Fuel Pump Relay or Oil Pressure Switch or/both on 1227747?

[PlayingWithTBI]

We've been having a discussion on the rolls of the fuel pump relay and the oil pressure switch near the dizzy. I know the ECM turns on the FP relay for a couple seconds with KOEO to prime the system. Then, I think it turns on the FP when cranking (when sensing ignition pulses from the ICM). Then the OP switch sends voltage to the pump when it sees pressure.

The question is, does the ECM turn off the relay after a while so the OP switch can shut off fuel if it loses pressure to save the engine from catastrophic failure due to no/low oil pressure or does it use them both for redundancy? I've seen articles claiming both, maybe someone here has hacked the code, or something like that, and can give us a definitive answer? TIA

Edit: This is on a 88-91 C/K 5.7L if it matters?

[Six_Shooter]

The ECM output for the fuel pump is on whenever there is a DRP (Distributor Reference Pulse, AKA and RPM signal), in addition to the priming pulse. The way GM set this up was that the fuel pump would get turned on by either the relay (controlled by the ECM) and by the OPSU once there is pressure, so that either worked as a back up for the other. There's also the idea that the OPSU will keep the fuel pump running for a few seconds after the engine shuts off to try to reduce fuel vapour in the throttle body, but that's never really been an issue would really be more of a problem later on, when the fuel lines and throttle body heat soaks with the now hot but not running engine.

[PlayingWithTBI]

Thanks bud, is there somewhere a guy could see proof in the code or something else? I'm of the same thought but, others think the OPSU is a safety for low pressure. One guy says, if he disconnects the OPSU, his engine dies. I'm thinking something is wrong with his FP relay, wiring, or ECM?

[NomakeWan]

Thanks bud, is there somewhere a guy could see proof in the code or something else? I'm of the same thought but, others think the OPSU is a safety for low pressure. One guy says, if he disconnects the OPSU, his engine dies. I'm thinking something is wrong with his FP relay, wiring, or ECM?

Whoever those guys are, they have a fundamental misunderstanding of electrical diagrams as well as how GM handled things back in the day. It's an incredibly common misconception, however, so it's worth addressing.

The OPSU is absolutely not a safety for low pressure. It is wired directly to the fuel pump. But what people seem to miss is that the fuel pump is also wired to a relay controlled by either the ECM or CCM/BCM (in models with such computers), and that that relay has priority. If the relay is closed, then battery voltage will continue to flow to the fuel pump even if the OPSU circuit is open (due to no oil pressure). The OPSU is incapable of overriding the relay. Thus, it is not a safety device. However, should the ECM/CCM/BCM and/or the fuel pump relay fail, the OPSU will allow the car to run (albeit with longer cranking time due to the failure of the normal system to prime the fuel at key on engine off). Only in this completely defective configuration can the OPSU be considered a "safety" device that will shut down fuel if oil pressure drops below the set point.

The only proof you (or anyone else) should need is the electrical diagram for your vehicle, which can be found in the factory service manual. Look at the wiring for the fuel system. You will see the pump, the relay (wired to the ECM/CCM/BCM), and the OPSU (wired directly to the fuel pump instead of the relay).

[PlayingWithTBI]

Thanks, I posted the schematic for the fuel pump and there's no argument that the relay and OPSU are wired in parallel. What they are saying is that the ECM turns off the relay some time after the engine starts and it's then totally dependent on the OPSU. One guy said, "if I disconnect the OPSU the engine dies". My thinking is he has an issue with his relay then. I have no proof either way and I'm not gonna crawl all over my engine while it's running just to disconnect the OPSU

[NomakeWan]

The system description can also be found in the FSM. For my vehicles, it's "Chart A-5." Your vehicle may vary. However, the way GM laid these out was pretty much identical from car to car in the late 80s and early 90s, and it's exactly as Six_Shooter described above:

charta5.jpg

The important part here being, "It will remain "ON" as long as the PCM is receiving low resolution reference pulses from the distributor ignition system." That's because on my cars there's both a low and high resolution pulse for the distributor, but on your vehicles there's just the one distributor pulse. Either way, as long as your ECM is receiving that distributor signal, it will attempt to energize the fuel pump relay.

[PlayingWithTBI]

The system description can also be found in the FSM. For my vehicles, it's "Chart A-5." Your vehicle may vary. However, the way GM laid these out was pretty much identical from car to car in the late 80s and early 90s, and it's exactly as Six_Shooter described above:

charta5.jpg

The important part here being, "It will remain "ON" as long as the PCM is receiving low resolution reference pulses from the distributor ignition system." That's because on my cars there's both a low and high resolution pulse for the distributor, but on your vehicles there's just the one distributor pulse. Either way, as long as your ECM is receiving that distributor signal, it will attempt to energize the fuel pump relay.

Thanks! That's what I was looking for.