art

I have a 3.0 sc in a '75 911 for which I need to convert the tach. Any input on which wire is the signal wire and best way to go about the conversion. I have an M&W ignition with their harness that plugs into the 6 pin SC connector.

I'm not sure what tach output I have now, but I recall that the output of the old style 3 pin CD system was a negative spike.

If I decide to wimp out and just buy an SC tach, does it just drop in?

Thanks for any help.

Lorenfb

The early 911 tachs used the high voltage pulse from the coil, which was a half
sine wave of about 200 microseconds and 400 volts. The later SC tachs used a
0 to 7 volts negative going pulse off of the TD pin of the CDI.

You need to determine the type of pulse provided by the other ignition unit. Call them
since they should have a tech support line to answer your question as to a tach
interface.

art

I checked the M&W site and they do show a +12v tach output. Thanks for telling me the obvious. It is always easier to post a question here than to think too hard.