil pressure (which would be zero) or crankshaft-rotation signal.

If The Computer gets a signal from the crank sensor or from the pickup coil, indicating the engine is cranking or running, it keeps the fuel pump relay turned on, or turns it back on.

IF there's a failure anywhere in The Computer, wire harness, or relay, so that the fuel pump relay doesn't supply power to the fuel pump, the oil pressure switch is wired in parallel. Once you get ~4psi of oil pressure, the switch provides a second source of power to the pump.

So the oil switch doesn't have "ultimate say", it's just a redundancy based on a second way of determining crankshaft rotation--because if the engine has oil pressure the oil pump must be turning, and that means the crank is turning.

If everything is working the way it's supposed to, with the engine running the power to the fuel pump would be split between the fuel pump relay and the oil pressure switch based on Ohm's Law and the various resistances in each part of the circuit. The short story is that the relay would provide "about" half the power, and the oil pressure switch would provide "about" half the power. But either one could supply full power if needed due to a failure in the other part of the circuit.

Ultimate say was a poor choice of wording. What I meant was either the PCM or the oil pressure switch could cut the motor off.

 

[Schurkey]

Ultimate say was a poor choice of wording. What I meant was either the PCM or the oil pressure switch could cut the motor off.

No, not really.

If the fuel pump relay is turned on, the oil pressure switch CANNOT turn the fuel pump off.

If the oil switch has pressure so it's turned on, the fuel pump relay CANNOT turn the fuel pump off.

EITHER ONE can turn the pump ON if the other one fails.

BOTH have to turn OFF to kill power to the pump.

 

[454cid]

Ok so you all are speaking of the 3 wire part..... is my one wire just for the gauge then? Just in case you haven't looked at my signature, this is a 99 454.

I'm not really understanding why GM would power the fuel pump with two circuits.