white1998gt

Have a 1968 Mustang w/ 302 stroked to 306. Has MSD ignition. Had a aftermarket tach in it that works. went to Mustang ECT. and bought their mini tach that installs where the clock used to be. Well, it doesnt work this is my second one they sent me. I have power from the ignition, checked with test light. Have a good ground. And am using my tach output from the msd box. Also tried a dummy wire from the negative post of the coil. Still doesnt work. My other tach works. But the ones from Mustang ECT do not. Am i supposed to use positive like the factory tach used ? I dont want to corrupt the thing and have them not take it back....Anything ?