Weird fuel gauge problem
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Joined: Dec 2010
Posts: 139
Weird fuel gauge problem
Yesterday somebody backed into my back bumper. Luckily the car is fine. But now, my fuel gauge is way off. With the key out of the ignition, the fuel gauge needle reads just over the full mark. When I turn the key, the needle goes clear over and past the "hot" on the gauge. What could be causing this?
.
Thread Starter
Registered User
Joined: Dec 2010
Posts: 139
Haha hopefully. Well I got under the car and found what has to be the ground(random wire coming from top of fuel tank, grounding on underside of trunk kind of above the axle). It looked fine, but I took it off and cleaned it. Gauge still reads the same. The gauge stayed the same even when I had the ground off if that makes any difference. Where is the next place to look? The slight hit should not have been enough to do anything to the sending unit. It was a slow hit, didnt shake the car too bad.
Registered User
Joined: Nov 2010
Posts: 131
From: Chicago
You can also check the brown wire going to the sending unit just over the fuel neck and over the tank , try jiggling it but not to hard if the wire is totally disconnected it will read way past Full , these senders are suppose to read 0-90 ohms maybe you nicked it with the hit ....
Thread Starter
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Joined: Dec 2010
Posts: 139
Well I disconnected the tan wire and grounded it. Nothing. Jiggled the wire and it seemed tight on the other end. No matter what I do the gauge stays the same. The part that I find weird is that when I turn the key to the on position the gauge moves from the "F" clear past the "hot". So maybe there is some other wire coming in contact with it and giving it power. Because it moves drastically when the key goes from the off to on position. Im starting to think the fender bender and this happening at the same time are just coincidence. I have wiring problems all the time. Thanks ahead of time for any advice. Wiring is not my strong point on vehicles.
Connoisseur d'Junque
Joined: Sep 2010
Posts: 21,183
From: The Hudson Valley
Okay, here's your fuel gauge circuit:
+12V (Instrument Power) ------> Fuel gauge (Driver's side + terminal) -----> Fuel gauge windings -----> Fuel gauge (Passenger side - terminal) -----> Tan wire -----> Tank sender -----> Rheostat (0-90Ω) -----> Ground.
The sender should read 0 - 0.6Ω at E, 44.9-45.1Ω at ½, and 88 - 90Ω at F.
That means that when you break the circuit, the gauge should read past F and when you short the tan wire (or the passenger side gauge terminal under the dash) to ground, the gauge should read E.
If you have a heavy duty (1W) 100Ω rheostat or potentiometer (maybe from an old tube radio - I'd suspect a tiny transistor radio control might not be able to stand the current), you can use it to make the gauge change. (Note that the gauge is designed to change very slowly, so as not to bounce around with every slosh). You could also make up a little test box with Radio Shack resistors, with a single "IN" terminal, and "OUT" terminals labeled "0Ω," "45Ω," and "90Ω," just like the factory made for the dealers.
Based on your having disconnected the tan wire (at the tank, I assume) and grounded it, and its having done nothing, it sounds as though there is a break in the tan wire, or a bad connection at the gauge or within the printed circuit that connects to the gauge. I'd try grounding the passenger side gauge terminal directly to see whether the gauge reading goes down to E. If it does, the gauge is good, and the problem is between it and the sender.
Good luck!
- Eric
+12V (Instrument Power) ------> Fuel gauge (Driver's side + terminal) -----> Fuel gauge windings -----> Fuel gauge (Passenger side - terminal) -----> Tan wire -----> Tank sender -----> Rheostat (0-90Ω) -----> Ground.
The sender should read 0 - 0.6Ω at E, 44.9-45.1Ω at ½, and 88 - 90Ω at F.
That means that when you break the circuit, the gauge should read past F and when you short the tan wire (or the passenger side gauge terminal under the dash) to ground, the gauge should read E.
If you have a heavy duty (1W) 100Ω rheostat or potentiometer (maybe from an old tube radio - I'd suspect a tiny transistor radio control might not be able to stand the current), you can use it to make the gauge change. (Note that the gauge is designed to change very slowly, so as not to bounce around with every slosh). You could also make up a little test box with Radio Shack resistors, with a single "IN" terminal, and "OUT" terminals labeled "0Ω," "45Ω," and "90Ω," just like the factory made for the dealers.
Based on your having disconnected the tan wire (at the tank, I assume) and grounded it, and its having done nothing, it sounds as though there is a break in the tan wire, or a bad connection at the gauge or within the printed circuit that connects to the gauge. I'd try grounding the passenger side gauge terminal directly to see whether the gauge reading goes down to E. If it does, the gauge is good, and the problem is between it and the sender.
Good luck!
- Eric
Thread Starter
Registered User
Joined: Dec 2010
Posts: 139
O, sorry I did not ground it at the tank, I disconnected the wire in the trunk. Ill have to drop the tank to do that wont I? Im starting to think this is somehow related to some other problems I have been having. All these electrical issues have been happening one after another in the last few weeks. For example, my right turn signal doesnt flash, it just stays on. My left turnsignal works, as long as the headlights are off. My headlights seem to be getting more dim, but that could just be my head playing games. And also when I turn my brights on my speedometer light goes out. Starting to think maybe somehow these things are connected? But idk, like it said I have almost no experience in wiring. Thanks for the helpful responses.
Registered User
Joined: Feb 2008
Posts: 11,798
From: Plano, TX
Do not drop that tank!
Since grounding it at the trunk connector did not work, the problem is upstream anyway. Next place to check is at the gauge itself.
Wiring can be a pain in the posterior. I hope you have a service manual with wiring diag - you will be needing it soon!
Since grounding it at the trunk connector did not work, the problem is upstream anyway. Next place to check is at the gauge itself.
Wiring can be a pain in the posterior. I hope you have a service manual with wiring diag - you will be needing it soon!
Thread Starter
Registered User
Joined: Dec 2010
Posts: 139
Unfortunately I dont have the service manual, I could have bought one at a swap meet for sixty bucks but passed on it for a new floorpan. Looking back on it I should have bought the service manual. Any guesses on any of my other problems?
Connoisseur d'Junque
Joined: Sep 2010
Posts: 21,183
From: The Hudson Valley
Until you get one, why not join WildAboutCars and download it?
- Eric
Registered User
Joined: Apr 2015
Posts: 27
Okay, here's your fuel gauge circuit:
+12V (Instrument Power) ------> Fuel gauge (Driver's side + terminal) -----> Fuel gauge windings -----> Fuel gauge (Passenger side - terminal) -----> Tan wire -----> Tank sender -----> Rheostat (0-90Ω) -----> Ground.
The sender should read 0 - 0.6Ω at E, 44.9-45.1Ω at ½, and 88 - 90Ω at F.
That means that when you break the circuit, the gauge should read past F and when you short the tan wire (or the passenger side gauge terminal under the dash) to ground, the gauge should read E.
If you have a heavy duty (1W) 100Ω rheostat or potentiometer (maybe from an old tube radio - I'd suspect a tiny transistor radio control might not be able to stand the current), you can use it to make the gauge change. (Note that the gauge is designed to change very slowly, so as not to bounce around with every slosh). You could also make up a little test box with Radio Shack resistors, with a single "IN" terminal, and "OUT" terminals labeled "0Ω," "45Ω," and "90Ω," just like the factory made for the dealers.
Based on your having disconnected the tan wire (at the tank, I assume) and grounded it, and its having done nothing, it sounds as though there is a break in the tan wire, or a bad connection at the gauge or within the printed circuit that connects to the gauge. I'd try grounding the passenger side gauge terminal directly to see whether the gauge reading goes down to E. If it does, the gauge is good, and the problem is between it and the sender.
Good luck!
- Eric
+12V (Instrument Power) ------> Fuel gauge (Driver's side + terminal) -----> Fuel gauge windings -----> Fuel gauge (Passenger side - terminal) -----> Tan wire -----> Tank sender -----> Rheostat (0-90Ω) -----> Ground.
The sender should read 0 - 0.6Ω at E, 44.9-45.1Ω at ½, and 88 - 90Ω at F.
That means that when you break the circuit, the gauge should read past F and when you short the tan wire (or the passenger side gauge terminal under the dash) to ground, the gauge should read E.
If you have a heavy duty (1W) 100Ω rheostat or potentiometer (maybe from an old tube radio - I'd suspect a tiny transistor radio control might not be able to stand the current), you can use it to make the gauge change. (Note that the gauge is designed to change very slowly, so as not to bounce around with every slosh). You could also make up a little test box with Radio Shack resistors, with a single "IN" terminal, and "OUT" terminals labeled "0Ω," "45Ω," and "90Ω," just like the factory made for the dealers.
Based on your having disconnected the tan wire (at the tank, I assume) and grounded it, and its having done nothing, it sounds as though there is a break in the tan wire, or a bad connection at the gauge or within the printed circuit that connects to the gauge. I'd try grounding the passenger side gauge terminal directly to see whether the gauge reading goes down to E. If it does, the gauge is good, and the problem is between it and the sender.
Good luck!
- Eric
Registered User
Joined: Dec 2011
Posts: 26
I could use some help. I'm having the same fuel gauge problem on my '71 Custom Cruiser.
The gas tank and sending unit have been removed from the car, as has the speedometer/fuel gauge cluster.
- Testing the ground for the sending unit on the beige wire makes the fuel gauge shoot past Full. And it stays there. Won't come back down.
- Same thing happens when a second identical fuel gauge is tested.
- Using a sending unit from a different car makes no difference. The fuel gauge stays past Full.
- Gauge only goes down when polarity is reversed.
But testing the speedometer cluster lights on the gray wire shows the lights work fine, but low and behold, the fuel gauge slowly starts to go down. What gives?!! Does the gas gauge only function when the lights are on?? Do I have a back cluster? Am I missing some sort of relay?
I'm stumped and don't know what to try next. Wide open to suggestions here.
Thanks,
CCFan
The gas tank and sending unit have been removed from the car, as has the speedometer/fuel gauge cluster.
- Testing the ground for the sending unit on the beige wire makes the fuel gauge shoot past Full. And it stays there. Won't come back down.
- Same thing happens when a second identical fuel gauge is tested.
- Using a sending unit from a different car makes no difference. The fuel gauge stays past Full.
- Gauge only goes down when polarity is reversed.
But testing the speedometer cluster lights on the gray wire shows the lights work fine, but low and behold, the fuel gauge slowly starts to go down. What gives?!! Does the gas gauge only function when the lights are on?? Do I have a back cluster? Am I missing some sort of relay?
I'm stumped and don't know what to try next. Wide open to suggestions here.
Thanks,
CCFan
Connoisseur d'Junque
Joined: Sep 2010
Posts: 21,183
From: The Hudson Valley
You've probably hooked something up backwards.
The gauge should read past F when the sender wire is not connected, and should read E when it is connected to ground.
The gauge needs a hot, a ground, and a sender connection.
- Eric
The gauge should read past F when the sender wire is not connected, and should read E when it is connected to ground.
The gauge needs a hot, a ground, and a sender connection.
- Eric
Registered User
Joined: Sep 2014
Posts: 208
From: France
Sorry, no contribution to this thread from me, but as my fuel gauge does exactely the same, very intesrested in this topic! 
The needle stays on the "H" from the Hot light.
Will check my ground connections by the end of the week!
The needle stays on the "H" from the Hot light.
Will check my ground connections by the end of the week!
Registered User
Joined: Sep 2014
Posts: 208
From: France
So....checked the connections tonight.
The needle was stucked on the "T" letter from "HOT".
I unplugged the connector in the trunk and connected the wire to the ground in the trunk.
==> the needle moved back on the E!!
I plugged the tan wire to the connector again and the needle did go back not on the "T" but on the "H" letter.
Filled my gas tank (full full) by the beginning of the week.
What do you think?
Thanks!
The needle was stucked on the "T" letter from "HOT".
I unplugged the connector in the trunk and connected the wire to the ground in the trunk.
==> the needle moved back on the E!!
I plugged the tan wire to the connector again and the needle did go back not on the "T" but on the "H" letter.
Filled my gas tank (full full) by the beginning of the week.
What do you think?
Thanks!
Registered User
Joined: Sep 2014
Posts: 208
From: France
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