Designed for current-limiting ignition systems (non-ballast resistor systems). If you are triggering an MSD 6 or 7-Series Ignition with its white wire or points terminal, you will need the PN 8910 Tach Adapter. The PN 8910 will correct the operation of most voltage-triggered tachometers or fuel injection systems that do not work directly off the tach output terminal of the MSD Ignition Unit.