You'd need a test sender with a confirmed range of 0 ohms empty, to 90 full, positioned outside the tank, or a variable resistor that can indicate 0-90. Position it well grounded and fed from the gauge ground feed. Attach a ohmmeter from the body of the sender across to ground. The beauty with the 66 (I'm assuming it's the 66?) is I think you can get to the sender feed.
Zero ohms (or close), float at the bottom of its travel, the gage reads empty. Swing the float up until the meter reads 90 Ohms (max up), the gauge reads at or close to the full mark.
A poor ground, poor continuity and more resistance in the ground feed from the gauge to the sender and beyond to the chassis, (higher resistance) could cause the needle to swing past the full mark??
Grounds are always where to start. Or perhaps an incorrect value sender was installed?
I don't think the gauge often fails does it?
Probably a far easier way then this.
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67 Grande Parisienne 4 door HT. 69 Parisienne Convertible.
I THINK I got it figured out. Took the sender out of the tank, manually moved to MT and full, checked the gauge and it coincides. Still not trusting it, senility, old age and experience.
I'll keep my "walking shoes" handy for a couple of months and monitor the mileage......LOL
I used to test mine by getting an old rheostat (volume control) from an old radio. I'd hook the feed wires to it and turn the dial and watch the gauge. If the gauge moved you had a sending unit problem. If not your wiring or gauge was the culprit.
As long as it reads correct near empty, the rest is a bonus on these old gauges. Mine goes over full when full, moves up or down when cornering, and is empty when it hits E. Just carry a small can of gas until you figure out if it works near empty.
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63 Parisienne sport coupe (The Big GTO), black, maroon interior, 409 4 speed; former owner of a 59 El Camino, 63 Corvette SWC, 62 Chev Bel Air SC. 1963- Pontiac top selling car in Canada
Mahone Bay, NS Still not old enough to need an automatic
