Do you mean pins 2 and 4 on the OVP? How does one go about checking them with the OVP relay in place?
Also, is it worth anything to check the pins in the plug at the back of the tach? Should the one that supplies voltage be hot at all times? I'm hesitant to touch anything behind the cluster with the battery connected, as I don't want to short anything else out. Is it OK to pull the harness off the tach with the engine running to check for a signal from the EDS (if it's even readable with a multimeter?).

On a side note, oddly, when I went to check things out in the car this morning, I found all the lights work normally now. I.e., the N40 unit that still didn't work yesterday, even though the circuit was apparently fixed, now works. I had assumed the short circuit damaged it. I'm extremely confused now, but I guess I shouldn't question it when something magically works.;)

Magic goes both ways
 

Ah, you caught me! Testing the OVP in place without special wiring harnesses is difficult. Get some insulated thin solid wire. Strip an inch or so of insulation from one end, bend it onto a loop just big enough to fit over one of the pins. Do that for pins #2 and #4 then plug the OVP relay back into its socket. Bring the wires out to where you can get at them and now you can measure voltage between each wire and battery ground. When finished, remove the wires and put OVP back.

Pins 2 and 4 should both have +12 Volts when the key is in "Run or Start," engine running or not. If no voltage, and assuming all of the factory wiring and the OVP fuse are both good, then the OVP relay coil has failed. Also, with the key OFF and in your pocket, your ohmmeter should get zero Ohms between pins #2 and #4 (because they are both the output of the OVP relay).

Safety note: OVP pin #1 is hot at all times. Be careful or disconnect the battery or both.

The tach's power lead (IIRC a "+" stamped into the metal case) is hot in "Run or Start." The signal lead gets a positive-going 10 Volt square wave that increases in frequency with engine speed. No telling what a voltmeter might think of it. Try setting the meter to 20 Volts range and see if you get a reading. Try both AC and DC and yes, it's safe to unplug the tach lead with the engine running so you can probe that lead. If you get a signal when the engine is running and it stops when you stop the engine then there is a chance that the EDS electronic box is working.

No telling why N40 now works. Maybe it has an internal circuit breaker that needs time to cool off and reset. Maybe it has a "smoke recovery" ability (electrical things work because they have smoke in them; when the smoke comes out they stop working). Maybe it was just irritated at you. Remember that things that start working by themselves can also stop by themselves; in the meantime "drive safely." .

Jeremy

I am a little worried that it will magically stop working, especially if it stops working in a way that causes yet another problem (I have a pretty powerful fear of things catching on fire in general). I'll be pretty carefully keeping an eye on everything once I get back to driving it. I suppose I could drive it without the tachometer now, but as I mentioned I don't really want to ignore it if it's a problem in the EDS, OVP or a short that could affect something else. I'll try the wire trick on the OVP and check for any kind of signal at the tach harness.

Awesome notes on the b pillar wires. MY left rear window is finniky as well.

B-pillar wiring
 

The broken wires in my B-pillar looked just like pictures in the thread. It's amazing how they can almost tie themselves in knots as they gradually break apart. I've always found it strange that the broken wires usually show up in the B-pillar although most door openings are at the driver's door. Possible explanations:
a) Different subcontractors for the front and rear parts of the passenger compartment;
b) Decision by M-B or sub to use coarser (less expensive) stranded wire for the rear doors since they don't get as much use;
c) what do you think?

Jeremy

Quick question re: the OVP: I noticed it rattles when I pull it out. I've seen other threads on here that suggest a rattling OVP is broken. Is that at all a reliable assumption? I have a hard time imagining it failed in total coincidence to the work I did behind the instrument cluster, though ... seems like the occam's razor is something in the cluster region. It's too late to go out and test more, but I'm just sitting here pondering it and was wondering if that rattle means a thing. 10A fuse looked fine but I tried a different one anyone, no change.

I can't really come up with a reason the rear door wiring would have more problems than the driver's door. Maybe people tend to open the rear doors all the way more (to put things in the car), whereas only open the driver's door partially, enough to get in and out? Maybe the wiring in the front shifts about more, and doesn't get as much wear on one single point? I was impressed how most of the wiring looked fine, other than that one very specific spot.

Rattles are not good. More specifically, a very slight rattle could be the relay contacts, which is OK. A louder rattle that you can hear and even feel, like something has come loose, is not good.

With the engine running I get 14v at the power lead and about 6 v DC on the the input lead ... it drops down slightly when I rev a little bit. I switched to AC and got a reading of 15.8, also dropped when I revved. I forgot to check as I shut down the engine. The fact that there's anything registering on that wire, and it changes with engine speed, leads me toward the theory of it being the ground in the cluster, rather than an input problem. Does that thinking make sense?

Tach problem
 

A bad ground is certainly possible. Notice in the following blow-up picture the path across the speedometer that the electrons must take to get from the tach (and the clock) back to the 3-gauge cluster, where the ground wire is located. If the ground strip does not properly contact the contacts on either side then the tach and clock won't work. BTW, does the clock work?

Your voltmeter measurements sound like there is indeed a signal coming from the EDS module, trying to get to the tach. As you rev the engine the frequency increases; your voltmeter is less sensitive at higher frequencies so it reads less, as you discovered.

Jeremy


http://www.peachparts.com/shopforum/...round_5159.jpg

Unfortunately the clock hasn't worked in a few years. Since I have one on my aftermarket stereo, I didn't bother to fix the clock. Is the ground strip connected to the rheostat? Maybe when the wires hit together while I was working around N40, it damaged the ground through the rheostat. I know there's nothing I can do about it now, but I wish I'd been smart enough not to pull the back of that plug off with the battery connected. It seems obvious now, but all I can do is try to fix whatever is damaged.:rolleyes:
So I have a gameplan before I take it all apart: If the ground strip is damaged, can it be replaced? Repaired? I'd really rather not replace the whole cluster or speedo/odo, because I want the original odo in there if possible.

You can easily swap in your original odo. Jeremy has at least one write up on it.

Cool, I will search for writeups. Guess I can fix that clock too while I've got everything apart.:)

Not saying this is the answer, but ....
 

For the light bulb problem on the tail light, and other electrical gremlins that might come along in the future, change all your fuses from the ones that pit and build up resistance, to the new copper ones.

Ground strip
 

I don't think a rheostat failure can damage the ground strip. Although the rheostat is installed close to one end of the ground strip, there is no electrical connection. Rheostat failures usually involve corrosion/wear/damage such that the rheostat stops conducting current so that the lights don't work.

I'll bet your ground strip is just dirty or a little corroded. Clean it with metal polish or very fine steel wool. If the ground strip is irreparably damaged (unlikely), you can use a short wire with a loop on each end to make a "jumper" that will be a good substitute for the ground strip. Notice in the photo that the two ends of the ground strip, the places that the red arrows (Red Arrows in the UK) point to, have holes in them. Two of the screws that hold the cluster together go through those holes; they also make sure the ground strip makes contact on both ends. No screws, poor contact. (Mercedes left that out of the owner's manual? Sorry.) The jumper wire can go anywhere on the metal back of the tach/clock but it has to go on the correct ground point (same place the strip ground goes) on the 3-gauge section. It may take some wiggling of the the jumper in between the two sections.

Fixing the clock takes a careful pair of hands. You have to remove the tachometer mechanism from the metal back plate in order to then remove the clock. Once the clock is out you need to unsolder and remove two 10 microfarad capacitors and replace them with new ones. I just did that job but didn't take any pictures, shame on me! The soldering is the easy part. I would be happy to guide you through the process and let me know if you need any parts.

Jeremy

Fuses
 

Yes, absolutely.