74 Lemans wont shut off
 

So i bought a new tach to install in my Lemans. My current set up is a GM Hei wired through a Relay to provide the proper 12v to the Hei, have has No problems. Hooked up the new tach, green wire to Tach terminal on Hei, Black Ground and the Red I plugged into Bat terminal in center of Fuse box, started car and everything worked fine, go to shut car off and it keeps running, with key in off position pulled Red wire from fuse box and still running, pulled Green wire from tach and still running, so tach completely eliminated had to unplug the 2 plug in wires at Alternator to kill car. Now with the Alt unplugged car will start and turn off but plugged in it will not, and swapped out Alt with another known good one. Any thoughts whats causing this?

Feed back through the brown/light/excite wire is whats causing the run-on condition. Common problem with relays and MSD boxes. Adding a diode to the brown wire is the cure.

Strange that it didn't show up till you added the tach??

Clay

Thnx for the Reply, that is kinda strange as i have used a relay in one of my other cars and havnt had a problem, thought yeah adding the tach seemed to bring it out. I will try the diode, thnx.

if im using the Brown wire which is the #1 terminal on the alt, does the stripe on the diode face the alternator or away from it, Thank you.

Bar on the diode marking towards the alt.

George

Thank you Sir, very much appreciated.