Temp gauge/Fuel Gauge problem

[65Vart]

Just took my 71 out, first time this year:glasses7:
I was having a problem last year with the temp gauge pegging when the lights are on, now same thing with the fuel gauge, both gauges any ideas? Interior is coming out soon so Im going to get under there and have a look. Its a non rally dash.

 

[cudascott]

Sounds like the voltage regulator for the gauges is going bad.

 

[67Dart273]

If it's the IVR then both gauges should be pegging together

 

[RedFish]

definitely crossed/shorted together circuits there somewhere. A couple of spots that are suspect come to mind but none would be found and corrected from underneath.
Get the 'how to' from a factory service manual and remove the inst' panel.

 

[BillGrissom]

Sounds like a bad ground somewhere. The only places the headlamps and gages interact is in the +12 V supply and the ground. Turning on the lights would only drop the supply, which would tend to make gages read low. A bad ground would do the opposite.

 

[Rustedwrench]

Interesting problem. Thinking out loud, normally the only ground needed by the instrument cluster is for the voltage regulator for the gauges. The gauges themselves ground through their respective sending units. When the headlights are on the illumination for the instrument cluster is on as well. So, if you had a poor ground for the instrument cluster it could be overloaded by the extra current for the illumination bulbs. That puts voltage on the ground circuit in the cluster and the result is no ground for the voltage regulator. If the regulator has voltage on the supply side and now on the ground side then the heating element will not heat and the contacts will never open, so full voltage goes to the gauges. If it were me, the first thing I would do is turn the lights on and then turn the dimmer for the instrument cluster illumination all the way down and see what happens. It is easy to do and I would not get my hands dirty taking apart things needlessly.

Or I could be full of sludge. Still, interesting to think about. I prefer to think first, work later.

 

[RedFish]

if the inst panel didn't have a chassis ground nothing there would work except brake warning lamp and amp gauge.