Fuel Pump will not prime - 1990 5.7 Chevy Sport

[kevvan]

New to these trucks so forgive the lack of understanding...

The truck died and I replaced the fuel pump, assuming it was the problem. However when I took out the old one and tested it, it worked when supplied power. I still went ahead with the fuel pump replacement and put everything back together.

What I know now is the fuel pump is not getting power. I tested it under the truck on a spot where I had access to the wires that run under the driver's door.

I know that if I supply starter fluid to the truck, it will run for a bit and I have checked that it has spark and everything on that front seems to be ok. I checked the fusible links, the fuse on the cowl, and the fuses in the fuse box in the driver's compartment.

Possible guesses, but I'm not too confident on how to troubleshoot: ECM or Fuel Relay

Kind of hoping that someone can point me to a troubleshooting guide or offer some words of wisdom while I do research on how to resolve the problem.

 

[Dover]

Fuel pump has its own fuse on the passenger side of the firewall next to the fuel pump relay. Check that, if the fuse is good could be a bad relay.


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[kevvan]

Fuel pump has its own fuse on the passenger side of the firewall next to the fuel pump relay. Check that, if the fuse is good could be a bad relay.


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Sorry, should have said this. I checked that fuse first. It is good, tested fine. Is there some place I could find directions to test the relay?

 

[F4U-1A]

Starter Fluid! My Jeep and the Korea war of circa 1952? (kidding) If you have a 12v supply, and a DMM, you can verify a faulty relay. 1990, could be an open somewhere. ECM may be bitched, My 1996 has a PCM. You have electrical abilities . Check for simply continuity first. There are some Primo Techs, on this Forum. And if all else fails hit the GMT with a rubber mallet. ( had to get that dig in for HotWheels). All Good.

 

[Schurkey]

If you crank the engine long enough to get oil pressure, the oil pressure switch should bypass the fuel pump relay, and the pump should run, and the engine should start.

If it does, the problem is in the ECM, fuel pump relay or wiring harness.

If it doesn't, the problem is most likely in the under-vehicle harness, the in-tank harness; or in the power supply to both the relay and the oil pressure switch.

A proper wiring diagram would help. This site has links to service manuals; I can't remember if 1990 is included...but it's probably the same for '88--'95.

 

[Pinger]

If you have air con - the relays are the same and sat side by side. Try swapping them.
If that doesn't work (or you have no AC) pull the relay - it's identifiable by the info on the FRC lid's inner - and bridge two connections. Cant remember their numbers but there's a YouTube video which shows them (they are diagonally opposite each other IIRC).
Or, unclip the FRC and look for where the grey and orange wires are beneath the relay and bridge them. Or, just run a live from the battery to the grey one. That by-passes the relay.

 

[Pinger]

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[kevvan]

This is the diagram for the fuel relay for my 5.7L. If I take a pin and connect #440 with #920, I bypass the switch and supply power to the fuel pump.
I think I have that right. If I am right, if I pin the two, I should hear the pump start in the tank - yes???

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[Schurkey]

There should also be a wire, which if connected to Battery +, will also power the fuel pump. I know where the wire is on my two Luminas, but I don't remember where the wire is on the GMT400s.

 

[PlayingWithTBI]

If I take a pin and connect #440 with #920, I bypass the switch and supply power to the fuel pump.

Yes, that looks about right. This is for an 88 but, as @Schurkey said, it should be the same.

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Here's a link to some manuals
https://www.gmt400.com/threads/88-95-service-manuals.43575/