Under the Hood: Give faulty fuel gauge a good, hard smack

Q: I have a 2002 GMC Sierra 1500 pickup with 100,000 miles on the odometer. Around the 76,000-mile mark, I installed a Delco fuel pump and level sensor module, which together cost $1,300. One year later, the truck's fuel gauge started acting up.

After a fuel-up, the gauge stays on full for about 150 miles before its needle finally starts to drop. After 300 miles, the gauge says the tank is about half full.

Two months ago, I installed a new fuel level sensor in the fuel tank, but it's still not working properly. A General Motors mechanic said the fuel gauge and the stepper motor were causing the problem. An instrument cluster repairman said that the stepper motors on 2002 GMCs never go out. He said you can't even buy one. The old tank sensor I took out reads 0.251 ohms on empty, 0.146 half, and 0.043 ohms full. The GM mechanic says the gauge is the problem. The instrument cluster mechanic says the problem is somewhere else. Please help me before I run out of gas!

-Chuck

 

A: Let's look at how this system works, and then perhaps we can do some diagnosing.

Your Sierra's fuel tank level sensor consists of a variable resistor with a swing-arm and float. It will gradually change in electrical resistance as the fuel level raises or lowers the float. The powertrain control module, or PCM, sends a reference voltage to the tank sensor and watches to see how the voltage is tugged by the sensor's varying path to ground. The PCM interprets the value and sends a network message to the instrument panel cluster, or IPC, which controls the gauge. According to my database, your 2002 Sierra was the final model year to use old school air core magnetic gauges, as opposed to the newer, stepper motor gauges. These, like the newer gauges, are inexpensive and
readily available.

The specifications you quoted for the original tank sensor are spot-on, except for a decimal place boo-boo (you were probably measuring in the K scale). Full should be about 45 ohms, and empty about 250 ohms. Since this sensor checks out and you seem to have identical symptoms with a second tank sensor, the problem appears to be elsewhere. An electrical resistance fault between the tank sensor and the PCM is unlikely, as this would make the gauge read low, rather than too high. It's also unlikely the network circuit has problems as other communications occurring in the same circuit are getting through fine. Barring an unlikely mistake by the PCM, this brings us to the instrument cluster and gauge.

How about trying this next time you know the gauge is reading lower than it should (we're going high-tech here): Using your fist, smack the dash pad hard, right above the instrument cluster. If the gauge changes, its movement is probably sticky. Another test would be to remove the cluster lens so you can touch the gauge needle. As it's reading wrong, with key-on-engine-off, gently move the needle to a lower reading. If it remains more or less in this spot the gauge was probably sticky. If it moves back to the higher position, the gauge works OK but is being sent the wrong instructions by the IPC brains.

 

 

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