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posted: 7/22/2012 5:26 AM

Fuel gauge malfunctions can be misleading

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Q. I hope you can get to the bottom of our Montana problem. When we turn the ignition on, the fuel gauge always reads full, no matter how much fuel is in the tank. While driving, it still reads full. However, when we come to a stop, about a minute later the gauge drops to empty and the low-fuel indicator comes on. Once we start driving, it'll go back to full. The electronic monitor tells us how much fuel has been used, distance to empty, etc.

All the readings correspond to the present situation. When the gauge reads full, it displays several hundred miles of range left. When the gauge drops down to empty, the range is zero miles. If I turn off the ignition and wait awhile, the sequence starts all over again.

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People keep telling me it's the sending unit in the tank. I don't understand how a defective sending unit can cause the gauge to go from full to empty while the car is not in motion. I've also disconnected the connector C305 underneath the van to completely eliminate the sending unit from affecting the gauge. With the sending unit/pump wiring harness disconnected the problem still shows up. I turn the key to the run mode, gauge reads full and after a short while it reads empty and the low level warning comes on. Turn the ignition switch to off, back to run and it does the same thing all over again.

We refuel by monitoring the miles driven and fuel used according to the electronic display. I've even contacted GM and they also point to the sending unit. I've heard everything from the C305 connector, sending unit, instrument cluster, bad ground, etc. What should the sending unit's resistance be when full and empty? Any ideas as to what the problem could be?

A. Sounds like you're having some fun with this one! While the fuel sender/fuel pump is a common problem on this vehicle it does seem as though you might be dealing with something else. I had one of my technicians study the schematic for your car and he discovered that the sender feeds the PCM and the PCM talks to the gauge.

Theoretically, with a good scan tool, you should be able to see what data the sender is sending to the PCM and determine where the fault is. This is one of those where you have to go through a process of elimination. The resistance varies depending on the size of the tank but empty would be 40-56 ohms and full should be 215-250 ohms for a 20-gallon tank. The low indicator light should come on at 76 ohms.

I hope this is helpful. Let me know what you discover.

• Douglas Automotive is at 312 S. Hager Ave., Barrington, (847) 381-0454, and 123 Virginia Road, Crystal Lake, (815) 356-0440. For information, visit douglasautomotive.com. Send questions to underthehood@dailyherald.com.

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