Tach problem

I seem to have a ground problem with my tachomer (1984 911) - it will work sometimes and then quit when I turn the lights on or a blower fan. Does anyone have any suggestions? Where could I hookup an aux ground? Is this a ground problem? This thing is making me nuts!
Thanks,
Paul

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1984 Targa

I believe there is a ground tab on the back of the tach that is part of the tach body itself. I'm going by memory here. It's been awhile since I had one out. Either way, find a way to run a wire from the body of the tach to chassis ground or battery ground,(neg. terminal). Maybe try tightening the screws on the back of the tach. as well. You'll need to pull the steering wheel and upper column cover to get the tach out. Unless you want to try to get at it from the back. I don't recommend that.