Huh, that teaches me to do research on a mobile phone while stranded. Thanks for pointing out the misunderstanding! I don't know where I got the impression the OVP would cause a no-crank no-start now.

I can't find the page that led me to believe that the OVP would cause a no-crank no-start if in a really sad state. It does look like it is, on occasion, implicated in rough cold starts. However, that same page indicates it is indeed not involved with the starter solenoid and motor. (To readers finding this page in the future: the linked page applies to W201 / 190E, but the 190E shared a lot of guts with the W124 / 300E from this era. The MAS unit and OVP relay are both found in 190Es and 300Es.) So, that rules out the OVP relay. May as well replace it as it's on the way, may fix the cold starts, and I can't cancel the order. Oops.

One common failure I read about were fuel pump relay failures, which will cause a no-spark (no-gas) starting failure. Since I'm not seeing cranking, I'm ruling out the fuel pump relay. It's located in the MAS unit, and as I haven't read about any other starting problems due to the MAS unit, I'm also ruling out the MAS unit.

This thread about a 300E cranking but failing to start on rainy days got me thinking - both no-crank no-starts have been on days with higher humidities. Yesterday had a relative humidity peaking at 90%. I don't know the exact date of the last failure, so I don't have relative humidity stats. If this is related to the issue I'm seeing, I expect I'll see the car start without problem the next time I get to it, when the humidity will be lower.

The voltage regulator module (VRM) is often implicated in starting issues, as it causes the battery to run flat. Alternator output voltage was tested by a mechanic, and I've attempted jump starts to no avail, so I've ruled out the VRM.

It sounds like diagnosing no-crank consists of:

1. Try ignition with a key, note any sounds. I hear the click of a relay or solenoid when I turn the key, but no cranking. Not at all sure which relay of solenoid.

2. Try ignition without a key using some connector above the fuse box (apparently called the X27 test point). If that one works, the problem is with electronics prior to that point: ignition key cylinder and neutral safety switch sound like the two suspects if this works.

3. Get underneath the car and try bypassing the starter solenoid, by jumping the starter motor itself directly to the battery connection. If this works, the solenoid is shot. If this doesn't work, the starter motor is shot. Proceed to replacing as needed. Is there a good video of testing the starter while it's still mounted in the car? I found quite a few that happily said "well once you've got the starter out, here's how to test it" - but removing the starter appears nontrivial.



Two questions:
1. Am I missing anything on this checklist?
2. Does anyone have a picture showing where the X27 test point is?