The easiest way to troubleshoot the fuel gauge is to disconnect the gauge and check the sending unit and gauge separately. To do this, disconnect the sending unit from the gauge by unplugging the tail light harness from the mid-body harness. There should be a connector in the trunk, up around the left wheel well. This will isolate the fuel gauge from the sending unit, allowing you to check each one. Working with the tail-light harness, measure the resistance of the tan wire to ground with the fuel tank full of gas. The reading should be 90 Ohms or slightly higher. If the reading is low, then there is a problem with the sending unit, check that the float is free to move up all the way. If you are reading around 65 to 70 Ohms, the gauge will read half full (lower resistance = less gas). If this checks out correctly (around 90 Ohms) then check the fuel gauge, I use two 50 Ohm 5W resistors (easily obtained at the local electronics parts outlet). Go back to the trunk, and with the tail-light harness disconnected, place a 50 Ohm resistor from the tan wire in the mid-body harness to ground, turn the ignition switch to on, the reading should be just above half tank. Now connect the two resistors in series, so that you have 100 Ohms. Connect the 100 Ohm string between the tan body harness wire to ground, the gauge should read just above full.
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