[Peter Serio]

GM car gauges are designed using a pair wire wound 12 volt DC electro-magnetic coils. When the key is not turned on; any GM car or truck gauge (Fuel, oil pressure or coolant temperature) on any car built after 1965 (A body), 1967 (F body) can come to rest at any place on the dial. This does not mean anything. When you turn the key on 2 things happen. 12 volts at the back of the gauge on one threaded post. 12 volts battery + energizes both coils. At the tail end of one coil the current goes thru a 90 ohm resistor (mounted at the back of the gauge) to ground. The steel housing is used as part of a ground pathway to get the connection back into the PC and then to the plug. (On all 2nd Gen F body cars there is a thin black wire in the multi-plug) which is constant ground.

The other coil exits out a different post thru the printed circuit into that same multi-plug. The wire in that plug which relates to the fuel gauge is tan. The tan wire goes thru several more plug-in body connectors as it makes it's way back to the trunk area. Then it goes underneath the car to the fuel tank sender. On the edge of the sender there is a black wire that connects to ground. This is what gives the gauge it's reading. It is a reference to either more or less "ground" at the trunk floor pan underneath the car.

More ground = trending towards the E mark while less ground trends the gauge towards the Full mark. In ohms one or zero = E . 44 or 45 ohms = 1/2 scale and 89, 90 or 91 ohms resistance at your tank sender = Full. (This standard of ohms as it relates to the fuel tank gauge/ sender was used on every GM car made after and including the year 1965). Prior to that GM cars had a zero to 31 ohms scale range to the fuel tank sender.

If your pointer goes way past the Full mark when you turn the key on the first thing I would check is the tan wire.

Follow that wire all the way back to the trunk checking the Fisher body plug-ins as you go. The next thing to check would be the sender ground. Often the attachment of this wire will become corroded on cars that sit. The black wire should be intact. That wire is grounded all the time, (even if the car is not running.) Never EVER PUT POWER to the tan wire, you will damage the sender and perhaps cause a fire. The tan wire (key on) should have about 3 volts in it. You can measure the tan wire with a voltmeter to determine if your dash gauge is bad or your tank sender is bad.


I would say you have an open circuit somewhere in the tan wire string.

It could be dash area (printed circuit) a bad plug-in connection in the body harness or trunk compartment; that or it could be a bad gauge, or bad tank sender. Old tank senders do go back after 50 years. These parts were not designed to last forever.