MediumKahuna

Well, I'm making steady (but slow) progress getting my TE shaped up. I got wheels (AMGs), and I tried the cheap clear corner lights with my stock US headlights. They look horrible (sorry Rooney) so maybe I'll buy Bosch corners and get full euros as soon as I can afford it.

Anyway the big news is that I finally fixed a bad short that came with the car. The symptom was that Fuse C would blow the instant you replaced it (so fast it would burn your finger). The Fuse C circuit powers the Lock Pump, Radio Amps, Antenna, Trunk/Tailgate Close Assist, 87 Courtesy Lights, some safety stuff and god knows what else. All good candidates for a short.

The wires that ultimately connect to Fuse C go all over the car, every door (twice), tailgate, under all four seats, behind the headliner - everywhere.

I'm an electrical engineer and if I ever met the guy (probably a teenager) that designed the wiring in my car, I would have to kill him in cold blood for the honor of all electrical engineers.

In my extensive research I found that the most common cause of this problem are the Exit Lights (bottom of doors) and the Power Antenna so I checked those first - nope. I proceeded to tear into the interior - nope.

The schematic (wiring diagram) shows that the circuit goes right from the fuse to a terminal block (X30) behind the drivers left kick panel and then splits 7 or 8 ways on its paths to all the far extremities of the interior. So I pinned my hopes on getting at X30 and narrowing the problem by a factor of 8 there. Having already torn up a good deal of the interior (and messed it up) I was afraid to yank the panel without instructions but all I could get was a few hints.

The "Driver's left kickpanel" is on the left side of the drivers foot area behind (left of) the Parking Break Pedal. Its the plastic piece you rest your left foot on in my car.
It covers the left side of the drivers foot well.
I finally got mine off, found the terminal, found my wiring problem and fixed it (well, disconnected it).
It was my trunk (well tailgate) closing assist motor.

To Remove the panel:
Remove driver's door sill cover.
Pull out that stuff that goes around the door starting at forward end of the sill.
Remove the screw in that area.
Remove black plastic retainers under carpet behind brake pedal.
Remove drivers left kick panel by sliding it back toward rear of car (it has a built-in clip that only unhooks that way).

Once I got that off I found the terminal block. Here's a picture of it:


Here's a close up of the of the terminal block with one plug (the bad circuit in my case) pulled:


Pull the plugs in the terminal block, one at a time, til your short goes away.
I hooked up a spare headlight to the battery + and the load side of the fuse, so the short made it light up. When I pulled the right plug, the headlight went out. My plug had 2 wires going to the pin of the bad circuit so I unsoldered them and figured out which wire went to the short. Later, after I disconnected the closing assist, I had to get the two wires back in the pin which was a pain.

Here's a picture of the kickpanel and the cover for the terminal block:


After I narrowed it down to one wire coming from X30 I noticed it was not a dead short. It had a few ohms of resistance which is more than the wire. If you apply a current to a circuit, the thing with the most resistance will get the hottest so I connected a battery cable to the plus on my battery and brought the other end around near my wire (that was disconnected from X30 at the time). I touched the wire to the battery cable, on and off, until it got hot. I was hoping that the thing that was shorted would get hotter and start to smoke, revealing its location. Instead the short went away. It turned out that my closing assist motor was only half baked, (now its extra crispy). After it cooled off the short came back partially and the motor would click (has a relay) when I touched the wire to +12. And that's how I found it.

bicylindrico

Good job! I always enjoyed problems like this in the shop and took pleasure in finding the problem while smoothly working through the process of elimination. Very satsifying. Congradulations on saving yourself a bunch of money!

shadowgriffen

agreed,

I'm currently trying to find the cause for my left rear, front pass side window fuse as it keeps blowing, but only when I try to lower the rear window...I have gotten as far as noticing it's something to do with the rear wiper motor on the tailgate....but I'm REALLY annoyind at all the work



I applaud your patience and hard work!

MediumKahuna

What year is your car? They changed the wiring a lot between 89 and 90.
Actually they didn't change it so much as they just glommed new stuff onto the old stuff, but the window motors have electronic switches and the wires from the window buttons go into a computer under the left rear seat from 90 on. Anyway pre 90 is easier to diagnose.

What precisely are the symptoms. I'm startng to feel like somewhat of an expert on interior wiring, maybe I can get you started in the right place.

There's another terminal block (called x8) above the extreme left rear corner of the headliner which has most of the circuits going to the tailgate and many which have no business being in that area of the car.

Here's a picture of X8 looking up while sitting in the 3rd (way back) seat. The disconnected wire with blue shrink on it is the one I disconnected to disconnect my tailgate closing assist motor (which is in the talegate):



In the above picture, the front of the car is toward the left and if the picture showed just a little more to the right, you'd see the tailgate.

shadowgriffen

wow that's really interesting, how would I know which wire to pull? By using the good ol' Le' old headlight trick?

Also, is the underneath what I like to call the "mini" headliner in the rear tailgate section of the TE? you know, the one that almost looks like a kickpanel on the roof of the tailgate itself?

It's an 88' by the way.

PS: this started happening about 2 weeks ago, and I can tell it's wiper motor related because I can hear the wiper motor clicking but it wont budge, this of course is part of the fuse which controls left-rear-front right windows

myfirstbenz

MK... Great write up. So have you lost the function of the closing motor? I love it, even though it makes some noise. In your studies of the system, you wouldn't happen to know where the switch is that turns on the interior lights when you open the trunk? I have looked everywhere, even tearing the rear of the car apart, and can't find it My lights don't comeon when I open the trunk, and I know the switch on the dash is fine.

BTW, your car looks great with those wheels and clear corners. The green really looks 10x better withouth the orange next to it.

MediumKahuna

Think of the headlight trick like a fuse that doesn't blow (if the fuse would have blown the headlight just comes on instead). Its a way to power up the circuit without wasting fuses.

Yes, I removed the "mini headliner" to take the picture in my post above.
When you remove the mini headliner make sure you pull it toward the rear of the car as it has a couple of clips that release that way (learned the hard way).


Use the headlight trick to get all the information you can about the problem.
Here's a picture of the trick in action on the fuse C circuit:



The headlight (must be a working headlight) acts like a 3-4 amp fuse but, instead of blowing, it lights up.
The circuit I was working on is powered all the time. So I just hooked one side to the battery. Fortunately Mercedes made the load side of the fuse holder the part that sticks up so you can clip onto it.

I got a feeling the window circuit (your problem) is only powered when the ignition is on. So either make sure the ignition is on or put a blown fuse back in and try to make the clips connect to either end of it so the headlight is connected across the fuse.

Anyway, after your headlight is hooked up:
Unhook one of the clips and then touch it on and off - find the location of any clicks you hear.
Then clip it on firmly and try all your buttons and what not. Make note of when the headlight comes on and how bright and if something happens in the car.
Get the complete action/reaction list and post it.
We'll go from there.

Also, do you have the wiring diagram? I can post the pertinent parts if not.

MediumKahuna

My closing motor is extra crispy at this point and disconnected so, yes, I lost its function. I haven't even taken the tailgate apart to look at it but I let the smoke out of its electronics so its history. I have a feeling that it may have just been stalled and I could have fixed it but all electronics have some smoke built inside them at the factory and, once you let it out, its done.

I have a TE so tailgate not trunk. I don't see any little buttons sticking out so there is no switch around the edge of the tailgate. But, by the way it behaves, I'm sure the switch is part of the lock/latch mechcanism for a tailgate and probably the same for a trunk.


BTW, myfirstbenz, you were the only guy to give me any good info on how to remove the drivers left kickpanel. Sorry I didn't give you proper credit above but
Thanks

shadowgriffen

yes, you are correct about the ignition "on".

I will see what I can come up with this weekend, thanks for your help.

myfirstbenz

We are all here to help, glad I did my part. I have looked around my tailgate everywhere and haven't found any switch, even by the struts, and it has led me to believe that it might be some kind of mercury switch that senses the tailgate is moving... hmmm.

loubapache

I just fixed a similar fuse blowing (#9) and MediumKahuna helped me by providing the wire diagram. The thread is at

https://mbworld.org/forums/e-class-w124/197059-1991-300e-fuse-9-kept-blowing.html

I did not use the headlight trick. Instead I used my multimeter (Ohm meter) to check the short.

If there is a short, the resistance between the top of fuse connector and car ground (battery negative terminal) will be either zero or very low. In my case it was around 1 Ohm. I used the shock tower as the car ground.

Once I found the short, the resistance went up to 200 Ohms. In my case, I did not have to disconnect many items because I hit the short on the very first try (was suspecting that was the case).

MediumKahuna

I doubt it. Any mercury switch will flash the lights with sever shaking.

My lights come on the instant the latch disengages while the door is still fully closed. I'm 99.9 percent sure the switch is part of the latch.

I don't want to wish bad luck on anyone but if somebody has to take their tailgate apart, please, tell us what's in there.

myfirstbenz

I'll do it, it isn't that bad, but don't you have the wiring diagram?

MediumKahuna

The wiring diagram for your car (below) shows the switch but not how the tailgate actuates it. The circuit for my car is almost the same.
I disconnected my closing motor at X8 (under mini headliner) but my tailgate switch still functions (works lights) even tho the circuit goes thru the closing motor.

As I said I strongly suspect that the "TAILGATE SWITCH" is part of the lock/latch. But the lock has other wires going to it as well for the central locking circuit. So check the wire colors if you get a look at it.

jinsoojoo

I know this thread is old af, but I'm hoping to get some assistance with the same problem....fuse C keeps blowing. I followed the instructions given by mediumkahuna, except instead of a headlight, I tried using a test light. The bulb continued to stay lit even thought I removed, one-by-one, the 7 plugs from X-30. So, instead what I did was insert a new fuse in each time I removed a plug one-by-one. Sure enough, when I removed the 6th plug down (second to last) and then inserted a new fuse, the fuse stayed intact and did not blow. So, I assume there is a short for whatever is related to the sixth plug down , which I think is the door lights (bottom of the door panels) since they would not illuminate. However, what's strange is that the rear lights on the b-pillars and c-pillars won't turn off completely now. They are dim when they are supposed to be off and when I switch them on with the dashboard switch, they go brighter. Is that why the test light would not turn off when removing the sixth plug? Should I assume that I have two shorts....one in the courtesy lights in the door panels and one in the courtesy lights on the c-pillars and b-pillars? What's also strange is that my front dome light will not shut off when I shut the doors. It will shut off when I switch the light completely off, but it stays lit in on-mode and door-open mode. Could this all be related? Thanks in advance for the help and apologies for my ignorance when it comes to automotive electrics!