I'm only telling you what i know first hand. All Gm tachs are voltage triggered. Factory original GM tachs do not need a adapter, only to bypass the original inline filter. Repo tachs, if built exactly as the originals would not need one either. Just try wiring your tachs trigger wire to the msd6al tach output terminal. Even MSD tell you this. It can not damage anything. If the tach doesn't respond correctly, then you would need a adapter. All a adapter does is keep electrical noise out of the tach. From the Msd6al instructions, pg 7 ...........................
If your tachometer fails to operate with the MSD installed you may need an MSD Tach
Adapter. Before getting an Adapter, try connecting your tachometer trigger wire to the tach
output terminal on the side of the MSD. This output produces a 12 volt, square wave (see
page 2). If the tach still does not operate, you will need a Tach Adapter. There are two Tach
PN 8920: If you are using the Magnetic Pickup connector (Green and Violet wires) to trigger
the MSD, you will need the PN 8920.
From the Msd6al instructions, pg 2 ...........................
The MSD Ignition features a Tach Output Terminal on the
side of the unit. This terminal provides a trigger signal for
tachometers, a shift light or other add-on rpm activated
devices. The Tach Output Terminal produces a 12 volt square
wave signal with a 20% duty cycle.
Some vehicles with factory tachometers may require a Tach
Adapter to operate with the MSD. For more information on
Tachometers and MSD Tach Adapters, see the Tachometer
Section on page 7.
If your GM vehicle has an inline filter it may cause the tach to
drop to zero on acceleration. If this occurs, bypass the filter.
Msd 8920 tach adapter instructions http://www.msdignition.com/pdf/8920_frm22049.pdf
Msd 6al instructions http://www.msdignition.com/pdf/6%20series/6series.pdf
If you need more help or have other questions post back.