No-crank no-start in 1990 300E 2.6L gasser; OVP suspected

[jgilik]

I have a 1990 Mercedes 300E - gasoline powered, with the M103 2.6L engine.

I had a no-crank no-start ~3 weeks ago. I let the car sit for a day before jump starting it with a booster pack, and it worked fine since then. I took it to a mechanic, he checked out the alternator, and we decided I probably left the trunk slightly cracked or did something similarly stupid to cause a light to drain the battery. This was likely an incorrect guess.

I had a no-crank no-start yesterday. I had my booster pack with me and jumper cables. Attempted jump-starting the car from both the booster pack and from a friend's massive truck: if the battery in that truck can start its massive truck engine, it can start a dinky I6 2.6L. Still no-crank.

Per my understanding, no-crank-no-start means I'm sweating my starter and related support circuitry: a couple of relays, a solenoid, ignition switch, neutral position switch, crankshaft position sensor, and the actual physical wiring leading to the starter.

Some additional background: I've been experiencing difficult cold starts since I got this car. These are indicative of a problem, but are a remarkably poor start for diagnosis: they can be indicative of anything from a problem with your engine control unit (computer) to dirty fuel injectors, with something like 20 other possible root causes in between. It's baffled more than a few mechanics.

I understand the overvoltage protection (OVP) relay used in this make/model/year has a failure mode where bad solder joints start cracking and electrical characteristics become incorrect - causing it to erroneously trip at times (which would cause no-crank no-start), and causing the car to be difficult to start at other times. I suspect this is what is wrong. I inspected the OVP yesterday prior to leaving my car, and it seems old as hell (gaskets fell apart when I opened it).

There are a few other possible causes, but none that are common to the make/model/year in question and only one that also explains hard cold starts (which I've been experiencing and trying to resolve fruitlessly until yesterday): if I have starter who is deciding to avoid turning, I'll rent a car, as replacing it will take time. This is unlikely and diagnosis is easy: you hit it with a stick to vibrate it out of the position it's stuck in; if the car starts, it's definitely time to replace the starter. The problem? I did not have a sufficient stick last night. Said stick needs to be sturdy and ~4 feet long. I will bring a sufficient stick today.

I understand the electro-hydraulic actuator (EHA) can cause no-spark no-starts and poor cold start behavior due to an unbalanced fuel mix, but the fact that I had a no-crank no-start made me rule it out. It was actually my primary suspect on the poor cold starts, as it's an analog part that periodically needs recalibration. I never got around to ordering the specialty tool to recalibrate it, so it may actually be the cause of the poor cold starts and cause the OVP to be a red herring as a result.

So, two questions related to the no-start:
1. Other than the OVP, starter solenoid, and starter motor - are there other super-common causes of no-crank no-starts in W124s, especially ones from 1990-1992 with the M103 engine?
2. Is there a way to easily field-reset the OVP? My understanding is that it's a relay that trips when ~18V is exceeded, causing it to act as a slow-acting voltage-based circuit-breaker. Is there a way to cause it to switch back to "eh, don't worry about it" mode to attempt starting again? I understand this is a bad idea if the OVP is not the root cause of the issue, but it sounds rather likely it is, and I would really like to avoid towing the car back home. My guess is that simply depriving the car of all electricity might do, so I'm going to try disconnecting the battery entirely when I visit the car tomorrow: any other ideas?