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Trying to troubleshoot my fuel gauge problem.

Terry Kitzman

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My 66 Chargers fuel gauge reads full all the time it returns to empty when the key is off. If I unplug the wire to the sending unit, the gauge reads full. It also reads full when the sending unit wire is grounded. The voltage at that wire bounces between 0 and 10v Any Ideas?
 
Sure sounds like a sending unit. You can measure the OHM's of the sender at various fill levels to get an idea. 73 to 10 Ohms is the range.
 
OEM sending unit, or more recent aftermarket unit?? I keep hearing some of the newer ones are trouble right out of the box.
 
He said the voltage is fluctuating at the wire. You would think if grounded there would be no signal.
It fluctuates because the voltage limiter cycles on and off. The gauges have a bunch of averaging in their design.
I agree check the wire to the sender. Or try running a test wire from the gauge to the sender. Just run it loose and see how it behaves
 
It fluctuates because the voltage limiter cycles on and off. The gauges have a bunch of averaging in their design.
I agree check the wire to the sender. Or try running a test wire from the gauge to the sender. Just run it loose and see how it behaves
Thats why I daid if the wire between the gauge and the tank was grounded the poster would not have the voltage fluctuation. If it was grounded he would have no voltage at the tank sender plug. I agree with the voltage fluctuation the limiter is working. Quick Ohm test like I suggested will tell him something.
 
Thats why I daid if the wire between the gauge and the tank was grounded the poster would not have the voltage fluctuation. If it was grounded he would have no voltage at the tank sender plug. I agree with the voltage fluctuation the limiter is working. Quick Ohm test like I suggested will tell him something.
Yes you're right. I was thinking he was checking at the gauge
 
The wire is being grounded somewhere between the tank and the gauge. The sending unit in the tank uses a rheostat that controls how much ground goes back to the gauge. You can’t be putting power to the sending unit because if something goes wrong the the tank could explode. To prove my thought on the sending unit sending ground to the gauge is when you ground the sending wire the gauge goes to full. If that sending wire had power you would get a spark and blow a fuse.
Your problem is most likely be that the wire going from the gauge to the sending unit is pinched somewhere.
 
Thats why I daid if the wire between the gauge and the tank was grounded the poster would not have the voltage fluctuation. If it was grounded he would have no voltage at the tank sender plug. I agree with the voltage fluctuation the limiter is working. Quick Ohm test like I suggested will tell him something.
I agree.
 
The wire is being grounded somewhere between the tank and the gauge. The sending unit in the tank uses a rheostat that controls how much ground goes back to the gauge. You can’t be putting power to the sending unit because if something goes wrong the the tank could explode. To prove my thought on the sending unit sending ground to the gauge is when you ground the sending wire the gauge goes to full. If that sending wire had power you would get a spark and blow a fuse.
Your problem is most likely be that the wire going from the gauge to the sending unit is pinched somewhere.
If the wire was pinched he would not be seeing fluctuating voltage at the plug on the sending unit. If that wire was pinched to ground it would kill the signal. If he ohm's the sending unit that will give him an idea. On the other hand if he did not have fluctuating voltage at the senders plug I would agree that the pinched wire to ground was causing the gauge to peg.
 
It is an aftermarket sending unit.
The things Pnora are telling you are accurate, and good advice. If everything he says checks out, then I wouldnt be too surprised to find that the sender is shorting internally (ohm readings are inconsistent, or inaccurate).
 
If the wire was pinched he would not be seeing fluctuating voltage at the plug on the sending unit. If that wire was pinched to ground it would kill the signal. If he ohm's the sending unit that will give him an idea. On the other hand if he did not have fluctuating voltage at the senders plug I would agree that the pinched wire to ground was causing the gauge to peg.
It should be easy enough to check. If there is power to the sending unit then a test light will light if you connect one end of the test light to ground and you put the probe into the wire socket at the tank making sure to disconnect the sending wire first. After that hook one end of the light to power and probe the sending unit socket again. If the wire is seeing ground the test light should light up. I’ll also check the wiring diagram in the service manual later to see if I can verify this. Too bad that this issue didn’t come up a few weeks ago when I was having issues with my gas gauge. My problem was that the gauge wasn’t working at all. Turns out the float was full of fuel. Luckily I had a new float from National Parts Depot from a Ford Mustang that’s a little shorter but was made out of thicker brass and that fixed the issue for me. FYI, I still haven’t found the leak in the old float although there’s still gas trapped inside. I’ve even heated the float and it still didn’t leak.
 
One other thing to make sure of is to be sure the sending unit itself has a good ground.
 
Let's all go back to post #1. "reads full when sender wire UNPLUGGED from sender" Why I said his wire from sender to gauge is grounded out somewhere. As for the 0 to 10v on and off the OP is reading at the sender wire when unplugged.. it makes sense since the gauge runs off a pulsed regulator and if he still has the old vr can it makes even more sense. Since this voltage is being read on a wire beyond the windings of the gauge the + side of the car system doesn't see the sender wire touching the body somewhere as a short.
 
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