I installed a Shiftworks Tach in place of the clock on my 72 SS. When I start it up, the tach goes to zero, but does not read any RPMs. I have a Delco Remy HEI and I crimped a spade terminal to the green wire to hook it up. Red is to the accessory terminal on the fuse block through an added 4 amp in line fuse and black is to the parking brake.

I thought I had successfully tested it outside of the car, but wasn’t sure since I couldn’t get a good ground.

Any help on how to troubleshoot?

I thought I had successfully tested it outside of the car, but wasn’t sure since I couldn’t get a good ground.

Any help on how to troubleshoot?
I would have made sure it worked before installing. Someone else had a problem with one of these just a little while ago.

Someone else had a problem with one of these just a little while ago.

What do you know. Me too.

The tach was bad. I called Ground Up and they said they would replace it immediately, but that I might want to call Shiftworks first instead of pulling the whole thing out.

Shiftworks sent me a replacement which they bench tested. Instead of pulling the whole tach, I just unplugged the original "brains" (which is a component that is wired to the tach in the dash) and plugged the new one in and it all worked fine.

Major props to Ground Up and Shiftworks for excellent customer service.

And major thanks to undee70ss for plenty of good, accurate, relevant advice in working through this.