Just wondering if you were able to diagnose this any further? I accidentally shorted +12Vdc to ground when installing some dash lights and I'm worried that I applied power to the Tach input from the CDI pin (pin 15?). My car was running strong before I did this, now the engine won't fire (no other issues, from what I can tell).

Just trying to figure out how to diagnose the CDI.

Update: I pulled the CDI box out and opened it up to find that D1 was a short circuit. D6 and D7 show normal diode behavior and have a voltage drop of ~.46V. I suspect this may be my (first) issue.

The unit is not a CDI! It is an inductive discharge ignition, i.e. an electronic set of points.

Has anyone saved a 'scope shot of the TD signal coming from the 6 pin CDI? I have a '75 911 that had a 2.7 liter engine, and that was replaced with a '78 911 SC 3.0 liter. The tach doesn't work, and I sent it to NHS and they did something to make it work with my new CDI. I got it back and it still doesn't work. Now I'm thinking something is wrong with the tach signal coming from the CDI but I don't know how it should look. I looked at it with a scope and I see a signal, but it has a fairly low level pulse about 40uS long with about a 6Vpp swing. Does this sound about right?

The SC CDI is triggered by an inductive pickup, this signal is required to be processed in the CDI box and that's the TD signal. The former CDI (3pin) boxes (up to 1977) use a breaker-like trigger signal coming from the dizzy. Why I am telling that? In these cases of elder CDI boxes and dizzy's as on the pre-SC cars you can test to directly use that signal directly from the dizzy with the tachometer. This works well only when the CDI box is still in place.

I suppose your setup is a SC dizzy and CDI box (6pins), and the tachometer is still the one from 1975, right? Then either you have to modify the tach or put a signal converter in the tach signal input. You wroite that yours has been modified already. Ask the supplier what they exaclty did and how you should connect the tach now. Otherwise you can replace the tach with a SC one. IIRC some tach's don't work in any ways with this change, some do. You cannot rely on that unfortunately.

Here are 2 articles from a german CDI specialist about the TD signal and the tachometer on Porsches, no pictures of the signal, but a detailed description on how they work:
https://www-911--hkz-de.translate.goog/TDSignal.htm?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=de&_ x_tr_pto=wapp

https://www-911--hkz-de.translate.goog/Drehzahlmesser.htm?_x_tr_sl=de&_x_tr_tl=en&_x_tr_h l=de&_x_tr_pto=wapp

Afaik the most comprehensive and most detailed information about them on net.
Both automatically translated by Google. Original in german only.

HTH

Thomas

That's correct. Hopefully, the tach has been setup correctly for this signal!

Accidentially found: KlassikATS also has good information about this, also some pics about the signal.
https://www.klassikats.com/technical-service-bulletin-cdi-tachometer/

Thomas