UberScheitzer

Just installed an MSD (6AL), matching coil, and pertronix ignitor. With the Tach attached to the output on the MSD without the adapter, we get a fairly acurate reading. Checked the rpm's with a tach/dwell meter and it is extremely close.

The question is, if we run it without the adapter will it damage the tach?

Thanks!

Jason
'74 911 S

art

I don't know the output signal of the MSD, but when I hooked up my SDS crankfire which has a +80v spike as the tach output and it did fry my early tach. I measured the output of the CD tach output with a scope and it was a large negative spike.

This was on a 914-6, but I think the CD system is the same as a '74

RoninLB

I can't give you much hard info but I read a member's posting to install the adapter anyway. I should have asked why, but I installed it "anyway". fwiw I believe a damaged tack may cause ignition problems that will make you nuts trying to locate. If I have any ign concerns the first thing I usually do is disconnect the tach.. and if I was running a computer fuel mgt I'd run the MSD noise suppressor 'cause computers need very clean voltage imo.