My '91 Classic Collection coupe (5.3L) has an intermittent gauge issue. The tach is fine and the speedo usually is, but the smaller gauges (drum-style) are unreliable. For some time, they have been affected by turning on the lights. Recently they began giving false readings even when the lights were off--specifically, the temp, fuel and oil pressure shoot up while the ammeter drops. When this occurs, if I switch on the lights, all the gauges peg at the upper limit. My mechanic cleaned all the grounds and the situation improved, but on a recent long day of driving, the problem still occurred, especially after I'd been driving for a while. It would rectify if I turned the car off and let it sit. One time, it even "fixed itself" while I was driving. As for the speedometer, it went through a spell of dropping to 0 then reading correctly, then dropping, etc. After several such episodes it was fine the rest of the day. Should I continue to assume the grounds are the issue, or do I need a circuit board--or something else entirely? The temp is my big concern, although recently even on hot days it's running around 190, per my infrared.

 

Yes, grounds.

Common issue.

Remove the cluster, clean all the circuit board contact points, and add a redundant ground wire to the cluster. You should be good to go after that

Cheers
DD

 

Also check the fuse box (under the drop down panel below the steering wheel on right hand drive cars); I sometimes lose all the instruments but they come back if I prod the fuses in the fusebox.

 

I would do a free check the alt. at Autozone or other retail store.
My internal reg went bad and fried the alt. giving me weird gauge problems.