Tach Wiring - Jeep-CJ Forums Tach Wiring. This is a discussion on Tach Wiring within the Chassis and Body forums, part of the CJ Forums category; I pulled this out of my dash and it wasn't connected to anything. Does it matter which wire goes to Factory CJ7 tach with mpi - YouTube Click to view on Bing4:07Factory CJ7 tach with mpi Christopher Spencer. Loading Solid grey wire out of ecu to solid red wire on tach, red/white wire on tach to other cut end of solid grey wire going to coil.Author: Christopher SpencerViews: 5 CJ Tach Wiring - jeepz Hence, the tach was connected in series with the primary side of the ignition system and carried the electrical load of that system. That is the reason for the heavy gauge wiring of the original CJ tach. The vast majority of modern OEM and aftermarket tachs count ignition system voltage pulses and carry very light current loads.