A lot of folks mistakenly blame an inaccurate fuel level gauge on the gauge itself—instead of its sending unit. That means they’ll likely go through the hassle of installing a new fuel gauge only to find out the problem persists.
So it’s important to know how to troubleshoot a fuel gauge sending unit.
Your fuel gauge is essentially an electrical resistance meter. It’s controlled by a variable resistor (AKA rheostat) in a separate module called your fuel gauge sending unit.
But not all fuel gauges and sending units use the same resistance values (measured in ohms), which can lead to confusion when it’s time to replace or upgrade your gas gauge.
The gauge gurus at New Vintage USA produced this helpful video that’ll show you how to test your fuel gauge using a multimeter. That way, you’ll know if your sending unit is working, and what resistance values it delivers, in case you wanted to swap in a new gauge.
Check it out:
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How do you test sender while in tank? Is that possible?
Hey John, you may be able to do some tests, depending on the application. But they probably won’t be as accurate and potentially misleading.
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For example, if you can access the top of the tank, you can put your multimeter on the connector pins to get your readings. Start by running the tank as dry as you can, and take your reading. Then top off your tank, and take another reading.
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Given that you’re taking resistance readings, it’s wise to get as close to the sender as possible.If you take your resistance measurements further along in the harness, it could be affected by natural resistance (or even problems) in the wiring.
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Perhaps the big advantage of taking the sender out of the tank is that you can manually articulate the sending arm to ensure your readings will be taken at the right level. This can be a factor, for instance, if the float/pontoon on the sender arm has been compromised and is filling with gas. Like, even with your tank topped off, your arm may only be halfway up its travel, make sense?
Nine times out of ten the fuel sender is the culprit. A very quick and easy way to prove out the gauge and the integrity of your wiring is make a gauge out of an old (known good) sending unit. Several things can happen with the sender resulting in inaccurate readings. Float arm separates from the pump housing, or float fills with fuel and sinks to the bottom of the tank. More times than none it’s an issue with the resistance gauge itself. The little fingers that ride along the face of the resister wear out resulting in no connection, or intermittent connection. You can either buy one of those resistance signal generators at Radio Shack or better yet take the sender from a pump which was replaced, and test with an ohm metter. They generally sweep from around 40-50 ohms to around 240-250 ohms. Disconnect the harness at the pump, plug in/adapt your tester and sweep gauge through it’s travel. You can probably find the resistance values which indicate full and empty. Gauge moves problems in the tank.
Don’t forget to check for faulty grounds. A loose or broken ground wire has been known to cause a myriad of symptoms including false fuel gauge readings. Grounding cables tend to corrode also causing havoc. Just a thought from an old bent wrench…