I bought the above noted module for my HEI as I wanted to use the Rev limiter function. I just hooked it up and the car runs great (same as before) and the tach works the same. When you set the RPM rev limit you are supposed to go to half the desired RPM and ground out a wire that comes out of the unit, and is tied into the tach wire. For example if you want a 5000 rpm redline you ground it out at 2500 rpm's. Well I tried every ground on my car including going directly to the battery and it doesn't work for my stock tach? Is it that the stock tach isn't compatible. It does go to zero while grounded, as the instructions state but wont sweep of do any of the other things the instructions say? Anyone else have this issue?
I have to wait to call the MSD tech support till Monday....Thought someone in here might have had the same issue?

 

I was going to see if I could remove the post as I have figured out my issue. I figure others might as well learn from my error. The module has a sweep setting that will sweep to the highest RPM setting that you have programmed into it. I first disconnected the filter that the Tach has just before the HEI on the firewall then I tried to set the sweep feature. It worked ! I then tried to set the RPM limit as described.....Here is the trick.
The Instructions say hold the ground wire for approximately one second to set the RPM, guess what? it is more like a half a second. Once I did that it set very easily.
Stupid mistake, but hopefully someone here will benefit from it.