W124 electrical fun: Two problems

[BodhiBenz1987]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.

[jay_bob]

Anyway I finally got a chance to download the diagram and take a look.

I'm fairly new to Mercedes and all my time so far has been on 123s but I think I see what is going on here. The 123 electrical manual spans about a dozen pages and it is crazy simple compared to a 124. I hate to think what's going on in my wife's 2012 Honda Pilot

Issue #1, no dash lights
I think the N40 relay is like a 'rheostat repeater'. It takes a 12 V reference signal on pin 4 (58d) from the dash rheostat. The items on 58d (mostly on the left side of page 130/3) are directly off the rheostat.

The reference signal on pin 4 has 12 V on it when you have your lights on because you are getting good voltage there, and you said your main cluster lights were working.

Pin 2 (58D) is the output. N40 takes power from pin 1 and electronically regenerates a signal on pin 2 that is a copy of what it is seeing on pin 4. I suspect they did this because there are too many bulbs for the dash rheostat to handle by itself (unlike in the 123, everything is off the rheostat, but I have about 10% of the bulbs you do). They wanted to keep the critical bulbs direct off the rheostat (instruments), but the less important items got relegated to N40.

When you tried to tie Pin 1 (12 V from fuse 9) to Pin 2 (the remainder of the lighting circuit that is not working) you blew fuse 9. That leads me to believe, as you do, that you have a short in your lighting circuit.

Unfortunately it is extremely difficult to use a multimeter to find a short in a circuit with incandescent bulbs. The cold resistance of an incandescent filament is nearly zero; as the filament heats up, the resistance increases to anywhere from several ohms (brake lights) to several hundred ohms (dash lights), inversely proportional to the wattage of the bulb.

Here is a trick to try:
Go to the parts store (or look around in your parts stash) for an automotive bulb socket with wire leads and a 12 V bulb. You want a bulb that is normally used for brake or turn signals. Wire the bulb socket across pin 1 and pin 2 of your N40 socket. This puts the bulb in series with the remainder of the lighting circuit. If you have a downstream short, the test bulb will glow brightly. The advantage is that the bulb limits the current, so you won't blow the F9 fuse while you track down the short, and it gives you a visual indication of what is going on.

Unfortunately you are going to have to bite the bullet and pull your console so you can lift wires at X6/1 one by one to find the shorted one. When you remove the shorted wire, the test bulb will go dim or all the way dark.

That damaged wire pack in your B pillar is certainly suspicious. There is a wire from the output of N40 in that wire pack that goes to the window control switch in the left rear door. (Was this the car that got hit in the left side?)

Once you get the short fixed, you can temporarily run without N40 by jumping pins 1 and 2, the lights will just be at full brightness. If you smoked your N40, try cracking the case, strip out the guts, and solder a wire between pins 1 and 2 for a good temporary fix. The fuse is 8 Amps, so #16 wire is plenty sufficient.

Issue #2, weird tail lights
That is a classic lost ground situation. Fix the bad ground connection and you will be all set.
What is happening is that your left side brake and tail lights share a common ground point. Normally the power for the left side tail lights flows like this:
fuse -> head light switch -> tail light -> ground.

And your brake lights go: fuse -> brake pedal switch -> brake light -> ground.

In both these circuits, the ground is the metal frame of the tail light assembly. There is a wire that brings that frame back to the actual ground on the body. I think you are on the right track that the wire is loose and probably needs to be reattached.

When you lose the left side ground, the power tries to get to ground any way it can. Now the power goes:
fuse -> light switch -> left tail light -> shared ground that isn't really grounded -> left brake light bulb -> brake light circuit (which is at 0 V since you aren't on the brake) -> right brake light bulb -> right side ground.

Your bulbs glow dimly because you have effectively put 3 bulbs in series. The brake bulbs are higher wattage so they glow brighter than the tail bulbs.

When you step on the brake in this condition, you bring the brake light circuit to 12 V. This causes the voltage across the tail light bulb to go to zero, because both sides of the tail light bulb now have 12 V on them. This makes the tail light go out when you step on the brake while the brake lights illuminate normally.

[ke6dcj]

Originally Posted by jay_bob

Anyway I finally got a chance to download the diagram and take a look.

I'm fairly new to Mercedes and all my time so far has been on 123s but I think I see what is going on here. The 123 electrical manual spans about a dozen pages and it is crazy simple compared to a 124. I hate to think what's going on in my wife's 2012 Honda Pilot

Issue #1, no dash lights
I think the N40 relay is like a 'rheostat repeater'. It takes a 12 V reference signal on pin 4 (58d) from the dash rheostat. The items on 58d (mostly on the left side of page 130/3) are directly off the rheostat.

The reference signal on pin 4 has 12 V on it when you have your lights on because you are getting good voltage there, and you said your main cluster lights were working.

Pin 2 (58D) is the output. N40 takes power from pin 1 and electronically regenerates a signal on pin 2 that is a copy of what it is seeing on pin 4. I suspect they did this because there are too many bulbs for the dash rheostat to handle by itself (unlike in the 123, everything is off the rheostat, but I have about 10% of the bulbs you do). They wanted to keep the critical bulbs direct off the rheostat (instruments), but the less important items got relegated to N40.

When you tried to tie Pin 1 (12 V from fuse 9) to Pin 2 (the remainder of the lighting circuit that is not working) you blew fuse 9. That leads me to believe, as you do, that you have a short in your lighting circuit.

Unfortunately it is extremely difficult to use a multimeter to find a short in a circuit with incandescent bulbs. The cold resistance of an incandescent filament is nearly zero; as the filament heats up, the resistance increases to anywhere from several ohms (brake lights) to several hundred ohms (dash lights), inversely proportional to the wattage of the bulb.

Here is a trick to try:
Go to the parts store (or look around in your parts stash) for an automotive bulb socket with wire leads and a 12 V bulb. You want a bulb that is normally used for brake or turn signals. Wire the bulb socket across pin 1 and pin 2 of your N40 socket. This puts the bulb in series with the remainder of the lighting circuit. If you have a downstream short, the test bulb will glow brightly. The advantage is that the bulb limits the current, so you won't blow the F9 fuse while you track down the short, and it gives you a visual indication of what is going on.

Unfortunately you are going to have to bite the bullet and pull your console so you can lift wires at X6/1 one by one to find the shorted one. When you remove the shorted wire, the test bulb will go dim or all the way dark.

That damaged wire pack in your B pillar is certainly suspicious. There is a wire from the output of N40 in that wire pack that goes to the window control switch in the left rear door. (Was this the car that got hit in the left side?)

Once you get the short fixed, you can temporarily run without N40 by jumping pins 1 and 2, the lights will just be at full brightness. If you smoked your N40, try cracking the case, strip out the guts, and solder a wire between pins 1 and 2 for a good temporary fix. The fuse is 8 Amps, so #16 wire is plenty sufficient.

Issue #2, weird tail lights
That is a classic lost ground situation. Fix the bad ground connection and you will be all set.
What is happening is that your left side brake and tail lights share a common ground point. Normally the power for the left side tail lights flows like this:
fuse -> head light switch -> tail light -> ground.

And your brake lights go: fuse -> brake pedal switch -> brake light -> ground.

In both these circuits, the ground is the metal frame of the tail light assembly. There is a wire that brings that frame back to the actual ground on the body. I think you are on the right track that the wire is loose and probably needs to be reattached.

When you lose the left side ground, the power tries to get to ground any way it can. Now the power goes:
fuse -> light switch -> left tail light -> shared ground that isn't really grounded -> left brake light bulb -> brake light circuit (which is at 0 V since you aren't on the brake) -> right brake light bulb -> right side ground.

Your bulbs glow dimly because you have effectively put 3 bulbs in series. The brake bulbs are higher wattage so they glow brighter than the tail bulbs.

When you step on the brake in this condition, you bring the brake light circuit to 12 V. This causes the voltage across the tail light bulb to go to zero, because both sides of the tail light bulb now have 12 V on them. This makes the tail light go out when you step on the brake while the brake lights illuminate normally.

I have an issue with the lamps that are controlled by N40 staying ON, when the ignition switch is OFF.

Once the car is started ('93 W124 500E) or the ignition switch in position 2 (right before starter cranking), the N40-controlled lamps go out.

I checked for a short (0-ohms) on pin#2 to ground of N40, and no shorts.
I also verified that pin#3 should be to ground, and it is.

I also removed the instrument cluster so that cluster rhetostat is removed from the circuit, and the N40-controlled lamps still stay on.

Is the failure status of N40 to always be ON (connect pin#1 and pin#2).

TIA,
:-) neil

[funola]

Good advice from jay_bob. Listen to him! The light bulb should be easier to find a short but a DVM will work if you have a discerning eye. The switch illumination bulbs are around 19 ohms ea and when you have a few in parallel, can drop to a few ohms. One of the loads is a heated window, which is the highest load of the bunch and the lowest resistance. Did you try shutting the heated window switch off and see if the resistance changed?