gas gauge problem

KreagerM · Nov 30, 2005

I have a 65 and when I turn the ignition to on/start etc. the fuel guage will always peg out to full (trust me the tank only has a few gallons in it). Even if I disconnect the wire going to the sending unit, it still pegs out to full. Should I try grounding out the wire to see what happens? If it stays at full when grounded will it mean it's the guage or a wire leading from the guage to the sender? What do you guys think the problem sounds like? Any help would be great. Thanks in advance.

reenmachine · Nov 30, 2005

The fuel sending unit reads about 76 ohms of resistance when the tank is full and about 6 ohms when it is empty. If you ground the send wire, you'll have very little resistance, and the gauge should read empty. Don't leave the key on for long like this. Odds are it's not the wire or the gauge, but it's pretty common for the sending unit to go out with age. It's cheap and easy to replace.

iskwezm · Nov 30, 2005

yeah,i dont think its the guage,there is a short to ground in the sender,i would read the resistance of the signal wire from the sender.Normaly when you ground the wire it will max the guage

jbuening · Dec 1, 2005

reenmachine said:
The fuel sending unit reads about 76 ohms of resistance when the tank is full and about 6 ohms when it is empty. If you ground the send wire, you'll have very little resistance, and the gauge should read empty. Don't leave the key on for long like this. Odds are it's not the wire or the gauge, but it's pretty common for the sending unit to go out with age. It's cheap and easy to replace.

I think you have it backwards. If you pull the feed wire from the sender and ground it to the body, the gauge will peg. If you pull the feed wire and the wire isn't grounding somewhere down the line, you will read below empty. I think the fellow has a cut in his sender wire between the gauge cluster and the tank and is grounding itself to the body. Those are a PIA to find.

reenmachine · Dec 1, 2005

jbuening said:
I think you have it backwards. If you pull the feed wire from the sender and ground it to the body, the gauge will peg. If you pull the feed wire and the wire isn't grounding somewhere down the line, you will read below empty. I think the fellow has a cut in his sender wire between the gauge cluster and the tank and is grounding itself to the body. Those are a PIA to find.

Yes, I had that exactly backwards. The higher resistance is at empty, so if you ground the send wire the gauge will indeed peg above full. It could still be a faulty send or a short anywhere between the needle and the gas

Do a resistance check between the post on the sending unit and ground to see if it reads somewhere in the correct range.

Capt Dan · Dec 1, 2005

If it stays full even when disconnected, doesn't this rule out the sender?

Must be the wire is shorted somewhere. correct?

jbuening · Dec 1, 2005

Capt Dan said:
If it stays full even when disconnected, doesn't this rule out the sender?

Must be the wire is shorted somewhere. correct?

You got it. He stated that he removed the sender wire and it still pegs, so somewhere between the gauge and sender a wire is grounding itself out. I'd check starting in the trunk, then remove the sill plates, then the behind the rear trim panels and dash. I'm not too familiar with the early cars, but the 69/70 mustangs have a rubber grommet where the wire passes through the trunk floor. I'd suspect that grommet has a cut in it and has worked itself through the wire.

KreagerM · Dec 1, 2005

Thanks for all the speedy replies and great advice. I had a feeling the problem was somewhere between the guage and the sending unit. I'll get cracking on that this weekend. Thanks again.

latamud · Dec 1, 2005

You could just run a new wire from the sender to the gauge.

KreagerM · Dec 2, 2005

That was my second option if I couldn't find the problem with the existing wire.

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