I've got a '96 xj. The temp gauge doesnt move. I've grounded it out to check the gauge and the gauge is good. I replaced the sending unit and it still doesnt read anything. The existing sending unit didnt seem like it was threaded in all the way, I just matched the depth with the new one. Could that be the problem?

 

use a multimeter to test resistance of old & new sender, should not be open circuit, maybe a few thousand ohms cold, chuck them in hot water, measure again.

that should prove out a sender fault

 

Interested by the next steps. I also have an issue on a 1987.
At first I though the gauge didn’t move. But actually it does, but slowly and seems to be out of range.
Like if it was calibrated wrong

 

turns out the wire in the connector that hooks to the sensor was loose. i just pushed it back in tight and was good to go

 

It's always nice when solutions are so simple ..