Troubleshooting problems with temperature gauge

Start the engine and allow to reach normal operating temp. Engine off. Pull purple wire off temp sender. Ohm meter leads on sender contact post and ground should show somewhere between 20 and 40 ohms if the sender is good. With ignition switch on a digital volts meter between the purple wire and ground will show a pulsing voltage that averages 5 to 6 volts if the wiring and gauge are good. If you do find the pulsing voltage a short wire with alligator clips from the wire terminal to ground should send the gauge needle quickly to 'H' with ignition switch turned to on. If you dont find the pulse voltage you'll need to check that purple wire at engine harness connector, bulk head connector, and all the way to the gauge. The gauges are held to contact a printed circuit board with stamped speedy nuts. Loosened nuts causing a dead gauge has been found a few times.
I know your model of fuel gauge measures 13.5 approx' ohms post to post. I dont know if the temp gauge ever changed from 20 ohm to 13 ohm type. In any case the gauge will have continuity through it or be open. Open or continuity between a post and the gauge can = replace it. In a 73 Valiant we had... a faulty gauge voltage limiter was causing wild needle swings in that temp gauge. A solid state regulator cured that but... the damage was done. The temp gauge died about 6 months later. I have to recommend you replace the limiter if/when you pull the inst' panel.