First: Create a "service loop" with all the wires to the cluster so that you can get it out of the dash. This is a must. Next: 90% of all electrical troubles in trucks is the ground. To say you have good ground is not to say you do have good ground, so be advised. You have to have shiny metal touching shiny metal. It's hard to even check for good ground. Now to trouble shoot temp system and to repeat what Bill said. Take wire off sender and be sure it's not touching anything, turn ign on, sender should read at low end of gauge. Then touch this wire to a GOOD ground, gauge should read high. In other words sender is a resistor, high resistance= cold, low resistance= hot. If it passes this test, the gauge is probably good. As stated, to test sender you need hot water or maybe a heat gun (no flame). Connect sender center post to gauge and ground side of gauge to threads of sender with a clip. place in hot water covering only the threads. Place thermometer in there too. Should read same as thermometer or fairly close. These aren't laboratory quality items (gauge and sender) they are old and beat up. I have taken gauge and sender into my kitchen a few times. If you can heat hot water out at the truck, that works too.
Beware the ground!
I have tested items on the bench and touched a shiny metal area and didn't work, touched another area, it worked. Ground is tricky. Sometime you have to dig into the metal with the test probe. Voltage will even arc and make contact, ground usually does not. if you are saying you have good ground because of a visual you have at some bolted joint, that may be wrong. The only fool proof way to check is to set up a wire from the battery neg to the post of the unit you are testing or, on the bench use a power supply like a battery charger with a safety fuse inline. In my opinion. I like to know for sure before I throw away a part or continue to look elsewhere in the circuit. Bypassing ALL existing wiring, with new temporary wiring, pos and neg ,is the only way to troubleshoot electrical on an old truck. Especially gauge and sender items. Also a temp gauge can be tested with a fuel sender or some pots and rheostats. Wish I still had my old slot car controller.
You must buy a compatible sender, I believe that is a T5.
