Does it immediately go to hot before you run the motor?

Pull the yellow wire off of the temperature sender on the head. Is the gauge still pegged? Short circuit in the engine harness. If not, measure the voltage there. Make sure it is 10V, like Texas Instruments intended.

If so, the next step would be to substitute a resistor (68 ohms) for the sender, that should give you a 3/4 gauge reading, thus checking the gauge itself. Or, if the resistor method seems a bit impractical, putting in a new sender would eliminate that possible cause.

Usually cars of this era (81-84 I assume) have rotten engine harnesses, and any short from that yellow wire to another one or ground could have your gauge pegged all the time.
--
Art Benstein near Baltimore

My dog is worried about the economy because Alpo is up to $3.00 a can.
That's almost $21.00 in dog money. -Joe Weinstein