Gauges increase rapidly when lights on (72 510)

[5150 Wll]

I did some searching but didn't find anything specific.  I recently purchased a 72 510 and its got an interesting problem.  The temp gauge and the fuel gauge both increase rapidly when I turn on the headlights or drive the car.  There aren't any hack jobs in the wiring and the car is pretty much stock.  I'm leaning towards some type of ground issue but may be off base on that.  Everything works on the car electrically BTW.  Any ideas on where to start greatly appreciated

[Roadster-ka]

Sounds like the dash lights are seeking ground thru your gauges. Look for corrosion on circuit board and/or round connector.

[datzenmike]

Both gauges are powered by a separate voltage regulator that supplies a constant unchanging 8v source. If it was run off the ignition it would run on 12 -14.5v and read up and down depending on if idling at a stop or driving around. Headlights, wipers and heater fan would throw the readings all over the place.

 

There was a $3 fix for this voltage regulator using a zener diode. Maybe someone can post a link to it???

[banzai510(hainz)]

I would reseat the volt reg connector

also the alternator connector and the ground screw near the volt reg

Maybe the alternator or volt reg is going out. as these feed the instrument power and its on both.

[5150 Wll]

Thanks for the responses guys.  I've done a bunch of connection cleaning and searching for grounds that have possibly come disconnected but all seem to be good.  I think I have a wiring diagram downloaded so ill start going through that.  I'm sure its something simple