Fuel Pump Only Runs Off Oil Pressure Switch

1owner1990 · Jul 23, 2022

PlayingWithTBI said:
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.

Yeah, I guess I agree. Thanks.

smdk2500 · Jul 23, 2022

PlayingWithTBI said:
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.

The 6.5 platform is this way for the lift pump. The lift pump draws more power then the contacts in the switch are designed for and burn it up causing the lift pump to not work and cause issues for the injection pump. Lucky for us the aftermarket has figured it out and there is a harness that is made that takes the load off the OPS. The OPS still controls the pump just does it by turning a relay on and off.

Caman96 · Jul 23, 2022

1owner1990 said:
which I'm proud to say is a work of art

That frame is too! :waytogo:

1owner1990 · Jul 23, 2022

Caman96 said:
That frame is too!

Thank you.

1owner1990 · Jul 25, 2022

Schurkey said:
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.

Still wondering what your thoughts are with this one... do you think it would be a good idea 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?

PlayingWithTBI · Jul 25, 2022

1owner1990 said:
do you think it would be a good idea 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?

FWIU it needs to be a 0-5VDC square wave going to the ECM. IDK what's being generated off your HEI "Tach" terminal.

Schurkey · Jul 25, 2022

1owner1990 said:
Still wondering what your thoughts are with this one... do you think it would be a good idea 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?

The oil switch probably is sufficient to run the fuel pump without a fuel pump relay to split the load.

I'd verify voltage at the fuel pump--as close to the pump as practical, which is pretty-much at the rearmost connector before the harness goes up and over the fuel tank. If there's excessive voltage drop, it may be due to deteriorating contacts in the oil pressure switch. You could do voltage-drop testing across the switch to see. Don't forget to check for voltage on the ground side, too. Every volt on the ground side is a volt subtracted from the power that the pump "sees".

Wild Guess: There's VD everywhere in the circuit. You're not going to find one location with a heap of VD, you're going to find small amounts of VD everywhere you test for it. GM used under-sized wiring to run the fuel pump.

IF (big IF) you find problems with the oil pressure switch, using the replacement switch to trigger a relay seems like a good idea. You could also power the pump (if needed) by using starter solenoid "S" terminal power to trigger that same relay. You'd have the fuel pump running when cranking the engine (relay triggered by S terminal power) and also when the engine runs (relay triggered by oil pressure switch.)

Given that you now have a carb with float bowls and therefore fuel storage that a fuel injection system does not have, a two-second prime plus running the fuel pump using the oil pressure switch is probably sufficient. No need for S-terminal shenanigans.

Perhaps a moderator will move this thread to the LSX + Carb Swaps sub-forum.

1owner1990 · Jul 25, 2022

PlayingWithTBI said:
FWIU it needs to be a 0-5VDC square wave going to the ECM. IDK what's being generated off your

PlayingWithTBI said:
FWIU it needs to be a 0-5VDC square wave going to the ECM. IDK what's being generated off your HEI "Tach" terminal.

Thanks, yeah I saw the square wave in your reply with the ignition module schematic, but voltage wasn't in the diagram (from what I saw). I'll check voltage output on the tach terminal (still, even if it's 5VDC, who knows whether it's a square wave, right?). Worst scenario, I guess I blow up a donor ECM. This all seems to be a lot. Hard to believe a fuel circuit is the biggest challenge in a nut & bolt restoration... really considering pulling the pump-related wiring and running an entirely new 'key-on' circuit w/ toggle switch cutoff and a new relay.

1owner1990 · Jul 25, 2022

Schurkey said:
The oil switch probably is sufficient to run the fuel pump without a fuel pump relay to split the load.

I'd verify voltage at the fuel pump--as close to the pump as practical, which is pretty-much at the rearmost connector before the harness goes up and over the fuel tank. If there's excessive voltage drop, it may be due to deteriorating contacts in the oil pressure switch. You could do voltage-drop testing across the switch to see. Don't forget to check for voltage on the ground side, too. Every volt on the ground side is a volt subtracted from the power that the pump "sees".

Wild Guess: There's VD everywhere in the circuit. You're not going to find one location with a heap of VD, you're going to find small amounts of VD everywhere you test for it. GM used under-sized wiring to run the fuel pump.

IF (big IF) you find problems with the oil pressure switch, using the replacement switch to trigger a relay seems like a good idea. You could also power the pump (if needed) by using starter solenoid "S" terminal power to trigger that same relay. You'd have the fuel pump running when cranking the engine (relay triggered by S terminal power) and also when the engine runs (relay triggered by oil pressure switch.)

Given that you now have a carb with float bowls and therefore fuel storage that a fuel injection system does not have, a two-second prime plus running the fuel pump using the oil pressure switch is probably sufficient. No need for S-terminal shenanigans.

Perhaps a moderator will move this thread to the LSX + Carb Swaps sub-forum.

Thanks @Schurkey. First, thank you for your detailed replies, they are very helpful at clearing this up in my quickly advancing age. I totally agree the wiring is undersized, which is my main drive to keep the tan wires run in the parallel factory configuration and to get this circuit functioning exactly as it was designed. I can't trust a constant draw across such a small (and impossible to access on the side of the road) switch. The switch is new, as is everything else, and I'm not awfully concerned with voltage drop. I came across this video today; rather interesting and it supports my theory of feeding the ECM a tach signal via the ppl/wht wire. The comments all seem to support that this is the fix. I think I'm going to try it (maybe this weekend); will let you know the outcome. I'd bet the ls/carb swap crew would find this to be useful.

Probably will have to copy/paste

xc_hide_links_from_guests_guests_error_hide_media

Fuel Pump Only Runs Off Oil Pressure Switch

1owner1990

Newbie

smdk2500

I'm Awesome

Caman96

OEM Baby!

1owner1990

Newbie

1owner1990

Newbie

PlayingWithTBI

2022 Truck of the Year

Schurkey

Supporting Member

1owner1990

Newbie

1owner1990

Newbie

Similar threads

Latest posts

Staff online

Members online

Forum statistics

Share this page