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Grounding the sender wire to test the gauge can damage the gauge. I seem to recall that the service manual has a recommendation on testing the 'full' condition.
The simplest way to test the gauge operation would be to test it by grounding the sender wire at the tank through a 10 ohm, 33 ohm and a 68 ohm resistor to check for full, mid (approximately) and empty indication on the gauge. If the gauge does not give the complete range of indication, then check to make sure that the dash voltage regulator is working correctly. I seem to recall that the voltage set point changed during the production run so you will need to check the service manual for your year. Also, check your electrical connections on the sender wire and that you have good grounding on the sender unit. A high resistance ground or bad connection can result in loss of sensing range on the gauge. If the voltage regulator is functioning correctly and the connection and grounding is good, then the gauge is likely damaged and you would have to check around to see if recalibration is possible.
If you don't have access to the resistors, pull the sender out of the tank, ground the sender body and then operate the float from the full to empty range to check for full range operation of the gauge.
If the gauge operates properly with the resistor test or the sender out of the tank, check the orientation of the sender in the tank. If you have an E / fuel injected model there are baffles and return lines in the tank that the sender may be hanging up on.
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