bobshollow,

Proper operation of the gauge required it to be grounded. This is usually taken care of by mounting the gauge in the cluster and no additional wire would be required.

Did you buy a gauge that was advertised as being suitable for 12 volts...probably. Troubleshooting the gauge from the connections on its back will work but it won't show any problems with the wire back to the tank.

If you have a ohmmeter, make sure there is continuity between the gauge case and the grounded dash/body.

If there is, I would attach the proper supply wire to the correct terminal, hook up the wire to the tank and do my test from the tank end.

Make sure there is a ground from the sending unit/tank to body.

You have done your homework well. Grounding the sending unit wire should swing the gauge to EMPTY and dangling that same wire in space should result in a FULL reading. It sounds like this actually happens when you somehow ground the case of the new gauge.

Stuart