Gas Gauge Problems…

by Randy Rundle, Fifth Avenue Antique Auto Parts

If you understand how a gas gauge is supposed to work, then finding your problems will be much easier. Here is how most (AC) type electric gauges work. Two parts make up the fuel gauge system, the dash gauge and the gas tank sender, located at the gas tank. Power to the dash gauge is received from the “switched” side of the ignition switch which means the dash gauge works ONLY when the ignition switch is in the ON position. There is a small counterweight located on the needle of the gauge which returns the gauge needle to the EMPTY position when the ignition key is in the off position.

With the ignition key in the ON position current flows from the ignition switch thru the small operating coil (located on the back of the dash gauge), on to a “CHOKE” coil. From there the current travels to a common terminal between the two coils. After the current reaches the common terminal between the coils, the current travels back thru the operating coils and then on to ground. At the same time current is also sent to the tank sender where current travels thru the sender unit and on to ground.

When the tank is empty, the float will be near the bottom of the tank so most of the current is sent to directly to the tank sender where it passes on to ground. As more fuel is placed in the tank, the float is raised and more current is sent to the operating coil of the dash gauge, causing the needle to move closer to the full mark. It is electrical friction (resistance) from the gas tank sender that guides the current to either the tank sender or the gauge.

1) The most common cause of fuel gauge trouble is a poor ground, especially at the tank sender. Make sure all wiring connections are tight and free from dirt and corrosion. A poor ground or loose connection to a fuel gauge system is just like loose or dirty battery cables to your starting system.

2) If your gauge needle remains on EMPTY when the ignition key is turned on and the gas tank is partially full, battery current is NOT reaching the gauge. Connect a jumper wire between the ignition switch and dash gauge. If the gauge works, replace defective wiring between the switch and gauge.

3) If a dash gauge's NEEDLE remains in ONE POSITION as the ignition switch is turned off and on, either the dash gauge or the tank sender is defective. Ground the gas tank sender terminal using a jumper wire. If the dash gauge shows empty, the gas sender is defective. If the dash gauge needle does not move, replace the dash gauge.

4) If the dash gauge reads EMPTY or LOWER than it should, check for defective wiring between the dash gauge and the tank sender unit. Also check for poor ground.

5) If a dash gauge reads EMPTY when the tank is actually half full, the wiring between the dash gauge and the tank sender is defective or the dash gauge is defective. Disconnect the wire at the gas tank sender terminal. If the gauge still reads empty, the dash gauge is defective.

6) If the dash gauge reads FULL when the tank is known to be only half full, the wiring between the tank or the sender is bad. Ground the gas tank sender terminal using a jumper wire. If the dash gauge still shows full, there is bad wiring between the dash gauge and the tank sender. If the dash gauge goes empty, the gas tank sender is defective.

Fifth Avenue Antique Auto Parts sells an amazing variety of quality automotive electric parts, especially for restoration, conversion and upgrading of electrical components. Examples are 6-12 volt conversion instructions and supplies, 6 volt alternators and pulleys, cooling fans, voltage reducers, etc. Randy’s address is 415 Court Street, Clay Center, Kansas 67432 or telephone him at 785 632-3450 or email him at fifthave@oz-online.net.