If you are using the magnetic pickup input (green and violet wires) to trigger your MSD Ignition, you will need the PN 8920 Tach Adapter. The PN 8920 will correct the operation of most voltage-triggered tachometers that do not work directly off the tach output terminal of the MSD control. This Adapter should also be used on current-triggered tachs (hooked in series with the ignition switch).