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Tip for Gas Gage problems

My 1972 Classic Chevelle gas gage would not read properly. Sometimes it would be full when the tank was really 3/4 or less. Sometimes it would read 1/2 when it was really closer to 1/4 full. Here is a short summary from the Chevelle Tech Archives:

I would guess that you have a bad sending unit in your vehicle. If your symptoms are not consistent, you may have a bad ground in the area of the sending unit or gas tank. A sending unit is typically nothing more than a variable resistor (rheostat/potentiometer). The float is attached to the wiper or tap of the "pot". As the fuel level in the tank changes, the resistance of the "pot" changes. This alters the current flow in the gauge circuit and changes the reading on the gauge itself. Sometimes the windings of the rheostat ("pot") within the sending unit can get bad spots on them (i.e., dirt or shorts). This can cause the gauge to read incorrectly or erratically, but the symptoms for this should be consistent (i.e., when the tank level drops to 1/2, the gauge bounces between 1/2 and 1/4, and then after a while it settles at 1/4). If you remove the sending unit, you may be able to test with an ohm meter. I do not recall what the normal max/min resistance is, but I think it is something pretty low like in the "hundreds" of ohms. This should be able to be measured between the sending output terminal and the case of the sending unit. As the float is manually moved up and down very slowly, the resistance should change gradually without seeing big jumps in the resistance reading. This may be tough to see if the bad spot is small.

Bottom line is if you can't clean it you will have to replace it.

Ref: http://www.chevelles.com/archive/files/badgasga.html

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