I get around 115-118 mVAC and 1.6VAC. I guess that points to my logic board as the issue...

I realize I'm a bit light on the mVAC, but that could be a result of my high end Sperry multimeter. If it points to something else, I'm ready for the next assignment.

Closing out the thread with the cause of my no-start issue. The speed sensor IC was toast. Big thanks to Ingo (ischmitz) for the diagnosis and fix!

A big thanks for the final post and the real solution to the issue. I was going to suggest trying your DME in a friend's Carrera. If a known good car will not start with that DME, there is your issue. Sending it for repairs was your fix. So again thanks for letting us know the issue.

Thanks Glen. I was hoping to try the DME swap, but my car is the "youngster" in my local crowd of Pelicans. I learned a lot along the way.

Thanks for the feedback. From what you write I guess it was the FI2438-03. I'm curious to know what was the failing mode for this IC. What was the reason for it to fail. Also what was the fix since this IC has been NLA for ages.

I agree, it would be good to know!:)

The failure of the early production of this IC has been known for over 20 years. The later mask process production appears to be more reliable, e.g. used in '87 and later DME ECMs.

I'll leave the technical, replacement and reliability details to Dave, Ingo and others with more knowledge than me. Here's a link that Ingo sent me regarding the IC:

https://www.richis-lab.de/ECU03.htm

The issue that led Ingo to this IC was my continued no-start after doing every test I could and replacing the speed and reference sensors. After proving my sensors were providing a signal and the DME was not registering those signals, the 2438 IC may be a logical check for someone experienced with the Motronic boards.

I can't confirm availability, but I found the following possible source for the IC:

https://www.jotrin.com/product/parts/2438_03?srsltid=AfmBOopRLo-tryTVdbtJ4dVArYbKSFKfforx-pKCtkkcLohFVQNGz-6x

This makes sense. If the speed sensor IC presents a low-impedance on the input pins it will reduce signal amplitude on the input.

I would caution about buying discontinued ICs. The sellers are often brokers who mis-represent availability. They will take payment and *maybe* refund when they can't find the parts.

They list it, but have NO stock, and never had any. Most likely, no excess stock was ever auctioned, for China to buy.

The failure of this chip has NO effect on the speed & ref sensors signals. Additionally, without knowledge of the actual outputs' (8/10) waveforms, the IC appears OK via a scope.

I wonder what the feedback signal to pin 11 is for. Is this some sort of watchdog timer signal that the MCU sends to lower the threshold after a few misses?
I may be totally wrong, but reason I ask is because this is what the MAX9924 appears to be doing in the same situation. This is described and illustrated on page 13 of the datasheet.
Wouldn't that newer IC be a good candidate for replacement of the custom IC?

Most likely pin 11 is just a reset for the TDC reference signal input, as it's from the 74LS02 NOR gate, which has a digital output (1/0).
It's not like the MAX9924 which optimizes a reference voltage based on a varying reluctance amplitude signal.
Remember, besides providing the the engine RPM, the DME's custom chip (2438) also provides the initial TDC reference point for the RPM signal.
Subsequently, the TDC is no longer needed, since the uP remembers where the TDC reference point is, based on the flywheel tooth count.

Thanks Dave.