Low voltage at fuel pump

[89chevy4scrap]

I have a 1989 k1500, I’m trying to figure out how to test the voltage to the fuel pump. I used a volt meter connected to the pump ground to test the harness that plugs into the pump but I’m a little confused. I tested the the purple wire in the harness and it only showed 8 volts so I figured that’s where my problem lies (intermittent running). But as I do more research it seems to me that the purple wire is only for the fuel gauge and my gray wire is my power. I am confused because when I hook my voltmeter up to the gray wire and turn the key it seems to show no power at all (even in the first two seconds of turning the key. The confusing part to me is how can my truck run at all (sometimes for 10 minutes or more), if my power wire has no voltage going to it. I’ve also either cleaned or replaced every single ground I can find on the truck including the one going from the pump to the frame rail and from the pump harness up to the thermostat housing. I’ve also tested power from the inline fuse and the relay for the fuel pump and they both show the same voltage as the battery. I’m no expert at all when it comes to electrical so any advice at all is greatly appreciated.

 

[JDGMC]

Engine and transmission type? Do you have dual fuel tanks? You may have an oil pressure-sending unit tied into the fuel delivery system to shut it down in case of low oil pressure (below 4 psi). Regarding wire track - get yourself the manuals here.

 

[89chevy4scrap]

It’s a 350 tbi, I’m unsure of the transmission type. It’s just the one fuel tank. I’ve read a few times about the oil pressure sending unit but I haven’t really looked into that much, maybe I will try and check it out tonight. Also regarding the wiring diagrams, I have a physical copy of the book with the wiring diagrams and I’ve found some of them online too. The problem is that in the diagrams it just shows one black wire and one gray wire going to the pump. It doesn’t show the purple wire. Just wondering if anyone knows wether or not my gray “power” wire should show voltage with the key turned over. Thanks for the reply.

 

[JDGMC]

What are the wire colors of the truck harness? The connector is on top of the frame rail. The picture shown is the fuel pump wiring.

 

[89chevy4scrap]

They are the same colour, as what goes to the pump, you can see the harness sitting in the top right of the photo. I was able to do some more testing and came to find that I do have decent voltage (11v) going to the power wire for the pump when the key is turned over. Although it drops down to about 9v or less when cranking. I read that this could be due to a weak battery and realized that I had no idea how old the battery I was using was. So I brought in a brand new battery. I plugged it in and thought It was problem solved. The truck sat and ran for 30 minutes straight and didn’t even stutter. Turned it off and back on again about 3 times just to make sure. Then maybe 3 hours later I go to start it up again and same as before, would fire up for a second or two and then die. I’m at a loss.

 

[GoToGuy]

Do you have the OE service manuals? If not in the engine thread there are a sticky for huge library of free pdf service manuals. The real ones, not generic clayner, or Hynes. With correct wire diagrams. And easy to understand troubleshooting procedures. Step by step. I don't have the '89 at home. Here is the 90 diagram how fuel pump is powered. Just pump power not gauge.

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[89chevy4scrap]

No I just have one of the hynes manuals, I’ll check out ones you’re mentioning and see what I can find. Thanks for the reply