Tach wiring with msd tach output wire

Has anybody used the gray tach output wire from an msd box to hookup their tach, or do you just use a wire from the negative side of the coil? I'm hooking up the rallye dash in my car and I had the tach converted to work with an msd ignition and the label on the tach says to use the negative coil wire. Wouldn't the gray wire from the msd box be the same signal wire as the negative coil wire? I already have the msd wire behind the dash and I would rather use that if it'll work instead of having to run an additional wire through the firewall to the coil.

I used the gray wire from the msd box.

Depending on the type of tach, sometimes an MSD tach-adapter is needed between the MSD unit and the tach.

- EM

My car has the rallye dash with the stock internals converted to work with an msd ignition. I just ran a new wire to the coil just to be safe.

If you are using an MSD ignition and your tach is calibrated for that use, use the MSD output. No wires other than the MSD coil wires to the coil.

I have the tach wired to the coil now. Do you think there will be any issues wired this way? The guy who converted the tach knew I had an msd setup and he said to use the negative side of the coil.

Yes, Msd says not put it on the coil too high of voltage. When they say it was setup for Msd it means you can hook it up to the Msd grey tach wire without an adapter. Good chance it'll burn up if you leave it on the coil or at best won't work.

The voltage to the coil from an MSD box is something like 480V IIRC. That's enough to let the magic smoke out of most electronic components.

My 6AL had a tach connection right on the box that you plugged into.

My buddy had to us a tach adapter to get his MSD Digital 6AL to play nice with his OEM 70 Challenger tach.

Kevin

OK thanks. Good thing I didn't start the car.