The sender is a variable resistance ground connection. As the temperature goes up, the resistance goes down. (Negative Temperature Coefficient) What you can do to test the gauge is (with the key off!) ground the gauge lead to the block, then turn the key on and see if the gauge deflects full scale (HOT). If it does, turn the key off IMMEDIATELY or you will burn out the gauge. At this point, the sender is probably bad. Try sanding down the connection and see if it works. If not, replace the sender. If the gauge doesn't peg hot, either the gauge is bad, or there is a bad connection or broken wire between the sender and gauge. You'll have to take the dash apart and (with the sender lead still gorunded) check for ground at the gauge. Also check for power at the gauge. If you have both, the gauge is bad. If you have no ground, check the firewall connections for corrosion, and check for broken wires. If you can't find the broken connection, just run a 18ga wire from the gauge to the sender.
Matt {peace}
Matt {peace}
