How to fix a gas gauge that stays on full all the time

Question:

The needle on the gas gauge of my 69 Grand Prix stays all the way to the tight—past full—with or without the power being on. I believe you did a piece on troubleshooting gas gauge problems a while ago, but I can’t seem to locate it. If you could revisit that, I'd appreciate it.

Answer:

The first thing to do is to get under the dash and disconnect the fuel gauge. If it drops back to zero with no power going to it, the gauge is functioning properly. But if it remains on full the gauge is the problem.

Remove the gauge from the dashboard and see if the needle has gone beyond the peg or bas rubbed against the face of the gauge and become stuck. Also have a look at the little coil on the back if there is one, to see if it is intact. Finding a fuel gauge for your Grand Prix will take a little detective work, but it should not be too hard to locate.

If the dash gauge is good, the problem is in the sending unit. To get at it you will need to drop the fuel tank. Use a hose and ahand pump to get the fuel out of the tank before you remove it from the car. Fuel tanks with a lot of fuel in them are heavy and unwieldy and, as everybody knows, gasoline fumes are explosive. Also make sure nobody is near the tank who is smoking, and make sure the tank is nowhere near an open flame or electrical spark.

As soon as the tank drops down a little, disconnect the wire from the gauge. A slip ring or several small screws are generally used to hold the sending unit in place. Clean around the area so as not to get dirt into the tank, and then unscrew the unit and lift it out. Check the float to make sure it is not full of gas. If it is, you will need to replace it. Move the float up and down through its arc from empty to full to verify that it moves freely and does not hang up. Finally, attach a multi-meter to the sending unit wire and the metal flange of the unit and set it to ohms. As you move the float up and down, the needle on the meter should move up or down,

If the float is full of fuel or the wire lever hangs up due to corrosion, it is advisable to replace the sending unit. The verdict is the same if the gauge does not respond to the multi-meter.

Sending units are easy to find, and usually cost under $50 at auto parts stores. Be sure to get a new seal with the sending unit, and use a little Permatex on both sides of the gasket to make a leak-free seal.