what is causing a mysterious electrical problem? on 1995 Volkswagen EuroVan

With headlights off, eurovan temp gauge gets a false overtemp. turn headlights on, while rotating dimmer wheel down then up, tachometer operation is directly affected. (dimmer wheel down,tach. works, dimmer wheel up,tach. does not works). Also dimmer wheel down,get a false overtemp with red blinking overtemp light,dimmer wheel up,temp. gauge creeps back to normal range but tach. drops to zero. correlation of tach. movement is directly affected by dimmer wheel movement. also, when headlights are on, there is dash area illumination except for all instrument cluster gauges. I have so far replaced temp sensor, inoperative horns, contact cleaned all fuse/relay plugs. Help please.

by in Dallas, GA on February 15, 2012
1 answer
ANSWER by on February 16, 2012
This sounds like a weakened ground problem in your dash. When you operate different devices, they can not utilize their normal ground, so they cannibalize the ground from another device, which makes that device go 'wacky'.