Here's a neat thing that happened with my tach yesterday.... When I went to start the car the battery was a little low. So I used my jumper box to start it. I'm sure the battery was low on volts. My tach was at 5000 rpm at an idle. When I revved the engine it went down to 2000 rpm. It was working exactly backwards with 6000 as a zero. After I drove around a while the battery must have regained it's charge because the tach started working normal again. Strange things these tachs......