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Needle in gas gauge gyrates wildly

#1

I have a 1939 Cadillac LaSalle frame off restoration. The dash is original and all gauges work well. But the needle in the gas gauge gyrates wildly when driving. The needle bounces between E and F and then settles about where it should and begins gyrating again. Only when the gas tank is full does the needle remain steadily at F. Is this a float problem in the gas tank, a ground issue or what.
Any ideas or suggestions are appreciated.

John P

#2

Interesting issue for sure. I would investigate the ground for the sending unit. In my experience a bad ground can cause the gauge to swing or read inaccurately. It is also an easier check before pulling the sending unit and checking the float.

Next step would be checking the resistance of the sending unit. This works best with it removed so the float can be moved to see the resistance change as the sending unit goes through its arc.