How to operate bike tach from MircroSquirt side

[R100RT]

On the older BMW motorbikes, the electric analog tachometer has its + and - leads, and a signal wire which shared the ignition signal from the original Bosch module & trigger canister. That is a ground signal, no doubt a very common method. I've removed that part of the old ignition and plan direct ignition control from the M.S. as per manual recomendations.
So, I'm seeking the method to comission the old tachometer.
Is terminal #35 of the M.S. suitable to utilize for this gauge, not quite sure from the schematics what is happening on that terminal @ each ignition event. If it provides a ground signal at each crank rotation , then I suspect that should work, if not then any ideas on how to achieve that service is appreciated from this end.
Lorne

PS - what I lack in electrical MS expertise, I make up for in creative fabrication. Forgive the layman questions.

[Matt Cramer]

Did the signal wire go to the side of the module that drove the coil, or to the sensor input side of the module?

[R100RT]

Hi Matt.
The tachometer signal wire was on the output side of the module, which provides the ground to the coils. In looking at the way MicroSquirt works, I guess I could just do the same and jumper the tachometer wire to the negative terminal (M.S. pin#12, Ign #1, White) of the primary side on ignition coils (that in essence is what BMW had done), but then I saw pin number 35 listed as TACH OUTPUT, the Green/Yellow wire - and pondered what happens on that circuit (doesn't really go into describing what you get there?)
Any help appreciated.
Lorne

[Matt Cramer]

The tach output circuit is a 12 volt to ground signal. I would use the coil negative instead; closer to how the stock tach worked.