How does the gas gauge work on a 1985 Cadillac el dorado
On them older cars (and maybe still to some extent) they had a tiny voltage regulator on the instrument cluster board, just before the gas gauge and the temperature gauge. It cut the voltage going through those two gauges to about 5-7volts, before sending the signal out to the sending units.
For the in-tank sending unit, it had about 5 volts current running from the dash to the sending unit. The float on the sending unit attaches to a "variable resistor", and then goes to ground. What the dash gauge measures is the "resistance to ground", and is thus translated to the needle on the gauge showing anywhere from empty to full. Pretty neat, huh?
So if you are working on the gas gauge, you would check for that small voltage at the fuel tank electrical connector. Folks used to test whether the gauge or the sending unit was the problem by taking the wire at the tank and just temporarily grounding the wire to a ground, a clean metal piece of the frame down there. If it caused the needle to move AT ALL, then you would know that the problem is not the dash gauge, but the sending unit in the tank. Get a helper to watch the needle while you ground it. Careful not to let it ground too long-you can damage the gauge or voltage regulator. You just want to touch it to a clean piece of the frame while someone watches for needle movement-even just a little movement. Often this test would cause the needle to jump to full, or maybe just halfway, but any movement of the gauge needle means the gauge is okay. For this test, the ignition key must be on (not start, not engine running, just key in on).
The variable resistor on the sending unit is a little flimsy, and easily disrupted-that is why so many failed. A new one should not be very expensive-it is all one piece. You don't buy a separate resistor, you just buy the whole apparatus with the float and arm attached.
Probably 80-90% of the time, when the gas gauge stopped working, it was the sending unit, not the gauge itself.
Cadillac Cars &...
15 hours ago