And now my temperature gauge is not working

[Rich69shortfleet]

I've been chasing various electrical issues and thought I had them all resolved today. Everything is working again in the dash except now the temperature gauge has quit. This has become a whack-a-mole game.

I grounded the wire at the sender and the gauge pegs hot. So the gauge and wiring are presumed good. I have an aftermarket temp gauge also so I know the engine is getting warm. I actually want to remove the aftermarket gauge once and for all so I need to get the factory gauge working again.

With the engine warm I do get continuity through the sending unit, though with some resistance as you would expect since that's the way the units work.

If the sending unit is okay, the wiring is okay and the gauge is okay, what is there left to check? Not sure what part of the puzzle is likely to be the source of the problem since they all seem to be working when tested independently. Everything else in the dash works; lights, other gauges, etc. so it isn't a ground problem. Is there any way to test the gauge independent of the sending unit to make sure it can do more than full cold or full hot?

In the past when the truck was cold the gauge sat at the lowest point on the scale. Now the gauge is sitting at a lower point (off the scale) to the left of the scale unless it is grounded when it goes to the peg to the right past the end of the scale. This is an original factory gauge, the underdash harness is a good original gauge harness and the gauge has worked fine for years (other than reading low probably due to a modern sending unit, plan to get the supposedly good one from Lectric Limited soon).