Fuel Pump Only Runs Off Oil Pressure Switch

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1owner1990

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As the title says, the pump stopped 'priming' with key on; only works when the oil pressure switch sends a run signal. Any ideas? 90 GMC, GM Performance 350/330 w/ holley carb and GM GP381 pump. About 4 hours of run time on the (brand new) 32 year old truck.
 

CorvairGeek

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The fuel pump relay is bad. The redundant circuit through the oil pressure switch is doing exactly as designed to keep the vehicle functional.
Since I see you have a carb, I don't know if you are still using an ECU for control. My assumption would still be the OEM configuration, with the ECU supplying a ground to turn on the fuel pump.
 
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1owner1990

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The fuel pump relay is bad. The redundant circuit through the oil pressure switch is doing exactly as designed to keep the vehicle functional.
Since I see you have a carb, I don't know if you are still using an ECU for control. My assumption would still be the OEM configuration, with the ECU supplying a ground to turn on the fuel pump.
Relay bench tests good. Any other ideas?
 

Schurkey

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Let me guess: Carb AND a non-stock, non-computer-controlled distributor?

If that's the case, the distributor is not communicating with the ECM, the ECM never receives the RPM signal that tells it the engine is cranking. Therefore, it never turns on the fuel pump relay.

Depending on how screwed-up the wiring is after gutting the fuel injection and stock distributor, that could be the reason it doesn't prime, either. Priming doesn't require a cranking/RPM signal, so lack of priming is likely a second failure, but maybe related to the carb/ignition conversion.

But even vehicles that haven't been tampered with can have fuel pump relay issues, which might be a defective ECM, defective wiring between ECM and pump relay, or a failed relay.
 

1owner1990

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Let me guess: Carb AND a non-stock, non-computer-controlled distributor?

If that's the case, the distributor is not communicating with the ECM, the ECM never receives the RPM signal that tells it the engine is cranking. Therefore, it never turns on the fuel pump relay.

Depending on how screwed-up the wiring is after gutting the fuel injection and stock distributor, that could be the reason it doesn't prime, either. Priming doesn't require a cranking/RPM signal, so lack of priming is likely a second failure, but maybe related to the carb/ignition conversion.

But even vehicles that haven't been tampered with can have fuel pump relay issues, which might be a defective ECM, defective wiring between ECM and pump relay, or a failed relay.
It has nothing to do with any deleted / altered wiring (which I'm proud to say is a work of art), as the system has been priming then switching over to the oil pressure sensor regulated circuit flawlessly for 5 months. After sitting in the garage for 17 years, I guess it's possible the ECM went out to lunch. Thanks for your help; hoping I remember to get back with you about the solution. And soon :)
 

1owner1990

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After wasting a lot of time performing exhaustive diagnostics, including an ECM swap and a bunch of unnecessary wire tracing, the culprit was in fact a 5 month old relay. @CorvairGeek, you were correct sir, thank you. I'm a 30 year commercial controls electrician and this simple circuit actually confused me a bit (judging by the amount of misinformation on the web, I'm not alone in my confusion). @Schurkey as for the ECM looking for an RPM signal from the distributor? I just don't know. The fuel pump functions and the truck runs. As you presumed, I did remove unneeded wires from the harness, leaving only a hot to the BATT terminal and adding a wire to the TACH terminal for the Auto Meter tach I installed. If the ECM is in fact looking for an RPM input, I'll have to assume the circuit is now only using the pump relay for the 2 second priming function and relying on the oil sending unit to keep the pump running after startup. If that's the case, is this proven to be a bad thing? Are the contacts on the oil switch not capable of sustaining this circuit permanently? Would it be prudent to splice in the PPL/WHT wire feeding the ECM pin B5 (distributor reference pulse in) to the TACH terminal on the HEI, thus probably reviving the circuit's intended functionality? Would ditching the ECM and factory electrics in favor of a keyed-on toggle switch feeding a dedicated pump relay be better?
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PlayingWithTBI

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I'll have to assume the circuit is now only using the pump relay for the 2 second priming function and relying on the oil sending unit to keep the pump running after startup. If that's the case, is this proven to be a bad thing? Are the contacts on the oil switch not capable of sustaining this circuit permanently?
If it's priming for 2 seconds, odds are the ECM is seeing DRPs (Distributor Reference Pulses) too or, it wouldn't fire the coil either and you won't get spark.

Here's where the ECM gets its DRPs. The pickup coil sends a sine wave to the ICM which sends the square wave (digital) Reference Signal to the ECM. Then it sends EST square wave to the ICM and thus, to the coil. When the ECM sees DRPs it turns on the Fuel Pump. One way you can confirm this is disconnect the Oil Pressure Switch and see if it still runs. The OPS is a backup to the FPR.

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1owner1990

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@PlayingWithTBI Unplugging the OPS... great simple idea. I will try that. If it won't run, it obviously points to the circuit relying solely on the OPS to run, to which again I ask: is this known to be a bad thing? With 18ish (maybe smaller) AWG wire, It's not like the pump is pulling off the chart amperage. Am I crazy to think the OPS could handle this?
 

PlayingWithTBI

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If it won't run, it obviously points to the circuit relying solely on the OPS to run, to which again I ask: is this known to be a bad thing?
IDK, I would wire the OPS to the FPR, or a different one, so it carries the higher current to the Fuel Pump. That is a lot to expect from the OPS even though GM did wire it that way but, IMHO it's just a backup, not the primary way to run it.
 

PlayingWithTBI

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I see you're running the large cap HEI which probably won't work the way GM designed this system. The TBI distributor is also an HEI system :33:
 
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