Fuel pressure relay question

[bp71k5]

It looks to me like the relay "A" terminal and the oil pressure switch are on the same circuit , and if either one of them is OPEN, then the fuel pump circuit is not complete.

Check it carefully again. If the relay is working correctly, the 12v hot from the battery will travel through the relay and straight to the fuel pump on the wire (in the diagram terminal "E") that's on the right and drops down and runs under the relay (in the diagram) straight to the pump, bypassing the oil switch. If the switch is also working and sees 4psi, 12v will travel across it as well before going straight to the pump.

The purpose of this scheme is to let the truck still start when the relay fails, but do it in a way that tells you somethings wrong. It also prevents the engine from dying from a relay failure which could be a safety issue if you were crossing over some train tracks.

 

[dyeager535]

Check it carefully again.

I think you are both saying the same thing. "440" is hot at all times, which is the battery feed to the oil pump switch, relay, and ECM, since they are spliced together. If the relay and switch are open, then the pump doesn't run. If you jumper the oil pressure switch or terminal a and e on the relay, the pump runs. B1 and C16 are simply battery voltage to the ECM, "465" (pin unlabeled in image for RV, probably same as CK shown below, A1) is the ECM control of the relay.

 

[bp71k5]

It looks to me like the relay "A" terminal and the oil pressure switch are on the same circuit , and if either one of them is OPEN, then the fuel pump circuit is not complete.

I think you are both saying the same thing. "440" is hot at all times, which is the battery feed to the oil pump switch, relay, and ECM, since they are spliced together. If the relay and switch are open, then the pump doesn't run. If you jumper the oil pressure switch or terminal a and e on the relay, the pump runs. B1 and C16 are simply battery voltage to the ECM, "465" (pin unlabeled in image for RV, probably same as CK shown below, A1) is the ECM control of the relay.

Maybe I'm misreading it. Entirely possible. Ultimately the point being that if either the oil switch or relay is working, the truck will run and start. Either circuit can be OPEN and it'll run as long as both circuits are not open.

 

[BOOGI3 M@N]

Yes the truck will run and start if your relay is not working. You will not get the initial pulse of fuel from your injectors to start the truck if your relay is not working and it will turn over for a while before it starts. That is what is happening to mine not sure about the oil switch.

 

[1-ton]

I think you are both saying the same thing. "440" is hot at all times, which is the battery feed to the oil pump switch, relay, and ECM, since they are spliced together. If the relay and switch are open, then the pump doesn't run. If you jumper the oil pressure switch or terminal a and e on the relay, the pump runs. B1 and C16 are simply battery voltage to the ECM, "465" (pin unlabeled in image for RV, probably same as CK shown below, A1) is the ECM control of the relay.

Yes, I can see what you are talking about. The hot 12V wire does run straight to the oil pressure switch, which would allow the system to power the fuel pumps once it reached 4-PSI. The same cannot be said for the fuel pump relay though because it needs power from the B2 terminal of the ECM to the "E" terminal of the relay, in order to complete the relays circuit. I am still a little confused how the relay works in this system. I would like to understand it down to the smallest detail.

 

[dyeager535]

I see where your confusion is now. B2 isn't source voltage FROM the ECM. It lets the ECM know the voltage to the fuel pump. A1(? at least on the CK diagram at the bottom) is how the ECM opens or closes the relay.

Fuel pump voltage is reported within the ECM, I'm not entirely sure why. Both fuel pump and battery voltage are part of the data the ECM receives, but from what I've seen, battery voltage is what is used to determine the injector offsets. Ahh...here it is (code) "54. Low voltage at fuel pump OR Low voltage at Fuel pump relay". Almost certainly voltage on B2 is being compared to one of the battery voltage inputs of the ECM.

Not going to type it all up, but went and looked, (emissions manual is where the code setting explanations are in my 1990 R/V manual set), but B2 is used to set code 54. Less than 2V for 1.5 seconds, or no voltage since last distributor reference pulse and code 54 will set. It would seem pretty difficult to ever see that code.

I think this one (even though for Camaro/Firebird) is a bit easier to read, although the description of the code setting is different than in the R/V manual:

GM states that the oil pressure switch is the back-up circuit, there is no reason to let anyone say that it's for safety, except that it will keep the engine going when the relay dies, which could lead to a safety issue otherwise.

 

[1-ton]

GM states that the oil pressure switch is the back-up circuit, there is no reason to let anyone say that it's for safety, except that it will keep the engine going when the relay dies, which could lead to a safety issue otherwise.

Yes, this makes it more clearer. The arrows showing electron flow helps, and the step-by-step wording is big help too. Your last post explains what is probably the problem that started thread in the first place.