W124 electrical fun: Two problems - PeachParts Mercedes-Benz Forum

BodhiBenz1987 · #1 06-08-2012, 11:25 PM

Problem #1: I've been studying wiring diagrams and disassembling things left and right to try to figure this out, but am hoping maybe someone has had a similar issue who can direct me ... I have some, but not all, switch lights out in the climate control area (not the CC lighting itseld), as well as a couple other areas. They all starting occasionally going out together, started working again together in between, and have now not been working AT ALL for a week. The dead lights are: Shiftgate, window switches, rear dome switch, antenna switch, rear headrest drop switch, exterior light switch (I actually don't remember this lighting up before, so it may be independent), center and right vent switches. It's just the illimunination that doesn't work ... all the switches function fine. From what I can tell on the wiring diagram, they all have connection to the X6 two-post terminal ("58d" circuit). I have tried cleaning all the involved switches (didn't think that would help but worth a try, and I have pulled out the CC, stereo and shiftgate wood to study the wires. I didn't see anything out of place, loose, bent, shredded, melted, etc. I'm pretty sure I found the X6 connection and both screws and all the wires going to them look fine. I'm guessing it's probably one bad ground ... but I have no idea how to find it in that mass of ground wires. Any ideas on how to proceed? I can try replacing the shiftgate bulb, in case it was a massive coincidence ... but I'm pretty sure it's not bulbs (unless one out bulb would cause a circuit failure). This is both annoying and a bit problematic when driving at night. I know where the buttons are and where the shifter needs to go, but it's still confusing not being able to see all those things.

My second problem, which I half figured out: I have one brake light that is always on, albeit dimly, when the exterior lights are on. For a while, it was actually going out when I hit the brakes. I discovered when I run a ground wire directly from the chassi to the metal strip the brake bulb is on, the light goes out (as it should). I noticed the part of that strip that connects to the ground wiring tab is corroded and not touching the ground tab. My terminology might be off, so I attached a picture with the ground flaw in question circled. Should I try to sodder the connection? I hesitate to do that, because it wasn't soddered in the first place. My alternative would be replace the tail lamp backing, or at least that one strip. Soddering would be easier, but I don't want to create a worse problem if there's more to that connection than I know about. Sorry I don't have a closer picture. My camera is "in the shop" so I only have a cellphone camera.

BodhiBenz1987 · #2 06-09-2012, 04:17 PM

Update: Even though my wiring diagram does not include it, I figured out based on another part of the FSM that the power to the x6 terminal comes from the N40 relay, which is behind the instrument cluster (ugh ...). Once I searched that, it looks like Jeremy5848 had pretty much the same issue in this thread: Another W124 broken wiring problem. Only different is I don't have lighting to the heated window switch, which makes sense.
I'm going to try a spare N40 relay I had sitting in a box (score another win for random parts hoarding) and check to see if pin 30 is getting voltage. Anything else I should try while I have the cluster out? It's not my favorite job so I'd prefer not to have to do it more than once.

BodhiBenz1987 · #3 06-10-2012, 12:49 AM

Here is what I found: There is 12v at pin 1 (power supply) of N40 with the lights switched on. There is also 12v at pin 4 (to X6). There is nothing at pin 2, the 58D (to X6/1) circuit. I tried my spare relay and it did not work either. Out of curiosity I tested the spare relay by itself ... when I connected pins 1 and 3 to 12v and ground, I found 12v at pin 4, but not pin 2. Do I have TWO bad relays here or am I understanding the wiring wrong? There should be 12v coming out of pin 2 (58D) and pin 4 (58d) when 12v is applied to pin 1, correct? One thing I found odd was that even with the relay unplugged, the CC and most of the switches (all the ones that were not part of the problem) still lit up with the switch on. I guess they are also powered from something else.
I tried putting a wire directly from pin 1 to pin 2, which was clearly not a great idea because it immediately blew fuse 9. Before the fuse blew, I got a brief lightup from the shiftgate bulb though, so putting juice there does get it to the switches in question.
Should I try buying a new relay? This seems like an odd failure to have in not just the one relay but also in the spare, so I'm wondering if I'm just testing wrong.

BodhiBenz1987 · #4 06-13-2012, 03:35 PM

A brand new N40 control unit didn't help. 0V is still coming out of 58D. I pretty much give up for now ... whatever it is is way above my head to find. It doesn't make any sense that it would be the rheostat because the cluster and several other lights work (and dim/brighten with the switch). I did try supplying a known good ground to N40 and it didn't help. Since it's blowing fuses when I bypass N40, but not with N40 in place, my guess would be it's blowing something in N40. The fact that I don't understand electricity, at all, probably doesn't help.
Any ideas about the taillamp issue I mentioned in the first post? Should I try to sodder it? It's occurred to me it could be related. Though when I jumped the taillight ground it didn't help the other lights. I want to fix the taillamp either way.

jay_bob · #5 06-13-2012, 03:39 PM

If someone on the board has a link to the ETM for Bodhi's car I would be glad to take a look. This is the kind of thing I do at work all the time but I am lost without a diagram.

BodhiBenz1987 · #6 06-13-2012, 03:48 PM

Here is a link: http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/ETM/ETM.pdf. My car has a build date of Dec. 86. Section 130 has the pages covering this circuit. I've been trying really hard to learn to read these things, and I have a good feel for what belongs in what circuit and roughly where problems might be, but I have a hard time understanding it when I actually sit down in the car with a multimeter looking at all the wires. I really had high hopes the N40 block would do it because the lack of voltage coming out of it explains all the lights that are out. Unfortunately now I need to find out an explanation for the lack of voltage from that pin of N40, since it's not just a bad N40.

funola · #7 06-13-2012, 04:22 PM

Quote:

Originally Posted by BodhiBenz1987

Here is a link: http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/ETM/ETM.pdf. My car has a build date of Dec. 86. Section 130 has the pages covering this circuit. I've been trying really hard to learn to read these things, and I have a good feel for what belongs in what circuit and roughly where problems might be, but I have a hard time understanding it when I actually sit down in the car with a multimeter looking at all the wires. I really had high hopes the N40 block would do it because the lack of voltage coming out of it explains all the lights that are out. Unfortunately now I need to find out an explanation for the lack of voltage from that pin of N40, since it's not just a bad N40.

I took a brief look at the link. Unplug N40 and with power off (ignition key out), measure resistance from pin 2 of N40 socket to ground and make sure there is no short, i.e. it should not read zero ohms. If you do. that can explain the blown fuse. If you get zero ohms, find and fix the short before testing another N40.

tbomachines · #8 06-13-2012, 04:26 PM

From what I see, your issue is not with 58d, but 58D. 58d heads over into the dash, which you say is working...so that circuit is fine. If you have it all apart, you can start testing the circuits for continuity (before they hit the 58D switch). So stick one pin on a ground and then test all the grounds for the rest of the switches...then stick one pin on the output of each 58D and check for continuity...unfortunately a huge PITA but that will tell you whether you have any open circuits or shorts in the wiring. Unfortunately all of the wires coming out of that switch appear to be gray/blue? Maybe also check for continuity on the gray/red wire in between the N40 module and the 58D switch

BodhiBenz1987 · #9 06-13-2012, 05:48 PM

When I connect the ground of the multimeter to the console ground and stick the other pin in pin 2 of the N40 harness, I got 0.00 ... but I also get 0.00 on pretty much everything I stick the pin into to, including the lamps that work. I tried the instrument cluster ground instead and got the same. My multimeter is low on battery so I'll replace those just to make sure I'm getting accurate readings. I'm pretty sure there's at least one short though, given the fact that pin 1 to pin 2 blows fuse 9 immediately. I should have thought of that before trying the new N40 unit and wasted it.
If all the switches read zero ohms, how do I narrow down where the short is? I haven't found anything that doesn't read zero yet.

It doesn't help that X6/1 is pretty much inaccessible without taking the console out.

funola · #10 06-13-2012, 09:02 PM

Quote:

Originally Posted by BodhiBenz1987

When I connect the ground of the multimeter to the console ground and stick the other pin in pin 2 of the N40 harness, I got 0.00 ... but I also get 0.00 on pretty much everything I stick the pin into to, including the lamps that work. I tried the instrument cluster ground instead and got the same. My multimeter is low on battery so I'll replace those just to make sure I'm getting accurate readings. I'm pretty sure there's at least one short though, given the fact that pin 1 to pin 2 blows fuse 9 immediately. I should have thought of that before trying the new N40 unit and wasted it.
If all the switches read zero ohms, how do I narrow down where the short is? I haven't found anything that doesn't read zero yet.

It doesn't help that X6/1 is pretty much inaccessible without taking the console out.

You have to set your multimeter to the lowest resistance range, i.e 1 ohm if your meter has one. When in the lowest range, short the meter leads together and note the reading. If it does not read zero, that's ok, just write down the reading. Let's say it's 0.2 ohms when you short the meter leads together, that is your zero calibration. Now measure pin 2 of the N40 socket to ground. If you get 0.2 ohms, you have a short. Find and fix that short by unplugging the loads on pin 2 one by one while watching the meter reading till the 0.2 ohm reading goes away, then you found the short..

BodhiBenz1987 · #11 06-13-2012, 09:33 PM

Quote:

Originally Posted by funola

You have to set your multimeter to the lowest resistance range, i.e 1 ohm if your meter has one. When in the lowest range, short the meter leads together and note the reading. If it does not read zero, that's ok, just write down the reading. Let's say it's 0.2 ohms when you short the meter leads together, that is your zero calibration. Now measure pin 2 of the N40 socket to ground. If you get 0.2 ohms, you have a short. Find and fix that short by unplugging the loads on pin 2 one by one while watching the meter reading till the 0.2 ohm reading goes away, then you found the short..

I was using the 20 ohm setting, but I will check and see if there's a lower setting, though. I did not think to calibrate the meter.

I wish it were a little easier to access the X6/1 connector, because I think that would be a good (central) place to remove each load. Of course all the wires are the same color there but if I found one that made a difference, I could trace its path from there.

funola · #12 06-13-2012, 09:37 PM

Is it possible to remove the loads by unplugging the connector at each load? If you still have a short, then you have to get to the x6/1 junction to isolate the short.

BodhiBenz1987 · #13 06-13-2012, 09:51 PM

Quote:

Originally Posted by funola

Is it possible to remove the loads by unplugging the connector at each load? If you still have a short, then you have to get to the x6/1 junction to isolate the short.

I should be able to do that by just unplugging each switch and taking the light bulb out of the shifter holder. Though I was thinking, if the short is in the wire on the way to a particular switch, wouldn't there still be am extra load on it after taking that switch out (from whatever circuit is crossing into that switch's circuit)?

funola · #14 06-13-2012, 11:02 PM

Quote:

Originally Posted by BodhiBenz1987

I should be able to do that by just unplugging each switch and taking the light bulb out of the shifter holder. Though I was thinking, if the short is in the wire on the way to a particular switch, wouldn't there still be am extra load on it after taking that switch out (from whatever circuit is crossing into that switch's circuit)?

The short can be anywhere. disconnect the easiest things first, keep going till you find it per the multimeter reading.

BodhiBenz1987 · #15 06-13-2012, 11:23 PM

Thanks! Jeremy pointed out to me that he had a problem with the wiring to the rear door switch lighting once because of the bent wiring at the B pillar ... which kind of sets off a light bulb (no pun intended) in my head because I have just been fiddling with the rear door window regulator, and having problems with the power supply to it. The switch lighting wire runs through the same place in the B pillar as the regulator wiring, so I am going to start there for a possible short. That would make sense timing-wise, too, because the lights stopped working just about the same time I was working on that door. I'll move along from there if I can't find a problem. Hopefully I will at least learn a lot about wiring from this.