Rick's Free Auto Repair Advice

Gas gauge wrong

My gas gauge reading is wrong

This problem shows up as a gas gauge that shows Gas gauge wrong, different fuel levels, needle swings, shows empty when actually full, or any number of other crazy behaviors. There are two known causes for this problem. But first, let’s take a stroll back in time.

In the old days the fuel sending unit had a variable resistor attached to a float arm. As the float arm went up and down, so did the voltage to the gauge. To troubleshoot the system, all you had to do was attach a gauge tester in place of the fuel sending unit. Then you would simply dial in the resistance to match full, ½, or ¼ tank and see if the gauge responded accordingly. If it did, that ruled out the gauge and the wiring and isolated the fuel sender as the problem. Fast forward to today.

In their infinite wisdom, car makers still use a variable resistor in the fuel sending unit. But the variable voltage feeds into a Body Control Module (BCM), which is actually a mini computer. The BCM then sends a digital signal to the digital instrument panel. In other words, no more mechanical gauges. So when the system goes nuts, the problem can be in the fuel sending unit, the wiring between the sending unit and the BCM, a bad BCM, a short in the data bus line between the BCM and the instrument cluster, or the instrument cluster itself. Are you having fun yet?

The ultimate diagnostic tool is a high end scan tool that will show you the actual readings from the sending unit and the output from the BCM to the instrument cluster. If you don’t have a spare $3,000 for that scan tool, you’ll have to do diagnostics the hard way, with a digital multi-meter and disconnecting connectors to get readings.

GM put out a bunch of bum fuel sending units, so it’s easy to automatically assume you have a bad one. But before you drop the tank and throw in a new one, check out the sending unit with your meter.

1) Find the electrical connector #C413 (if you have a wiring diagram). This connector is usually located in front of the right rear (passenger side rear wheel) tire. Disconnect the two halves.

2) Test the resistance between the purple wire and black wire going to the sending unit. The reading should be between 40Ω (tank empty) to 250Ω (tank full). The reading should also be steady. If it is not steady and varies as you move the connector or wires, look for a short in the wiring harness.

3) With the key in the run position, test the voltage on the other half of the connector between the purple and black wire. It should read battery voltage (above 12 volts). Wiggle the connector and wiring to see if you get any voltage dropouts. If you do, fix the wiring harness.

If the resistance and voltage values check out, suspect a problem with the BCM or instrument cluster, rather than the fuel sending unit.

 

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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