hot rod,
The electric gauges rely on a variable ground that is supplied through the sending unit. I don't know what kind of setup you have, but you might be able to isolate a gauge/wire problem from a sender problem by disconnecting the wire at the sender.
If you disconnect the wire at the sender with the key switched ON the gauge should peg either MTY or FULL. When you do this, don't let the wire touch ground!
The next part of the test requires you to touch the same wire to a good ground, like a engine component. This should make the gauge read the opposite of what it did on the first test.
If you got the gauge to swing from one extreme to another...HOT to COLD or vice versa, it probably means the wire and gauge are fine and it's the sending unit that has gone south.
This troubleshoot is similar to one for a fuel gauge. The operating voltage for the electric gauge has to be the same as the trucks voltage, and the sending unit resistance value must be matched to the gauge.
If you bought the gauge and sender as a package, this should be the case. If the wire going to the sender is chaffed or burned through, it might be going to ground. This would cause a false reading.
The location of the temperature sending unit must proper. The threaded portion of the gauge must not be insulated from the block with tape or pipe dope.
If the gauge worked at first, and doesn't now, one would think some component failed. Do some more detective work and see if a clue pops up.
This assumes the engine isn't actually heating up!
Stuart
