HELP 1991 GMC Jimmy Lost electrical power to fuel pump

[RaTcheTmasTer]

So I have tracked down a couple of facts.
Pump ran when I used a motorcycle battery to connect to the frame in back and positive wire to the pump plug and the pump works.
But it doesn't work when I turn on the key. No pump sound and does not fire up.

The truck turns over and will start with starting fluid for a second.

I checked the Fuel pump relay and it is good.

But I know just noticed that the headlights don't work and the interior lights don't work either????
Anyone have a diagram for the 91 Jimmy Blazer full size square body?

An ideas or tips?
Someone said it could be the computer?

Thanks . Tim

[RustyPile]

"Someone said xxxxxxxxxxx" is a guess and a guess won't fix your problem..

Here's a link to wiring diagrams:
https://www.rockauto.com/en/catalog/...g+diagrams,718

[RaTcheTmasTer]

Originally Posted by RustyPile

"Someone said xxxxxxxxxxx" is a guess and a guess won't fix your problem..

Here's a link to wiring diagrams:
https://www.rockauto.com/en/catalog/...g+diagrams,718

Thanks for the info. But I can only see a list for parts. Im not too good at using the internet for searches on websites.
can you explain on how to locate the wiring diagrams?
Thanks

[kwmech]

Does the fuel pump cycle for about 2-3 seconds when you first turn the key on? Is the oil pressure sending unit plugged in? Is there power to the fuel pump when the key is on?

[Dead Parrot]

I would do a fuse check. Stuff that was working and now isn't but tests good points to open circuit somewhere. Fuse panel is a good start place. Fairly common for fuses to protect more things then what the panel label suggests. i.e. a blown headlight fuse might also supply the fuel pump relay control side.

Another thing to check is any fusible links.

[Greenlee]

It’s been a like 15 years since I rebuilt my truck so my memory is a little fuzzy. But it’s a 350 TBI and I remember when I first put it all back together my fuel pump wasn’t working just like you’re describing. It turned out I forgot to hook up one wire that ran all the way from the alternator area to the battery. I want to say it was the smaller Pos wire that runs along side the Pos battery cable. So might be worth checking for a damaged or disconnected wire in that area. Given that your headlights aren’t working seems like you could have something like that going on.

[RustyPile]

Originally Posted by RaTcheTmasTer

Thanks for the info. But I can only see a list for parts. Im not too good at using the internet for searches on websites.
can you explain on how to locate the wiring diagrams?
Thanks

That's not a list for parts, it's a list of written materials. Find your truck on the list and click on it..

I found it by searching: "1991 GMC Jimmy wiring diagrams"...

[v30crewcab]

I would check the fusable links that come off the starter. I've had those go bad and cause all kinds of problems.

[RaTcheTmasTer]

Well I solved it!!!!!!!

I hope someone can benefit from my results.
two years ago the truck would not start in my driveway. it would turn over but would not start.

I had changed the coil, fuel pump, ignition module, fuel relay, fuel filter, cap and rotor, about 2 years ago. a new pump worked.

Now to current issue.
My truck stumbled and kept going a few times. Then one day it stumbled and died.
I had no power to the fuel pump. No power to headlights or interior lights.
I checked cap, rotor, ignition module, pump relay, ground on fuel pump. all seemed fine
So I changed the fuel pump. Would not run. I had changed the coil, fuel pump, ignition module, fuel relay, fuel filter, cap and rotor, about 2 years ago as I mentioned.

Well after two weeks messing with it, I had pulled out fuses, disconnected the battery for 30 minutes. I then took a small motorcycle battery and connected the battery to the fuel pump directly at the pump wire. When I grounded it the pump ran! But truck key was not giving power to the pump and it did not run.

Then the next day I got in and cranked it and it started!
I figured I must have done something and that must have fixed it.

Then three days the truck stumbled a few times over 10 minutes and then died.
I noticed the headlights and interior light were out and not working. Truck cranked but would not start.

So I started checking the wires around the lights, battery etc in the dark at 10:30 at night. I did have a flash light!!
Then I grabbed the wire loom at the alternator. I had left the headlights on incase I did something that would kick the lights on. Well when I moved the wires at the alt. post, the headlights and interior lights came on!!!
so I wiggled them again and they went out and a wire broke from the post. I had only been hanging on by a thread!! Luckily I had a wire cutter, a crescent wrench, and fixed the wire!! And it STARTED!!
I couldn't believe what looked like a 14 or 16 gauge wire that was somehow running the computer and/or the headlight and interior lights! Including the fuel pump!!!

Pictures show the wire. Red with fusible link and black insulator that was not on the wire anymore. Connected to the alt post. There is a much bigger wire coming off the bottom. I wonder where that one goes...
I tested wires in the engine compartment and everything seemed normal accept that one wire!

crazy but the gremlin is gone . . . problem solved.

[RaTcheTmasTer]

Originally Posted by v30crewcab

I would check the fusable links that come off the starter. I've had those go bad and cause all kinds of problems.

Very close to the answer!!

Alt wire. Do I have to keep the fusible links or can I remove them?

[RaTcheTmasTer]

Originally Posted by Greenlee

It’s been a like 15 years since I rebuilt my truck so my memory is a little fuzzy. But it’s a 350 TBI and I remember when I first put it all back together my fuel pump wasn’t working just like you’re describing. It turned out I forgot to hook up one wire that ran all the way from the alternator area to the battery. I want to say it was the smaller Pos wire that runs along side the Pos battery cable. So might be worth checking for a damaged or disconnected wire in that area. Given that your headlights aren’t working seems like you could have something like that going on.

YOU NAILED IT!! That wire was braking and barely had a connection!!
!!!!

[RaTcheTmasTer]

Originally Posted by Dead Parrot

I would do a fuse check. Stuff that was working and now isn't but tests good points to open circuit somewhere. Fuse panel is a good start place. Fairly common for fuses to protect more things then what the panel label suggests. i.e. a blown headlight fuse might also supply the fuel pump relay control side.

Another thing to check is any fusible links.

That was it!! I checked the fuses.

Then I checked the wires on the alternator and the small wire was mostly broken and barely hangin on it.

[RaTcheTmasTer]

Originally Posted by kwmech

Does the fuel pump cycle for about 2-3 seconds when you first turn the key on? Is the oil pressure sending unit plugged in? Is there power to the fuel pump when the key is on?

IT had no power to the pump.
Thanks for the ideas.
Its now been solved!! check my post above!