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Dancing Fuel Gauge
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Jim L. in OR
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Hi Everyone, I'm still breath'n.

And having fuel gauge/tank/ and so on.

THE PROBLEM: Drive to filling station and fill the tank - needle rose to F but as fuel was consumed needle would sink to halfway point and go no further until ignition switch is turned off.

STEPS TAKEN: Wiring checked and it's OK. Removed sending unit from tank and it read .000 for Full, .073 for Empty. Checked gauge by grounding out put wire and it rose fitfully to F then moved fitfully up and down. Fitfully, but with a pulse. Cut hole in floor of trunk for access to Sending Unit with tank in place. I am tired of playing with the fuel tank proper. Put everything back together and went and filled the tank up.

RESULTS: Not much. Established that when tank is full needle will, with a steady pulse rise to F then dance up and down All over the place. Incidentally, Temperature Gauge barely registers. Checked Gauge out several years ago by grounding method. It's OK suspect sending unit - just never got around to replacing it.

I suspect the Instrument Voltage Regulater isn't playing the game but would like to hear what more experienced people have to say. Don't want to have to go back behind the Instrument Panel if it's something else.There are things with sharp teeth lurking back there.

Thank you in advance for any help you can give.

Jim L in OR

Posted on: 2020/11/15 23:21
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Re: Dancing Fuel Gauge
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It is possible the instrument voltage regulator is faulty but the temp gauge also works off the same unit and even though reading low, should be doing a version of the same dance if regulator was acting up. To check the temp sender and gauge remove the wire and touch to ground just long enough to see if the temp gauge moves to hot. I really suspect the fuel sender is the cause of your fuel gauge issues.

The 73 ohms at empty is good but it should be around 10 when full and not 0 ohms. Unless your meter is not very accurate at low values then that reading on the full side is suspect. Even though it might test good out of the tank there have been many instances where a sender again proved to be faulty once back in normal operation.

The not going all the way back to empty is a puzzle. Assuming the gauge itself is working properly and does go to empty when the sender wire is disconnected or key is off, for it to stay at half full when the tank is not would indicate something in the tank is preventing the float from dropping or else there is something amiss inside the sender that is providing a fixed resistance value that is enough to keep the gauge reading even though the float has dropped past the half full level.

One other thing that many have found after tanks were removed and replaced is a poor ground between the tank and body can become an issue. The poor ground will also cause a dancing gauge but would not be responsible for it failing to go past half full. To completely rule out a ground issue you might drill a small hole in the excess metal at the center weld seam on one of the corners and attach a small wire to provide a solid ground connection between the tank and frame or body. Alternately, you could also figure out a way to attach a wire on the sender flange or retaining ring working thru your access hole.

Posted on: 2020/11/16 10:57
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Re: Dancing Fuel Gauge

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Does it only do this when the car is moving, or does it "dance" when sitting still?

It when moving, that would point to an intermittent ground or loose wire somewhere.

If it does it while sitting, and an ohmmeter between the post on the sender (while it's still in the tank) and a good body ground is stable (not dancing) then I would suspect it's the gauge itself. Some heating up and cooling down inside.

I'd pull the cluster and do the tests from the gauge side as that will help diagnose any issue in the wire between the gauge and the sender as well.

Posted on: 2020/11/16 12:29

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