By William C. “Bill” Anderson, P.E.
The fuel gauge is one of an automobile’s most important instruments. An accurate gauge helps keep one from running out of fuel, or at least indicates when more is needed. However, some believe that cars generate gas or that someone else will fill the tank. Does that sound familiar?
Faulty fuel gauges are a common problem in collector cars. Sometimes this is due to sitting in an unfavorable environment, and sometimes it is due to old age; everything wears out sooner or later. And, it is a sensitive instrument. But, it is simple to test and most problems are easy to fix. My experience with fuel gauges has been primarily with General Motors cars, but much of what follows is generally applicable to many others.
The fuel gauge is composed of two components: a sender in the tank and a gauge on the instrument panel. The sender consists of a float on an arm that varies the resistance the sender applies to the circuit with gauge. GM cars from the 1930s to the 1960s operate on a 30 ohm scale. At zero ohms in the sender, the float is on the bottom of the tank and the gauge should read empty. At 30 ohms, the float is at the top of its travel and the gauge should read full. The sender must be grounded to operate properly. The lack of a good ground is often the reason the gauge does not work properly.
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Gauge Reads Full At All Times
If the fuel gauge reads full at all times, the probable causes are:
• The wire between the sender and gauge is broken and/or the connections are not good.
• The resistance wire in the sender is broken.
• The sender is not grounded and/or the tank is not grounded to the chassis.
To determine what is causing the problem:
• Remove the wire from the terminal on the sender and ground it to the chassis. If the gauge now reads empty, the sender is not grounded or the sender is bad.
• Ground a test lead and touch it to the sending unit terminal on the back of the dash gauge (it is often marked with a red paper tag). If the gauge now reads empty, the wire between the sender and gauge is broken, or there is a poor connection at the gauge.
• Remove the sender from the tank and connect one lead of an ohmmeter to the sender terminal and the other lead to the sender housing. Then, move the float arm and observe the resistance as the float arm is moved; it should vary from 0 to 30 ohms.
If the foregoing tests do not identify the problem, then the dash gauge is bad and it must be rebuilt.
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Gauge Reads Empty At All Times
If the fuel gauge reads empty at all times, the probable causes are:
• The wire between the sender and gauge is shorted to ground.
• The sending unit is shorted internally.
• The float has a hole and no longer floats.
To determine what is causing the problem:
• Remove the wire from the contact stud on the sender. If the gauge now reads full, the sender is faulty.
• Disconnect the wire from sender terminal at the dash gauge. If the gauge now reads full, the wire between the sender and gauge is shorted to ground.
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Repairing faulty wires or connections is straight forward. To ensure a good ground, I always install a short wire from the sender to the frame (note the white wire in Photo 1).
If the sender is replaced with a new unit or an aftermarket replacement, it is advisable to check that it performs correctly before putting it in the tank. Use the test procedure described earlier.
If the dash gauge needs repair, this work should be done by those who specialize in instrument repairs; many offer this service.
William C. “Bill” Anderson, P.E., has been involved with the automotive hobby for more than 40 years with experience ranging from hotrods, to sports cars, to restoration of vehicles from the 1930s through the 1980s. Through Anderson Automotive Enterprises — — he restores and appraises cars.
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