Tach not working after installation of Bosch ignition box
 

Hi,

my '80 ´911 SC came (to me) with an MSD 6 AL ignition installed. Since the installation of it as well as the rest of the electrical components on the panel in the engine compartment was a "hack", I have restored everything to the factory original condition. Today I have hooked up the Bosch ignition box, and to my great surprise, the car now idles much better than with the MSD box.

Problem is that my tach is no longer working now. I'm sure I have connected the cable for the tach signal correctly. What I suspect is that there may be a tach adapter hidden somewhere in the signal path that was needed for the MSD multi spark design.

Before I start ripping things apart I would like to ask if the MSD would definitely need such an adapter and, if the adapter is installed somewhere, it would keep my tach from working with the now installed Bosch ignition box?

Thanks, Manfred

Manfred:

SC tach output from the six-pin "TD" terminal is a square wave from about 1-6V. (A 5V square wave).

MSD 6AL output is a 12V square wave with a 20% duty cycle.

If someone introduced a tach adapter this could be the problem, you will need to search in the car to find it.

If someone has modified the tach, this will need to be reversed. Can you borrow an SC tach?

304065,

I hope that the tach is still original. Tomorrow I will first try to find out if there is a tach adapter anywhere behind the dashboard. Judging from the poor installation of the MSD box I can't imagine that they had the know how to modify the tach (at least I hope). As of now, I don't know anyone who could borrow me a known good one for a cross check.

Thanks, Manfred

follow the black/violet wire from the 6 pin CD plug. the MSD either had that wire cut and run to the tach terminal on it or a jumper was run to the old CD plug.

Found and corrected the problem. There was an additional wire to +12V at the tach terminal black/violet at the back of the tach, something like a "pull-up" that was probably needed with the MSD box. I removed the +12V leaving only the black/violet wire from the Bosch ignition box and the tach works now.

Thanks for the replies,

- Manfred