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?

[Frank Reiner]

The only apparent gap in your knowledge is that the OVP is not in the starter motor circuits.

So, fix the cranking problem first. It is a discrete problem.

[optimusprime]

JG check this out if you can go to GOOGLE search and put in the box put in there --

---- Mercedes Benz Model 124-m103 maintenance manual . -----
If its any good to you let me know .Every thing you need to know and much more..

[jgilik]

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?

[jgilik]

Found documentation on the X27 test connector's location through this benzworld.org post.

[optimusprime]

Dont know why i bother giving you a link to the location of a workshop manual .In it you will find all information you need .

[slk230red]

I went through the same thing on my '93 10E 2.3....intermittent no crank. Turned out to be a defective starter. Replaced the starter with a rebuilt Bosch and no problems since.

[jgilik]

@optimusprime: I'm well aware of the factory service manual; I've even provided links to its contents in a few spots. The issue isn't with knowing that it exists, the issue is with knowing what to look for within that manual and where to look for it. I have yet to find OCR'ed copies of the PDFs, so it's difficult to search through them automatically, and the indexes are lacking at best, often taking hours of poring through to find something even tangentially related to a topic when you're a newbie like me. Trust me, the factory service manual is the first place I look, but I often miss parts of it. For instance, the X27 test connector's location was squirreled away in the electrical troubleshooting manual instead of being colocated with all of the other articles related to the starter / ignition system. Often times, people with far more experience than me are going to be able to double check my assumptions, and when they point out I'm wrong on something (as Frank did), I learn.

@slk230red: Thanks for the feedback! I'm certainly starting to suspect the starter - I'm guessing that it will either be the neutral safety switch or the starter. Since I tried starting it while wiggling the shifter around neutral, I'm almost expecting a starter issue.

My mechanic told me that you can sometimes get a starter that's getting ready to die on you to 'wake up' by pounding on it with a dowel. I'll plan on doing that if the jumping the X27 point fails. Either way, if the X27 point fails later today: sounds like I'm replacing the starter.

[slk230red]

Originally Posted by jgilik

@slk230red: Thanks for the feedback! I'm certainly starting to suspect the starter - I'm guessing that it will either be the neutral safety switch or the starter. Since I tried starting it while wiggling the shifter around neutral, I'm almost expecting a starter issue.

My mechanic told me that you can sometimes get a starter that's getting ready to die on you to 'wake up' by pounding on it with a dowel. I'll plan on doing that if the jumping the X27 point fails. Either way, if the X27 point fails later today: sounds like I'm replacing the starter.

I've had the neutral safety switch problem before on my 240D. But the experience on the 190E was different, it was intermittent....sometimes it would start and sometimes it wouldn't. I even took the starter to 4 different parts stores to test it and sometimes it passed and sometimes it failed.

I said heck with it and bought a new one.....no problems since.

Good luck,

Dave

[jgilik]

Quick update:

I attempted jumping +12V to the center terminal of the X27 jump point: I used a jumper cable clamped onto a small Phillips screwdriver I didn't care much about. The test point is actually a connector. You disconnect the Molex connector, and then have a choice of where to connect +12V to. I touched the screwdriver to the male portion of the connector - the part closer to the engine, physically attached to the wall, remaining stationary after you disconnect the connector. Got a nice spark, heard the starter solenoid click, but did not get cranking.

Got it towed, got the starter replaced, and it runs great now.