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Old 05-15-2022, 12:55 AM   #1
Fraud
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Fuel gauge issues

I'm certain this is a dumb problem and I'm missing something obvious. I've read through the various threads here and can't pinpoint my issue.

My fuel gauge is pegged at 3 o'clock.

Pull the sender wire at the fuse block, no change.

Pull the sender wire at the tank, no change.

Ground the sender wire at the fuse block, no change, I would have expected this to send the gauge to empty (gets ign switched voltage of ~7.8v)

Ground the sender wire at the tank, no change (also ign 7.8v)

Measuring resistance between the sender wire and the tank I see about 85 ohms, and I put 15gal in it yesterday without knowing how much was in there beforehand, so "near full" seems about right.

For the sender wire to get switched voltage that implies the gauge is working, correct? And to see that voltage at the tank implies that there is continuity on that wire... But if it's not going to empty when I ground it, what am I looking for next?

The ground on the gauge/instrument panel?

Thanks for your help!
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Old 05-15-2022, 04:08 PM   #2
pjmoreland
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Re: Fuel gauge issues

It does sound like your sender wire has continuity to the gauge since you are measuring voltage on it. Here are some thoughts:

1) Are you certain you are touching the sender wire to a good ground?

2) Is the metal case of your gauge cluster grounded adequately?

3) You might learn something useful by disconnecting the gauge from the flexible circuit board so you can measure the internal resistances of the two windings inside the gauge. Maybe the winding that pulls the needle toward Empty is fried. Maybe the ceramic resistor on the back of the gauge is bad.
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Old 05-15-2022, 06:31 PM   #3
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Re: Fuel gauge issues

Quote:
Originally Posted by pjmoreland View Post
It does sound like your sender wire has continuity to the gauge since you are measuring voltage on it. Here are some thoughts:

1) Are you certain you are touching the sender wire to a good ground?
I'm using the door latch, which seems to be pretty solid/consistent with the test light, but I admit I could be less lazy!

Quote:
2) Is the metal case of your gauge cluster grounded adequately?
Probably the next step, but I'd assume if it wasn't grounding it wouldn't power up at all... No?

Quote:
3) You might learn something useful by disconnecting the gauge from the flexible circuit board so you can measure the internal resistances of the two windings inside the gauge. Maybe the winding that pulls the needle toward Empty is fried. Maybe the ceramic resistor on the back of the gauge is bad.
That was my next -next step, just trying to rule out things that involve so much pulling apart before I get into it.
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Old 05-15-2022, 07:09 PM   #4
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Re: Fuel gauge issues

Yeah, I wouldn't expect the needle to move to the Full side if the gauge cluster wasn't grounded.
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Old 05-16-2022, 11:04 AM   #5
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Re: Fuel gauge issues

Have you checked out this thread?

http://67-72chevytrucks.com/vboard/s...d.php?t=640615
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Old 05-16-2022, 01:19 PM   #6
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Re: Fuel gauge issues

Yeah I went through that one but it didn't seem to point to what I'm seeing :/
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Old 05-21-2022, 08:31 PM   #7
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Re: Fuel gauge issues

Ok so I'm a damn fool. Grounded it properly from both ends of the tan wire and the gauge does indeed go to empty, so the gauge works as expected and I'm even more confident I have continuity all the way to the sender.

So got the multimeter out again, and measuring resistance from the connector on the sending unit to a good earth. I was really just testing for continuity because my working theory was that a connection came loose in the sender and it wasn't grounding, but I got a reading...

Unfortunately I'm seeing a little over 1k ohms. Zeroed out the meter again to be sure...

So... Dead sending unit?
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Old 05-21-2022, 09:27 PM   #8
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Re: Fuel gauge issues

1000 Ohms is way outside the expected 0-90 range of a good sender. Are you touching one of the leads to the mounting flange and the other to the electrical connection post on the sender?

How did you get the 85 Ohm measurement previously? Was the tan wire still attached to the sender when you took that measurement? The wire should be disconnected when measuring the sender resistance.
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Old 05-21-2022, 11:16 PM   #9
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Re: Fuel gauge issues

Quote:
Originally Posted by pjmoreland View Post
1000 Ohms is way outside the expected 0-90 range of a good sender. Are you touching one of the leads to the mounting flange and the other to the electrical connection post on the sender?
One lead to the sender connection on the tank (tan wire disconnected), one lead to ground.

Quote:
How did you get the 85 Ohm measurement previously?.
By reading the wrong scale on my analog multimeter, see damn fool comments on previous post
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Old 05-22-2022, 12:27 AM   #10
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Re: Fuel gauge issues

Ah, that makes sense.

Is your tank grounded adequately? If you touch the tan wire to the mounting flange of the sender, does your gauge move to empty, or do you have to touch it somewhere else to get it to move?
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Old 05-30-2022, 07:55 PM   #11
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Cool Re: Fuel gauge issues

Ok just got back to it, haven't had a lot of time lately...

Touched tan wire to the tank itself (via the sender mounting flange) and the gauge goes to empty (or a little under 1/8th)

If I measure resistance between the sender wire terminal and the mounting flange for the sender I get a little over 1k ohms like I was seeing before...

I should just bite the bullet and buy a new sender huh? I can't work out what would cause it to go so far out of range but I feel like I've isolated everything else now.
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Old 05-30-2022, 07:57 PM   #12
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Re: Fuel gauge issues

I agree that there is a problem with your sender. Your gauge and associated wiring seem to be working correctly.
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