The MSD 8920 Tach Adapter will correct the operation of most voltage-triggered tachometers that do not work directly off the tach output terminal of the MSD control.
There are two style tachometers commonly used ; Voltage triggered and Current triggered, this adapter can be used to create a tacho signal for either type.
Voltage Triggered: If the trigger wire of the tach was originally connected to the coil negative terminal you have a Voltage triggered tachometer.
Current Triggered: If the trigger wire of the tach was originally connected to the coil positive terminal, it is a Current Triggered Tachometer.