Not sure:
Excerpt:
http://www.msdpowersports.com/pdf/frm22049_pn8920.pdfIf 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.
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.
Current Triggered: If the trigger wire of the tach was originally connected to the coil positive
terminal, it is a Current Triggered Tachometer.
Wiring with a Magnetic Pickup Distributor and a Voltage Triggered Tach.
AUTOTRONIC CONTROLS CORPORATION • 1490 HENRY BRENNAN DR., EL PASO, TEXAS 79936 • (915) 857-5200 • FAX (915) 857-3344
CAUTION: Never connect any wires to the Coil Terminals when an MSD 6, 7, 8
or 10 Series Ignition is installed.