Hate to jump in, but I have a 96 Mustang GT and my temp gage sits far above H whether its running or not and never moves. I have replaced the coolant sensor, tested the electrical and removed the cluster to see if that was an issue. I manually moved the temp gage needle to L and reinstalled the cluster. As soon as I started the car, the needle went straight to H. My car is not overheating, but I would still like to know my gage is working. What is the best solution? New Cluster? Thanks for any advice that can be offered...

 

Buy a A-pillar gauge pod and hook up a real gauge and while your at it get a real oil preasure gauge. the factory units are just dumby guages

 

sounds like you might have an open ground