[PDF] MSD Tach Adapter PN 8920 - MPS Racing MSD Tach Adapter PN 8920 If your tachometer doesn’t work correctly after installing an MSD Ignition Control, always try to connect the tach’s trigger wire directly to the tach output of the Ignition. If the tach still registers incorrectly, install this Tach Adapter. Tach Adapter Wiring Instructions | Mustang Tech Articles Tach Adapter Wiring. Using a Rally Pac with a MSD Ignition . When using a Rally Pac or aftermarket tachometer with a MSD Ignition, you need to use a tach adapter to prevent damage to the tachometer. The following diagram shows how to wire the tachometer, tach adapter and ignition together.