Temp Gauge Issues

If the IVR is the original type I'd get a solid state replacement. ALSO the contacts where the IVR plugs in can get corroded and you must solder across them to bridge to the PC board traces. Check the connector pins for loose, and repair. Last the nuts on the gauge nuts need to be loosened/ tightened to "scrub" them clean

There's an old thread on this and it's a good one

Printed circuit pins repair
Hey Guys,
I’ve got a 72 Demon with non rallye dash, temp gauge has never worked, all other gauges working including fuel.
Have replaced sending unit and tests cable going to it and it is all good.
So I narrowed it down to a faulty gauge, pulled the gauge and replaced it with a new one and it’s still not working, also now my fuel gauge is playing up which it wasn’t before. Do you guys think it’s an issue with the voltage regulator or the circuit board itself?
Thanks in advance for your help
Cheers from Oz
Mike
i would pull the wire off sending unit and take voltage reading to ground. if voltage is present, it is sending unit and they differ in resistance if generic is used.you can also short that wire to ground quickly gauge should jump. if no voltage check at block in fire wall for bad connection same way as at sending unit. although because the gas gauge is also acting up sounds like voltage regulator. easy fix. had this same problem in a 64 were neither gauge worked, unfortunitly the regulator is built into the gas gauge, not easy fix.