So i know this part is rice, but I have always wanted one since back in the 80's and now i got one cheap. So heres the deal. I have a 97 JDM ITR. I installed a tach on the dash, ran the wires as follows.

Red power - connected to yellow power wire coming out of gauges.
yellow lights wire - connected to yellow power wire coming out of gauges.
black ground - connected to black ground coming out of gauges.
green tach sense - connected to blue wire coming out of gauges.

So all wires are connected to the far right plug where the tach is on stock gauges. The stock gauges still work perfectly, but the new tach stays at redline the whole time it's on. it jumps around when i hit the brakes, and when i have the blinker on it jumps with the blinker. I am wondering if the ground wire is not working? any help is appreciated.

 

It is part rice, but not as bad as it could be. Anyways, sounds like the ground. Everything else should work fine on being butted in, but not so much when you have a ground butted in. A ground needs a solid connection without a middle man. I would remove the ground you have now and secure it directly to a source of soild metal, with no paint on it.

 

Lengthen the ground lead and fish it down below the gauge cluster.

You can then attach it to the steering column support frame with a self-tapping sheet metal screw. That way you wont have remove paint from the chassis.

To be sure that you have a solid ground, crimp a spade terminal onto the end of the lead. Drive the STSM screw through it, then into the steering column support frame.