anyone know how gas gauge wiring is on 88 Olds Frienza?

I have an 88 Olds Frienza I drive to work in. Lately the gas gage goes way high, about 1 inch above full almost every time I turn on the ignition, but
is intermittent. Banging hard on the top of the dash a few times usually brings the reading back to normal. Sounds to me like a maybe a ground is loose, or maybe there is a mechanical vibrator-like voltage regulator that is sticking closed. Any ideas on what to check?
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Not sure about the 88 Frienza, but from 90 Toronado...
The guages have: Grnd, Signal, Ref 12v. The only way to have a guage go beyond full is if the signal is higher than the reference. I would check for a +12v line problem....is something drawing to much current in the IP circuit?

but
mechanical
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Thanks, Mike. Now I have to figure out how to get to the terminals on the back of gauge. Space is really tight under the dash on this compact car. If I lose the 12 v ref voltage due to a loose connection, that sounds like that would produce those symptoms, then.

way
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I recommend improving grounding. A few jumpers can make a big difference in an old car driven on salty roads. Start with a jumper from block to frame. Protect that expensive alternator by making sure bracket paint has been scraped from contact areas.
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