Why do some tachs work without MSD, and others do not?
 

I just wired up MSD on an 86 930 today, and was surprised to find the tach working without the adapter. I at first assumed that the cars with points would always work without the adapter, but later debunked that myth. My 76 930, and my 1980 SC needed the tach adapter but a few months back, I did a customers 79 sc , and it did not need the adapter. I have found no rhyme or reason for the differences to this point.

Yup, it's been this way for years. I wrote this I'm the 101 projects book 13 years ago. I think it's related to the tolerances on the electronics in the VDO tacks.

Wayne

MSD and Tachs
 

I put an MSD 6A in my 1980 SC several years ago when the Bosch CDI went out and the tach worked/works just fine! It's been a while since I checked the wiring schematic but I think there are two choices for wiring the tachometer. I used the (I think) white wire that comes out of the left end of the unit to feed the tach connection. With points, I believe there is another color wire (don't remember color) that comes out of the right end of the unit that must be used.