Fuel Pump will not prime - 1990 5.7 Chevy Sport

[Schurkey]

I guess I didn't remember where the priming wire was, because it's routed to the ALDL on the GMT400.

The wire hangs free near the battery on my Luminas.

 

[Pinger]

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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As you stand at the driver side looking into the engine bay at the space where the relay was - terminal 30 is top right. That is, toward the centre line of the truck and toward the rear. From there it's a straight run to the pump. Jump it directly from the battery and the pump has an indisputable live feed.
If not directly from the battery try terminals 86 and 87 direct to terminal 30. One is hot at all times, the other via the PCM. One of them should work.

 

[HotWheelsBurban]

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.

Hey now, that "smack- the-tank" trick does work sometimes! That was also one of many things done to the Burb last summer when passlock decided to go kookoo. I wasn't sure that it wasn't the fuel pump, and I knew that had worked on my previous Burb. That's how we got it to run to get that Burb to the shop to have it changed LOL.
Anyway, good luck with your truck!

 

[thinger2]

Your aldl connector should have a test pin for the fuel pump. On my 94 that is terminal "g" but triple check that it is the same on a 90.
If you put 12 volts to the test pin you should hear the pump turn on.
Dont run it that way just test it.
If it turns on, everything past the pump relay and oil pressure sensor is good.
You can have duelling intermittants at the relay and the sensor at the same time.
In other words it may flip between running off of the relay and then the oil pressure
They provide "alternate paths" to the pump to keep it from dying while you are driving.
But, if they are both bad or both intermittanly failing it should show a code 43 but only if you can get it to run long enough to kick a code at all.
Fuel pump relays come in a couple of different flavors
The 15.99 at the parts clown store is made in china and supposedly rated for 15 amps. It has aluminum contacts
NAPA sells a 30 amp for about 30 bucks made by Echlin that has copper contacts and is made in the USA.
Get that one
Theoretically, the pump shouldnt be able to draw enough amperage to fry a 15 amp.
But that assumes that the Chinese 15 amp rated part will actually handle 15 amps.
It will only handle 15 amps for a very short time.
Same for the oil pressure.
If it passes the test pin power up replace the relay drive it around and see if you get a code 43
(fuel pump circuit low voltage)
If it does, replace the oil pressure sensor

 

[kevvan]

Replaced the distributor* + ICM today, the Fuel Relay, and the oil sensor (Since I was replacing the distributor and could get to it). I buttoned everything up only to find that it still will not start.

I do hear the fuel relay clicking now, which I did not hear before. Went through the grounds, and they are good. If I spray starter fluid in the truck I can make it run. Still seems like it is not getting power to the fuel pump and it does not prime. Tested the fuse when turning the key and see it comes on for a few secs and then turns off as it should. Really thinking I have to trace a broken wire back to the fuel pump but not to clear by the diagrams how or what wire to test for continuity.

I would guess that the power comes from the fuel pump relay through the harness to the bulkhead connector (C100 on the diagrams) and then down along the frame rail to the fuel pump. Not sure though how to read the diagram and figure out what wires are involved.

*I went to take the distributor cap off and the bolt broke that holds it on. It is the original distributor for the truck. I figured since I was in there I would swap it out for a new one. Stupid reason, but sometimes it is time to do stupid things.

 

[Schurkey]

I do hear the fuel relay clicking now, which I did not hear before.

Progress.

Went through the grounds, and they are good.

Including the ground wire for the fuel pump, at the back of the vehicle???

Still seems like it is not getting power to the fuel pump and it does not prime. Tested the fuse when turning the key and see it comes on for a few secs and then turns off as it should.

As suggested, you could have power to the pump, but no ground, and you'd have the same symptoms.

Really thinking I have to trace a broken wire back to the fuel pump but not to clear by the diagrams how or what wire to test for continuity.

Two wires: Tan/White from fuse to pump, Black/White from pump to ground.

I would guess that the power comes from the fuel pump relay through the harness to the bulkhead connector (C100 on the diagrams) and then down along the frame rail to the fuel pump. Not sure though how to read the diagram and figure out what wires are involved.

C100 does not seem to be involved. C105 is, by the fuel tank.

 

[kevvan]

Progress.


Including the ground wire for the fuel pump, at the back of the vehicle???
Response> When I replaced the fuel pump in November, I took the tank down and did the job that way. I knew how important that ground was at the time and doubled down on cleaning the connections when putting it back together. I didn't test it, but of all the grounds, that one I have supreme confidence in.

As suggested, you could have power to the pump, but no ground, and you'd have the same symptoms.


Two wires: Tan/White from fuse to pump, Black/White from pump to ground.


C100 does not seem to be involved. C105 is, by the fuel tank.

Response > The guide I am referencing, [1990 Light Truck C/K Models Electrical diagrams and diagnosis manual], shows C105 at the headlights... In the diagram I have, C406 is the plug at the tank: which is inaccessible to me because it is above the tank and would require me to lift the bed to get to. Something I want to avoid if I can.

If I start at the fuel tank, and trace the branch back to the engine compartment, the wires come up on the drivers side under the master cylinder and appear to go into the junction box (Labeled C100 in my diagram and called the bulkhead connector in references I've seen. To me, that block looks like it mates up with a block at the inside of the truck, and possibly the fuse panel.

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[kevvan]

This is the diagram I am studying. From what I think I know, the basics are power is supplied to the Fuel pump relay, it opens and the power to run the fuel pump is delivered from the [920 1.0 Pink/BLK] pathway to the oil pressure switch [C135] and the inline fuse [C136]. From the inline-fuse it travels out the TAN/WHT wire to multiple places. At Splice [S126] it branches off to plug [C406] which leads to [C407] at the fuel pump.

If you know how this diagram works, have I traced it right?

In addition to what I think is going on, there is the [C100] block in the diagram. It has the PPL/BLK wire that feeds to plug [C407] at the fuel pump. So in some way, the [C100] bulkhead block is in the mix through [C107] which is connected to it.

I will share that this stuff reminds me of when I learned to build a computer from scratch. Learning DOS, and how systems boot, how the motherboard is designed to work with RAM, the CUP, the GPUs and so on. Fun but infuriating. All new learning curves. I like a challenge but I also would really like to see my truck turn over!

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[kevvan]

Thanks for this. I am going to try it today after work.

Your aldl connector should have a test pin for the fuel pump. On my 94 that is terminal "g" but triple check that it is the same on a 90.
If you put 12 volts to the test pin you should hear the pump turn on.
Dont run it that way just test it.

 

[Schurkey]

power is supplied to the Fuel pump relay, it opens and the power to run the fuel pump is delivered from the [920 1.0 Pink/BLK] pathway to the oil pressure switch [C135] and the inline fuse [C136]. From the inline-fuse it travels out the TAN/WHT wire to multiple places. At Splice [S126] it branches off to plug [C406] which leads to [C407] at the fuel pump.

If you know how this diagram works, have I traced it right?

Almost.

Power is provided to circuit 440, which supplies BOTH the oil pressure switch and the fuel pump relay. If EITHER the oil switch or the relay is activated, the fuse gets power via circuit 920, and then back to the pump via ckt 120. In normal operation, both switch and relay are active when the engine is running, so the current flows through the relay and the switch and is combined at the splice before the fuse.

In addition to what I think is going on, there is the [C100] block in the diagram. It has the PPL/BLK wire that feeds to plug [C407] at the fuel pump. So in some way, the [C100] bulkhead block is in the mix through [C107] which is connected to it.

C100 and C107 and that purple/black wire are only involved to make the gas gauge in the instrument cluster function. C100 has nothing to do with running the fuel pump.