Thanks Eric. I've checked voltage and I'm at 13.5v on the above pins. However, that reminded me that I need to double check the grounds in the engine bay.

Try calling these guys (25+ yrs Porsche electronics) for personal help; www.systemsc.com

Is that Loren's company?

Hi Josh, have you checked your battery voltage whilst cranking? Batteries can be misleading like that, all fine when sitting there doing nothing, but as soon as you put some load on it!! Some older or lesser quality electronics do not like even minor voltage variations.

Probably a good idea, however that generation of Motronic is quite tolerant on battery voltage. +12V line is apparently sampled by the ADC, and probably monitored by the MCU. And the 5V line seems to be supported by a low dropout +5V regulator that can still function with voltages as low as 6V.

Josh, have you checked and listen to the ICV in the engine compartment for humming when ignition is on and engine is off? Any humming / buzzing is a good sign that the MCU has a heartbeat. Sorry if you have already mentioned it.

I have nothing technically helpful to offer. But offer my moral support. I recently had a not start issue on my car that thankfully was resolved much easier than your seems to be. (bad CHT sensor)

Good luck. I hope you get it resolved soon and get your car back on the road.

Best
Foster

Thanks all for the ideas. My battery lives on a tender, so it should be good. I did test while cranking, but now am unsure if that was just with the test light or multimeter (likely both). I'll check again. One test I haven't done is to jump the CHT and try cranking it. I'll add that to the list to try as well. The ICV is buzzing happily away, when the ignition is on.

Ok so your MCU is alive.
What about the tach? Is it bouncing when cranking?

Got some testing in before work. Jumped the CHT with no luck. However positive voltage on the coil dropped to 10.8v - 10.9v while cranking. Tach didn't budge at all during any tests. I can't say I ever remember it moving until start-up.

Battery voltage is good. The drop is normal. No tach bouncing means no ignition signal (hence the no spark). No injector signal either, since your test with the noid light failed.
That means the ECU probably doesn't get a reference position to start the engine.

Either your ref and/or speed sensors are bad or disconnected, or the ECU circuit that is processing their signals is bad. Keep your fingers crossed as this circuit is NLA.
But your next step is to check both sensor signals at the harness side while cranking, preferably with an oscilloscope. Or, with a voltmeter in AC position, speed sensor should show 1 V or above, ref position sensor should show less voltage but not zero. I guess you have already checked their resistance, which should be around 1000 ohms for both.
Then if your sensors test OK, that leaves the fault to your ECU. You should send it for repair.
Plenty of skilled people here on this forum.

Yes, Dave's company

Gilles - I've replaced both sensors and confirmed I get pulses from them at the DME harness (LED test). I also did the test with a pocket oscilloscope and think I had good readings (it made waves), but I've hit some settings on the scope and can't seem to get anything other than a tiny fuzzy line. I might see if I can reset it back to factory settings, as my skills are not so scope-tastic with that thing.

may be you can redo the testing this time at the big connector that plugs into the ECU. That way your test would include the harness wiring that goes along the car up to under the seat.
The ref sensor is accessed on pins 25 and 26. The speed sensor is accessed on pins 8 and 27.
ECU disconnected, of course. These are passive sensors, so they don't need power and can be checked just by cranking.

I've done the full pinout testing at the DME harness plug, per the factory manual for everything and it was all within spec. I still replaced the speed and ref sensors, just to eliminate those as potential issues. I'll retest again tonight with the multimeter, but the LED test showed both "flashing" during crank. The speed sensor flashed more quickly than the ref sensor, if that helps with anything.

Yes, it sounds like your sensors are good. The ECU doesn't get or interpret their signals correctly, I'm afraid. Reason why you get no spark and no fuel.

Yes, that's where you focus the troubleshooting!

+1 for me.

Did you guys (Lyle & Scott) swap with a factory Porsche connector or modern 3 pin connectors? If swapped with the Porsche connector, do you have a part number? I'll do some more jiggling tonight. TIA

Potentially a waste of time, effort, & money! Measure the signals at the ECM connector while cranking;

1. pins 25 & 26, 200 - 300 mVAC
2. pins 8 & 27 - 1.0 -1.5 VAC

If OK, then the problem is on the logic board as wazzz refers to.

Thanks Dave! Will measure those tonight when I get home!