Check your ground points!
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Variable voltage type gauge theory of operation - just as general information:
VARIABLE VOLTAGE TYPE
The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines current flow through its coil, and therefore pointer position.
When resistance is high in the sending unit, less current is allowed to flow through its coil, causing the gauge pointer to move toward the directly grounded coil. When resistance in the sending unit decreases, more current is allowed to pass through its coil, increasing the magnetic field. The gauge pointer is then attracted toward the coil which is grounded through the sending unit.
A special tester is required to diagnose this type gauge. Follow instructions included with the tester.
Gauge failures are often caused by defective wiring or grounds. The first step in locating trouble should be a thorough inspection of all wiring, terminals and printed circuits. If wiring is secured by clamps, check to see whether the insulation has been severed, thereby grounding the wire. In the case of a fuel gauge installation, rust may cause failure by corrosion at the ground connection of the tank unit.
Basic schematic. Note: this is applicable to all dash indicators (i.e. gas tank level, coolant temp, oil pressure):
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Trouble shoot matrix:
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GM GAS GAUGE DIAGNOSIS:
Most gas tank units consist of two parts, the gauge mounted in the dash and a tank sender mounted in the tank.
The following trouble-shooting instructions apply to the AC-type fuel gauge, which is one of the most common type of units. Your service manual will give you any added instructions necessary for your specific type of fuel gauge.
The gas gauge, often located in the instrument cluster, consists of two small coils spaced 90 degrees apart with an armature and a needle placed at the intersection of the two coils. A dampener is also located on the armature to prevent excessive needle movement on rough roads.
The tank sender unit is mounted to the outside of the gas tank (with a float on the inside, of course) and is made up of a metal housing that contains a rheostat (which is simply a resistance unit), and a brush that comes in contact with the resistance unit. The opposite end of brush unit is attached to the float arm located inside of the gas tank. The movement of the float arm is controlled by the amount of fuel in the gas tank. The variations in the amount of fuel in the tank cause the arm to move. This changes the resistance of the tank unit, changing the amount of current at the gauge unit coils, which in turn moves the needle located between the coils.
Beginning around 1950, some vehicles have a small voltage regulator behind the dash that supplies a stable voltage supply for the instruments - something less than the battery voltage of six (or 12) volts. This isolates the instruments from the effects of varying battery and generator voltage. So if you are checking voltage at the dash gauges on these cars and you read something less than battery voltage, it may be normal. Follow the power wires from the instruments and you'll probably find the regulator.
The most common cause of gas gauge trouble is a poor ground, especially at the tank sender unit. It is important that all wiring connections are clean and tight, and free of dirt and corrosion. A poor ground or loose connection to a gas gauge is just as likely to cause problems as loose or dirty battery cables assuredly will with your starting system.
For example, if the gas gauge needle remains on empty when the ignition key is turned to the "on" position and you are sure the gas tank is partially full, battery current may not be reaching the gauge. To check further, connect a jumper wire between the ignition switch and the dash gauge. If the gauge now works, replace the defective wiring between the gauge and the ignition switch.
If the gauge needle remains stuck in one place, try turning the ignition switch off and on several times in succession. This will allow you to determine if it is the dash gauge or the tank sending unit that is defective. If turning the ignition switch off and on (a half-dozen times is a good test) does not seem to help, and you've verified that the dash gauge is receiving power, try the following troubleshooting steps:
A functioning sending unit will have a high resistance when the tank is full and a low resistance when the tank is empty. If the gauge reads higher than it should, make sure the wire attached to the electrical terminal on the tank sending unit is making a good ground connection. If necessary, clean the connection and reinstall the wire to the terminal, then check the gauge operation. If that didn't fix the problem, use a jumper wire to ground the electrical terminal on the tank sending unit to the tank unit housing (leave the wire to the gauge connected to the tank unit). Turn the ignition switch on. If the gauge reads empty or below empty, the sending unit is defective.
If the gauge still reads high, try grounding the tank unit electrical terminal to a clean portion of the frame. If this brings the gauge to empty or below, the tank unit is OK, but there is a bad ground between the gas tank and the body or chassis (or less likely, between the tank unit and the gas tank). Either way, find and correct the bad connection or merely run a permanent jumper wire from the frame to one of the screws holding the tank sending unit to the tank.
If the gauge still reads incorrectly, the wire from the tank unit to the gauge may be faulty. Check by using a jumper wire to ground the tank unit terminal on the gauge (the one not connected to the ignition). If the gauge now reads empty or below, the problem is in the wire leading to the tank (or its connection to the gauge). If grounding the tank terminal still doesn't cause the gauge reading to drop to empty or below, the gauge is faulty.
When the gauge reads lower than it should, check at the gauge by disconnecting the wiring from the tank unit. Turn the ignition on and if the needle reads full or above, the gauge unit is probably good and the tank unit or the wiring leading to it are probably the problem. Reconnect the tank unit wire to the gauge before proceeding.
The next trouble-shooting task begins by disconnecting the wire to the tank sending unit. If the gauge still reads full, there is likely a short to ground somewhere between the sending unit and the gauge. Look for worn insulation allowing the wire to contact the body or frame. If the gauge reads full or above with the wire disconnected, the problem is in the tank unit rather than the wiring. It could be an electrical fault, or the float may be stuck or may have sunk because of a hole caused by corrosion. Remove the tank sending unit and check it. If the float is bad, you'll likely hear gas sloshing around in it or see gas dripping out. If the float is OK and the float arm seems to be swinging freely through its full range, replace the tank unit.
Ground the gas tank terminal post of the tank sending unit using a jumper wire. If the dash gauge now reeds empty, the gas tank unit is defective. If the dash gauge needle fails t move, the dash gauge is defective.
If the gas gauge works, but reads lower than it should, check for defective wiring between the gas gauge and the tank sending unit. Also check for a poor ground at dash gauge and the sending unit by using a jumper wire. If the dash gauge reads empty when the tank is actually half full or greater, the wiring between the dash gauge and the tank sending unit is defective. Disconnect the wire at the gas tank sending unit terminal. If the gas gauge reads still reads empty, the dash gauge unit is defective.
