[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. MSD 8920 Tach Adapter, Magnetic Trigger Installation User install this Tach Adapter. There are two style tachometers that may have been used on your vehicle; Voltage triggered and . Current triggered. The wiring diagrams show both installations. Voltage Triggered: If the trigger wire of the tach was originally connected to the coil negative terminal . you have a Voltage triggered tachometer.