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TR2/3/3A Fuel gauge problem

BobbyO

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I've read several other threads concerning this problem but am not sure any of them were the same. My fuel gauge was working properly and reading 1/2 tank when I added fuel (about 5 gallons). At that time the gauge pegged to full and stays there no matter the amount of fuel in the tank. Nothing was done to the gauge so I suspect the sending unit. If so, what happened? Is there a way to check it? Thanks!
 

martx-5

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If it just happened, it's probably not the gauge. The sender should read about 0 ohms empty and about 80-90 ohms when full. If it stays pegged chances are you either lost the ground or the green/black wire from the gauge has lost continuity. There is a connector along the way. You can check the operation of the gauge by grounding the green/black wire. It should go to empty. Check it at the sender. An ohmmeter will tell you if the sender is working. Bad grounds on the sender are the big offender.
 
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BobbyO

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Checked the green/black wire at the sender by grounding and the gauge went to empty. Ran a long jumper wire from the ground on the sender to the ground strap on the battery, no change. Estimating somewhere around 1/2 tank ohm reading was 66 ohms at the sender. Open to any and all suggestions.
 

dklawson

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As stated above, when you disconnect the green/black wire from the sender or gauge the gauge should go to (above) full. When you short that wire (or its gauge terminal) to ground the gauge should read (below) empty.

Based on the tests you have already done, this is unlikely... but check that the gauge itself has a good ground. It's case must also have a good ground for the gauge to work properly.

For more information on the magnetic gauges, visit the MGA Guru website. Barney Gaylord has pages that explain how the gauge is supposed to work and steps on how to calibrate it.
https://mgaguru.com/mgtech/electric/fg_01.htm
 
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BobbyO

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My bad. Still had the wires connected when I measured resistance of the sending unit. Removed wires and have an open in the sending unit, bad unit. Odd how it failed when moved from about half when I added fuel. Guess a 50+ year old unit finally gave up the ghost. Thanks for all the help anyway.
 

TR3driver

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Whoops, I see you solved your problem while I was writing a post. Good on you!

Not so strange, the variable resistance is just a coil of resistor wire. It wears as the arm slides against it, eventually creating a thin spot that breaks. Most of the wear is near the center of the sweep. So they do wear out, and your symptom is typical of one worn enough to break.
 

CJD

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Just a thought before you order a new sender...you might want to look down into the filler neck with a flashlight to make sure it's not just the float arm sticking in the up position.
 
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