1970 Chevrolet El Camino Fuel Guage

1970 CHEVROLET EL CAMINO
24,000 MILES • V8 • 2WD • AUTOMATIC
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MIMACYMI
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I changed the speed odometer and replaced the printed circuit. After doing that I found one of the grounding wires had attached to one of the three terminals on the back side of the fuel gauge. It has been removed but now the gauge does not work. There are three terminals, two that connect to the printed circuit, one that appears to attach to nothing? Am I missing a connection? Could having the grounding wire on one of the terminals that is connected to the printed circuit ruined the fuel gauge or the circuit?

1970 El Camino 396SS
Lance
Nov 27, 2015 at 8:07 AM
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CARADIODOC
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GM fuel gauges need three connections. One goes to 12 volts, one to ground, and one to the sending unit. I doubt you damaged the gauge.

If the ground connection is missing, the gauge should go to "full" regardless of how much gas is in the tank. There's two electromagnetic coils in the gauge. The 12 volts feeds both of them. One is a weak coil. The second end of it goes to ground. That coil pulls the needle to "empty". The second coil is a strong one and it pulls the needle toward "full", but the other end of it goes through the sending unit. When the float in the tank goes to "full", the resistor becomes very low resistance so a lot of current flows through that strong coil. It overcomes the weak coil and pulls the needle to "full".

When the fuel level is low, the sending unit becomes a higher resistance. Less current flows through the strong coil so it becomes weaker. It can't over-power the weak coil, so that weak one pulls the needle toward "empty".
Nov 27, 2015 at 8:52 AM