Chrysler Repair: Gas gauge irratic, gas gauge, variable resistance

Question
I recently acquired a 85' Le Baron with 36,000 miles and in good condition, everything was in tip top shape and the gauge showed half tank of fuel. After putting in a full tank of gas, the gauge went to empty, but returns to half when the ignition is off. It worked fine before. Any idea?

Answer
Hi Pete,
The reading when "off" is not relevant due to the magnetic design of these units. The reading when "on" is inaccurate because there is some corrosion/oxidation on the surface of the variable resistance in the tank which provides the changing voltage in response to the fuel float level. It may improve on it own through normal usage or you may have to remove it from the tank and clean the surface to restore the normal function of the fuel gauge sender unit. In the meantime be aware of the limitation and keep track of fuel input to the tank and miles driven subsequently.
Roland