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Fuel gauge not working properly

Tiny
MARK UNDERWOOD2
  • MEMBER
  • 1995 GMC C1500
  • 5.8L
  • V8
  • 4WD
  • AUTOMATIC
  • 250,000 MILES
Replaced fuel pump and gauge only works right when full.
Wednesday, August 5th, 2020 AT 5:43 PM

19 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,758 POSTS
Hi,

When you replaced the pump, was it a complete unit including the float and rheostat assembly for the fuel gauge? Does it stay on full and never move?

If it is staying on full and not reacting from that point, it's easy to snag the float when installing it causing it not to move. Take a look at pic 1. When you reinstalled the pump assembly, was the highlighted component free?

Let me know.
Joe
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Wednesday, August 5th, 2020 AT 9:26 PM
Tiny
MARK UNDERWOOD2
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Thanks Joe, yes it was installed as complete unit, gauge stays on full (with full tank), after I run about 10 gallons out of tank, it starts fluctuating wildly, fill tank back up and it stays on full.
Thanks
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Thursday, August 6th, 2020 AT 6:20 AM
Tiny
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Hi,

I have to be honest. It sounds like the new level sensor (rheostat) is faulty. If you would have told me it never changed from full, I would have suspected the float got stuck during installing it. Since it does what you described, it sounds like it is bad.

Let me know if you have other questions.

Take care,
Joe
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Thursday, August 6th, 2020 AT 11:37 PM
Tiny
MARK UNDERWOOD2
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Thanks Joe, can I just plug in another pump (not installed) and see if it changes?
Thanks,
Mark
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Friday, August 7th, 2020 AT 6:45 AM
Tiny
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Hi,

Yep, you will need to move the float up and down to see if the gauge reacts.

Let me know what you find.

Joe
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Friday, August 7th, 2020 AT 11:01 PM
Tiny
MARK UNDERWOOD2
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Hey Joe, I crawled underneath truck and 1st thing I saw was a broken cable that ran from frame to filler tube, I repaired cable to no avail. What was that? For static electricity? Anyway looks like ground to fuel pump is not as easy to get to as I thought.
Any suggestions?
Thanks,
Mark
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Sunday, August 9th, 2020 AT 1:16 PM
Tiny
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Hi,

Is the fuel pump working? If it is, then the ground is good. They both ground at the same point. As far as the ground, it is at the top of engine. See the attached pic.

Joe
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Sunday, August 9th, 2020 AT 10:33 PM
Tiny
MARK UNDERWOOD2
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Ho Joe, fuel pump is working, the gauge is not working properly. The ground for my gauge is what I'm thinking. Does gauge work off fuel pump float?
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Monday, August 10th, 2020 AT 5:10 AM
Tiny
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That ground is on the left side of the instrument panel. See the attached pic.

Let me know if that helps.

Joe
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Monday, August 10th, 2020 AT 9:23 PM
Tiny
MARK UNDERWOOD2
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Hi Joe thanks! Couldn't make out last word of ground location. I can make out " right behind left side of?" Can't make out last word even with magnifier.
Can you please help?
Thanks,
Mark
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Thursday, August 13th, 2020 AT 7:29 AM
Tiny
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Hi,

I see that. It can't be read. It says of IP which is instrument panel.

Let me know if that helps.

Joe
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Friday, August 14th, 2020 AT 12:16 PM
Tiny
MARK UNDERWOOD2
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Hi Mr Joe! Thanks again for info.
I found grounds as diagram shows. The ground to rear of right cylinder head was broken in half. Can I ground this wire elsewhere? Exhaust manifold? Intake manifold?
Thanks!
Mark
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Monday, August 17th, 2020 AT 10:30 AM
Tiny
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Mark,
Don't ground it on the exhaust manifold. It will burn off. Just find a good ground place on the engine where it isn't near anything too hot. You could even ground it to the body of the vehicle.

Joe
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Monday, August 17th, 2020 AT 2:18 PM
Tiny
MARK UNDERWOOD2
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Hi Joe, I replaced and cleaned 2 grounds (actually it was just a frame to body to engine ground) that were under hood and now gas gauge just stays pegged out. Any ideas? Suggestions?
Thanks,
Mark
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Tuesday, August 18th, 2020 AT 8:45 AM
Tiny
JACOBANDNICKOLAS
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Hi,

If it was a body ground, I'm surprised it changed anything. Did you locate the ground under the instrument panel?

If it is pegged, the only other thing I can think of is the float is stuck or the rheostat is no good.

Joe
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Tuesday, August 18th, 2020 AT 6:32 PM
Tiny
MARK UNDERWOOD2
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Hi Joe, yes I redid ground wires under instrument panel and body ground on left side of engine (body to ground). With still pegged out fuel gauge. Is there anyway to check grounds that I redid? I want to check out all possibilities before I pull truck bed.
Thanks,
Mark
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Thursday, August 20th, 2020 AT 5:58 AM
Tiny
JACOBANDNICKOLAS
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Mark,

The only thing you can do is check for continuity to ground from the opposite end of the wires that you repaired. Is there any chance you could have gotten the float stuck when you replaced the fuel pump? If this wasn't an issue before the pump replacement, then I suspect that either the float is stuck, the sender rheostat is bad on the new unit, or you have a short to power. If you disconnect the connection at the pump, does the gauge drop to empty?

Joe
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Thursday, August 20th, 2020 AT 10:06 PM
Tiny
MARK UNDERWOOD2
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Hi Joe thank you for your info. You helped me greatly by pointing out the grounds and such.
I have decided against dropping the tank for now.
For other questions do I need to go back to main page or ask here?
Thanks,
Mark
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Sunday, August 30th, 2020 AT 4:47 AM
Tiny
JACOBANDNICKOLAS
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Mark,

If the question is related, I would be happy to help. If it is on different topic, I ask that you start a new thread. We try to keep things specific to one issue per thread. I hope you understand.

If you start a new thread, I hope to get it.

Take care and let me know.

Joe
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Sunday, August 30th, 2020 AT 8:57 PM

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