gas guage doesn't work

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Feb 14, 2006
Woody P
Things stuck on full, "out of the blue" says PO, "one day it just stayed at full". The guage says full, actually more than full, all the time, even when its not running. All other guages work, could it be a wire? the float? where do I start? How would I check if it was a loose or broken wire (besides using my eyeballs)? Thanks for the help

I would make sure the wire at the sender is connected...and then, make sure it has continuity to the back of the gauge....

Do all the other insturments work?

What year truck? Stock tank or aftermarket?(long range rear tank, something custom, etc...)

Ohm meter is your friend here...

Good luck!

Thanks Poser,

Yeah all other guages work, Its a new tank on a 79. I'll check the wire at the sender and at the guage. I have no idea about an OHM meter though.
It is the setting....'Ohms resistance'...on a multimeter....very helpful when diagnosing electrical drama...

Just remember that you want to isolate whatever it is you are testing...if you wanted to check wire from the sender to the back of the insturment cluster, you would unplug the cluster from the harness, and put one test probe into the socket on the connector where the wire from the sending unit comes into it, and the other test lead on the wire that is removed from the middle of the sending unit...if you get .004 or around have continuity. :) if not, you have what is called an open, or could be a broken wire, or poor connection....

You can pick up a pretty decent meter at radio scrap or similar, and they usually come with some instructions....sometimes.. ;) The Toyota manual will elaborate on the use of one when trying to diagnose other electrical issues.

Or pick one up, and post up questions out here....there are many people out here far more sharp on sparky stuff than I....but someone will help you.

Good luck!


Perhaps your float is stuck in the up postion. To check this, meter the resistance (ohms) right at the sender with the wires disconnected. A LOW reading will say that the tank is full, a high one will say that the tank is empty (not sure of the actuall #'s though.

The way this works... At full, the resistance of the float is 0, or close to it. This causes NO resistance in the circuit, so the gauge gets full voltage and goes all the way up.

As your tank becomes more empty, the float drops, increasing the electrical resistance, causing a voltage drop in the circuit. This does not alow the fuel gauge to get a full 12V, so it reads lower.

Eventually the resistance of the float reaches a level high enough (when the tank is empty) that the voltage drop is so great that the needle gets almost no voltage, and barely moves (shows empty)

HTH, not confuses you more!!
Sounds like the wire is shorted if it reads way over full, but if it reads over full with the key off, the gauge is toast.
all good stuff gents, I'll tackle it this weekend and post back if I still cant get it.

Mr MoMo, I found what the OHM resistance should be in a Haynes manual, says it should vary between 17+/- 2.1 Ohms at the lowest float setting.


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