http://67-72chevytrucks.com/vboard/s....php?p=5410890
Ran throughout the bench test
Originally posted by hatzie on 7/1/12
The problem you're describing is usually a damaged or loose calibration resistor. You'll find the resistor (usually white with a brightly colored center that corresponds to the resistance value) on the back of the gauge can between 2 of the three studs. Time, vibration, and moisture in the air can corrode and/or wear through the thin metal contact surface of the resistor.
When the resistor makes proper contact the needle will sweep to the full range of the gauge. When the resistance changes, from loosing contact, the needle will no longer move to the top of the gauge but bottom is unchanged. When you change the gauge out it works again because the new gauge has a fully working resistor not because the gauge itself was bad. It's possible to fix the resistor by lightly buffing the contact with fine steel wool (for corrosion) or adding washers that are slightly larger than the bottom of the retaining nuts to get by the vibration damaged contact surface.
I'm a cheapskate, I try to go after the cheapest part first