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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Re: GM Fuel Gauge Operation

Postby zeke » Tue Jun 05, 2018 2:08 am

A couple pics showing the variable, 90 ohm wire wound resistor in the sender unit and the wiper contact.
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Re: GM Fuel Gauge Operation

Postby zeke » Mon Aug 12, 2019 6:00 am

The 75 Monza and Vega fuel gauge looks a little different as the car doesn't use a printed circuit board to connect the dash components. Operation is the same.
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Re: GM Fuel Gauge Operation

Postby gradford » Thu Apr 02, 2020 4:52 pm

HI Zeke

I'm having a problem; it occurred after my 80 Sunbird was stored for several months. The fuel gauge needle went far past the full mark, and stayed there, regardless of whether the ignition was on or off.
I looked at your sketch, and used the resistances marked on the diagram to check out the fuel gauge itself; with the external resister removed, I see an open circuit between + and either of the other two terminals; I think this means there is an open winding in the gauge. I disassembled the gauge from its enclosure, but there is no visible damage to the windings.
What is your opinion?
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Re: GM Fuel Gauge Operation

Postby gradford » Thu Apr 02, 2020 4:55 pm

another note; I checked the circuit going to the tank sender and have a reading of 53 ohms at the instrument cluster terminals, and very similar at the trunk connection between tan and ground. It appears that the sender and circuit are OK.
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Re: GM Fuel Gauge Operation

Postby zeke » Fri Apr 03, 2020 3:51 am

gradford wrote:HI Zeke

I'm having a problem; it occurred after my 80 Sunbird was stored for several months. The fuel gauge needle went far past the full mark, and stayed there, regardless of whether the ignition was on or off.
I looked at your sketch, and used the resistances marked on the diagram to check out the fuel gauge itself; with the external resister removed, I see an open circuit between + and either of the other two terminals; I think this means there is an open winding in the gauge. I disassembled the gauge from its enclosure, but there is no visible damage to the windings.
What is your opinion?


Yep, sounds like a bad gauge. Very uncommon. Should be continuity between all 3 terminals.
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Re: GM Fuel Gauge Operation

Postby gradford » Fri Apr 03, 2020 10:02 pm

HI Chris,

Thanks for confirming my suspicions. Do you know which years GM used this 90 ohm gauge? I've taken the gauge off the dash face, so I can change it out with a working unit, but don't want to get something from Ebay that's incompatible.
I see some ohm values on listings for gauges that are supposed to fit GM, that are not 90 ohm.
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