97 Lesabre fuel guage HELP

[alwaysomething]

Please can someone help me figure out this problem. My fuel guage reads full all the time. When you shut the key off it drops to 3/4. I have heard check the ground, check the purple wire. On the 97 Lesabre where is the ground?. Any help would be great, I do not want to spend $300 for a part that I might not need. Using the trip od is not good I ran out at 256 miles.

[Blue Bowtie]

The fuel gauge purple wire may be grounded, or the level sender may have a problem. You can disconnect the harness near or at the tank and measure the resistance of the fuel sender.

[Smith1000]

The resistance reading on the ohm meter should read between 0 and 240 ohms. The full reading will be on the high end. If problems with the wiring are ruled out, you can check to verify that it is the fuel tank sending unit by substituting variable resistors with known values to the wire that goes to the computer, which ultimately sends the fuel level to the gauge. Radio Shack sells packets of variable resistors for about $2 or so.

For instance, 120 ohms should be a half tank. For mine, I twisted several resistors together to equal 120 ohms, turned the key to the on position and the gauge shot right up to a half tank. I ended up putting in a salvaged sending unit which took care of the problem.

The sending unit is probably the problem. We have 2 '97 Lesabres and they have both had sending unit problems. One, once the gas level gets very low, it jumps up to full. The other, I replaced the sending unit. To get to it, the tank must drop down. For later years, I believe 2000 on, the sending unit can be replaced without dropping the tank.

[Blue Bowtie]

Smith is correct. The newer sensor is 40-240 ohm and operates opposite the older 0-90 ohm units. I mislead you in stating that the purple wire may be shorted. Actually, that circuit may be open. My error.

I agree that the sender is the likely culprit, but it makes sense to diagnose it before dropping the tank to see. I am in the habit of testing the dash gauge first with a scanner to command a given fuel level and see how it complies. That works on many '94-up GMs. If that makes the gauge indicate properly, the problem is likely before the PCM, I.E. in the wiring or tank sender.

FWIW, the sending unit can be repaired if you are careful: