One fault owners can almost certainly expect to develop on older machines and trucks is a fuel level gauge that eventually stops working.
Repairing that problem isn’t too difficult, even if OEM replacement parts are no longer available. There are plenty of aftermarket components that can do the job. However, even though all fuel gauge systems use the same two basic components, a gauge on the dash and a sending unit in the tank, there are some differences in how each one functions, depending on its design. And they need to be compatible with each other. Getting the right pair isn’t just a matter of just picking up any old parts off the shelf at the local auto parts store.
Here’s a look at how these systems work, how to determine exactly which parts are at fault and what you need to replace them with.
In older machines and vehicles there are two common types of gauges: thermostatic and balancing coil. (Newer equipment and vehicles may have gauges controlled by a microprocessor.)
Current flowing through a thermostatic gauge heats a bi-metallic strip, causing it to bend. That bending motion is linked to the gauge pointer and causes it to move a distance equal to how much fuel is in the tank.
Balancing coil gauges, on the other hand, use competing magnetic pull from two small electrical coils to move the needle one way or the other. The difference in current flow, and therefore magnetic pull, between the two coils is determined by how much resistance is created by the sending unit in the fuel tank. Resistance from a sending unit also determines the amount of heat created in a thermostatic gauge.
The only practical difference between these gauges is the balancing coil gauge needs an additional wire run from it to a chassis ground. No big deal.
The sending unit
As the sending unit float moves up and down riding on top of the fuel in the tank, the resistance to current flow between the gauge and chassis ground increases or decreases.
But how much resistance a sending unit creates and whether it creates more or less when the tank is empty varies. Sending units in some older GM vehicles, for example, create maximum resistance when the tank is full. Older Ford and Chrysler vehicles are exactly the opposite. Even if a sending unit creates resistance in the correct way for a gauge to work, it needs to operate within the same range the gauge is calibrated for.
The good news is either type of gauge, thermostatic or balancing coil, will work with a sending unit as long as the resistance range is compatible.
Diagnosing the fault
A couple of simple tests will determine whether the gauge, the sending unit or both are faulty.
To test the gauge, disconnect the wire leading to the sending unit and connect it to a good chassis ground. Or disconnect it at the gauge and run a separate wire from the gauge terminal to a ground. (The sending unit wire terminal on the back of the gauge will likely be marked with an “S.”) Turn on the ignition key and ensure voltage is available at the other gauge terminal (likely marked with an “I”). For gauges designed to work with low resistance when the tank is full, which is the most common arrangement, the needle should move all the way to the full position. If it doesn’t move, the gauge needs to be replaced.
The best way to test a sending unit is to remove it from the tank. Using a multimeter set to read ohms (resistance to current flow), place the positive lead on the connection terminal and the negative lead on the body of the sender. Move the float up and down to see if there is continuity (current can flow) and note what the resistance range is. If the sending unit is working, knowing the resistance range measured by the multimeter will be important in order to match it to any new aftermarket gauge.
It is possible to do a limited test on a sending unit without removing it from the tank. Just remove the wire leading to the gauge and attach the leads from the multimeter the same way as before. If the sender is working there will be a resistance reading. And if you know approximately how much fuel is in the tank, you can make a good guess at the resistance range.
For example, if the tank is half full, you get a reading of 45 ohms and you’re working on an older GM truck, you probably have a 0-90 ohm sending unit.
If the multimeter reading shows the sending unit has no continuity (won’t let current flow through it), the sender is faulty.