Bruce Longenecker's

Primer on Fuel & Temp. Gauge Repair

I read that 2 out of 5 classic muscle car owners drive around with a 2 gallon gas can in their trunk because their fuel gauge doesn't. I recently went 10 rounds with my fuel gauge and before it was over, I completely understood the entire system. I would like to share what I have learned. Below is a drawing of the circuit that controls the fuel gauge and the temperature guage.

The Voltage Limiter is a device that takes the voltage from the battery and turns it into a series of brief voltage pulses. The periods of on time is short and the off time is long. By pulsing the voltage, the sending units can remain cool. This is very important in the fuel tank. If the voltage limiter would fail, it could cause both gauges to completely fail and not move off the left peg. The voltage limiter could also short, this would cause the gauges to go all the way to the right at the same time the steady voltage would could over heat the sending units.

The gauges quite simple, they are slow acting heated bimetal meter movements. A fine nichrome is wrapped around a piece of bimetal. As current passes through the nichrome wire it heats the bimetal bar. As the bimetal bar gets warm, it bends, as the bar bends it moves the indicator arm to the right. When it cools, the arm moves back to the left.

The wires that connect the gauges to the sending units are interupted be two cable connectors each. The fuel gauge line first passes through a round connector that mates with the instument panel circuit board. Then the second connector is located behind driver side kick panel. Either of these connections could disable your fuel gauge. The temp. gauge shares the same round connector. The second connector firewall connector. \

Notice the voltage limiter, it's the rectangular box at the top center of the circuit board.

This is where I found the problem on my Demon. The round connector and the female voltage limiter connector were sloppy. There connections need to be tight. I used a small wire brush and 400 sand paper to clean the connections, then I soldered the connections. This was the root cause of the Demon's fuel/temp gauge issue.

Make sure these connections fit snuggly when you reassemble the connectors. Sorry about the focus, the dash is back together and I don't plan on taking it apart again tp retake the photo.....

This is the connector that is behind the drivers side kick panel. The blue wire is the fuel sending unit line.

This is a photo of the fuel tank sending unit. The piece in the upper right corner of the photo is the float. It moves up and down with the amount of fuel in the tank. When the tank is full. the sending unit's resistance is about 15 ohms when the tank is empty, the value is about 80 ohms.
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:Hemi_Demon@hotmail.com