I have a Sunpro super tach II that worked flawlessly for a while. Suddenly, it stopped reading at all and then would work intermittently. I figured it was the guillotine connectors corroding so I soldered up my connections and I grounded the black wire to the firewall directly. Green wire to the neg. side of coil and red to the fusebox.

Now, my tach will not read any low rpm. In fact, when I rev the engine it jumps from 0 to over 3,000 even when I don't think I'm at 3,000 rpm yet.

I grounded the battery to the block, block to frame, and block to firewall.....but the tach worked before any of these additions.

Any ideas of why this might be? I think it's getting interference but I just cannot figure from where.

 

Double check that its on the 8 cylinder setting. I assume you have it connected to the tach port of the coil? Make sure you have a good connection there. I had an issue where my ring terminal wasn't making good contact intermittently and would occasionally not work correctly.

 

Krause,,
You know I was thinking that very thing on the drive home. I did have trouble with that ring not holding tightly when I installed the motor. I'll double check and follow up.