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Fuel pump not getting enough power

Tiny
JASONL77
  • MEMBER
  • 1988 GMC S15
  • 2.8L
  • V6
  • 4WD
  • MANUAL
  • 180,000 MILES
Truck listed above has a V6 TBI. My fuel pump isn’t cutting on when I turned the key over. I just replaced the fuel pump not long ago. I checked the power going to the pump and it seems to only be getting half the power that’s needed. I traced the wires and checked them, there were two wires that were burnt in one area. I replaced those. I also changed the ECM B fuse that was blown. I’m stumped as to what to do next. Any help would be appreciated. I’ve been trying to get this truck running for quite some time now.
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Monday, May 24th, 2021 AT 2:23 PM

9 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

The pump is designed to prime when the key is first turned on (for a second or two), and then it turns off until the engine starts. Once it starts, it should get 12v. Are you only seeing 6v?

If that is the case, you need to go to the fuel pump relay and check pins A, b, and d. See if there is 12v there. There could be an issue with the ECM.

If you look below, I provided the diagnostics for the pump relay circuit. Follow the directions and let me know what you find.

Here is a link you may find helpful when testing:

https://www.2carpros.com/articles/how-to-check-wiring

Let me know what you find or if you have other questions.

Take care,

Joe

See pics below.

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Tuesday, May 25th, 2021 AT 8:56 PM
Tiny
JASONL77
  • MEMBER
  • 12 POSTS
Fuel pump is cutting on now after I fixed a couple of wires coming from it. Now I’m having trouble with the injectors not spraying fuel. I’ve checked the injectors and they are good but are not getting the signal to spray for some reason.
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Saturday, July 3rd, 2021 AT 9:51 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

The Electronic Control Module (ECM), located in the passenger compartment, is the control center of the fuel injection system. Before we start trying to diagnose what can be the cause, I need you to check for diagnostic trouble codes. The vehicle has an OBD1 system. It doesn't require a scanner to get the codes. All you need is a short jumper wire or even a paper clip. Here is a link showing how it's done:

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Try this and let me know what you find.

Joe
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Saturday, July 3rd, 2021 AT 8:46 PM
Tiny
JASONL77
  • MEMBER
  • 12 POSTS
I'm a little late on this but I had to take a break from working on this truck. I've checked for the codes and I'm not getting any. I turned the key over and one of the injectors was spraying and the truck sounded like it wanted to start. When I turned it over again neither of the injectors were spraying and still aren't. Any help is greatly appreciated. I'm close to throwing in the towel on this one.
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Monday, April 11th, 2022 AT 9:31 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

Are you sure the fuel pump is providing the pressure needed? I looked for a wiring schematic and did find an old OEM schematic for the fuel control. I noticed it is tied to the oil pressure switch. Also, there are two relays for the fuel pump. The issue for me is the schematic isn't specific to where the items are located and I'm not there to look.

Take a look through the schematic below. See if it helps. Confirm the relays are good and the fuses indicated are also good and have power to them.

If you do have power and the relays are good, I suspect the ECM is the likely issue. The injectors are controlled by the ECM.

Take a look through the schematic and let me know if you have already checked these items. Also, pic 2 explains how to test the injectors.

Take care,

Joe

See pics below.
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Monday, April 11th, 2022 AT 6:26 PM
Tiny
JASONL77
  • MEMBER
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You mentioned the oil pressure switch, so I started looking around and checking some things out. I found these 3 (images attached) underneath and two of them were hooked up but the one with three wires was just hanging there and I'm not seeing a place for it to go. Any and all help is appreciated. I'm still checking the other things you listed and will let you know what I come up with.
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Sunday, April 17th, 2022 AT 10:18 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Looking at pic 2, I am struggling to determine wire colors. I see the orange, but the others are difficult.

I did note the oil pressure switch and fuel relays all have an orange wire with a tan/white wire going to them. Is there any power to any of the wires?

Let me know.

Joe

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Monday, April 18th, 2022 AT 1:32 PM
Tiny
JASONL77
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I thought I was getting somewhere. It acted like it was going to start a couple of times. But each time only one injector was spraying, and it was a different one each time also. Now I'm back to neither of them spraying again. I've replaced the oil sending unit, coolant sensor, knock sensor, I've replaced so many different parts I probably could have already bought an old truck that runs.
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Sunday, May 8th, 2022 AT 2:41 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

I'm sorry to hear this isn't running yet. Have you checked to see if the ignition is working properly? In other words, does it get spark to the plugs? If there is no ignition signal, the fuel pump won't work.

https://www.2carpros.com/articles/how-to-check-for-ignition-spark

Also, have you confirmed fuel pressure? It only requires between 9 and 13 PSI, but if that is too low, the injectors won't properly work. See pic 1

Let me know.

Joe

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Sunday, May 8th, 2022 AT 8:00 PM

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