I can't figure out what's wrong.

With the temp gauge hooked up to the temp sending unit, the gauge
doesn't move no matter how hot the motor is (Okay -- I haven't tested
it at red hot). If I put the temp gauge wire against a part of the
motor, the gauge swings into the red. I've replaced the sending
unit with one from a junk yard (don't know if it works or not but
I'll assume it does). I've run a wire from the base of the sending
unit to some other part of the motor to ground it -- no change.
The gas gauge works normally.
chris