GM Fuel Gauge Operation

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GM Fuel Gauge Operation

Postby zeke » Tue Jun 27, 2017 9:18 am

GM Fuel Gauge 101 by zeke

The GM fuel gauge used for many years operates with two coils wound 90 degrees in relation to each other, effecting a magnet mounted on the needle shaft. The balance of the magnetic field is controlled by a variable resistor in the tank sender unit.

Figure F represents the equivalent electrical circuit.

When the ignition switch is turned on, a very small current (~250mA) flows through both coils to ground and to ground through the sender unit.
If the tank is empty, both sides of the full coil will be grounded. The empty coil will receive full voltage, thereby attracting the magnet and moving the needle to empty.

As the tank fills up with fuel, point B moves away from ground causing an increase in voltage on the full coil and less on the empty coil - needle swings towards full.

The shunt resistor on the back is not for dampening the meter movement contrary to popular belief. It is there to bias the full coil and provide the factory a way of calibrating the gauge to compensate for manufacturing tolerances. Different color coded ceramic resistors are used at the gauge factory. The dampening is achieved mechanically in the meter movement so corners and bumps don't cause erratic swings in the needle.

If the external resistor is not making good connection with the speed nuts, the meter will only move to 1/4 full when the tank is actually full. Remove it and lightly sand connection pads on the ceramic chip resistor.
If the sender circuit goes open, the tank will peg the gauge over full. Check sender itself, tank ground, all connectors that the Tan wire goes through, the dash PCB connection point and the copper traces on the PCB to gauge connectors. Likewise, bad connections will add resistance to the sender circuit and cause it to read high.

If the gauge itself loses it's ground, (point C) the tank will show empty.

#GM #Fuel #Gauge #Operation #Wiring #Diagram #HBody #Vega #Monza
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1979 Chevrolet Monza 2+2

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