Ingo,
Interestingly enough I was looking back over some old posts. I was asking last year about hooking up the upshift light on my car. You posted this in response:
http://forums.pelicanparts.com/porsche-911-technical-forum/501857-tach-wiring-question.html
"BEWARE: If you hook up the wire coming from pin 11 of the 35-pin DME connector to the wrong terminal of the tachometer you will damage the DME. It carries the load signal of the DME. It is essentially the injector duration. Unfortunately, if that pin gets hooked up the tachometer input (with its internal low-impedance pull-up) it will mess with the peak&hold driver in the DME.
First your mixture will be out of whack and eventually the DME will be damaged and the car won't start. I had several DME's on the bench that needed repair due to this exact blunder.
If it was me I would not even bother getting the upshift light to work. Pin 21 of the 35-pin connector carries the RPM signal. It is usually a black/purple wire. Use an Ohmmeter to find it. Then find out on which spade the tachometer reacts properly to it. That's it.
Ingo"
Now I am wondering if during my early installation I did this same blunder? If I did does that mean that the S400 chip is shot? Is there a way to test it? From another one of your posts this appears to be the peak hold driver as you mention. As of last year it didn't appear that anyone had found a pin for pin replacement.