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04 E55 AMG: Third brake light out?

Posted: Sat May 08, 2021 4:58 pm
by kevm14
Picked Ian up on Friday. When we got home, he got his backpack out of the trunk and closed it. As soon as it closed and I pressed the brake, I got this.
You might think....oh it's a European car, it just goes through bulbs. Sure. I mean I haven't had a single bulb burnout in 11 months but sure. Then I remembered that the trunk brake light strip is LED. Which means it probably just didn't burn out but maybe?

Of course due to my forum experience in the back of my mind I said "I bet a wire just broke from the trunk harness." This is a common thing. By common I don't mean that it happens all the time on the same car but as far as I know it afflicts every single car if you open and close the trunk enough times. The harness flexes when the trunk opens and closes and eventually wires just break. Super common, lots of threads talking about the power trunk and whatever else. But I figured, well, before I play wires let me at least examine the light and maybe test it, or test the plug for power, or something like that.

Removing it takes many steps as you'd expect but nothing you couldn't do in 10 minutes if you had done it before. I didn't look up how to get to the bulb. I did the following things:
- Remove all plastic push rivets that hold the trunk lid liner on.
- Remove one rivet for each of the trunk hinge covers
- Remove the 5 or 6 torx screws holding the plastic cover on the bottom of the lid that covers the latch and has the power trunk buttons and stuff.
- Pop the plastic cover off and unplug power trunk buttons.
- Work the trunk liner out and unplug the light and emergency release button as access allows.
- Work trunk liner out of the hinge covers and it comes out.

Then to remove the third brake light, just squeeze the plastic tabs and the light releases from the outer lens. Fish the light out from the hole, unplug it and that's it.
First I tested the light. It worked. Of course.

Then I tested the harness plug. No power when brake lights are on. So that confirms it. I wanted to look in the left rear area of the trunk for other reasons and so this is a third reason to pull back the trunk cover and get in there.

To do that, remove the black plastic trunk piece that covers the bottom latch. There are like 6 push pins and it snaps out. Unplug the two lights. Then unscrew the luggage hold down things. Remove two more plastic push rivets. Remove the left trunk hinge cover. Two plastic tabs unsnap and it slides out.

Pull the left rear trunk liner out away from the side to access whatever.

Didn't take very long at all and I immediately saw a broken wire. However, this was not the culprit because the brake light started working even with it disconnected. Then I noticed everything else. MANY prior repairs, all covered with electrical tape.

I'll spare the extended process but basically I was able to pull a few of them right apart. Others I could not but when I slid the electrical tape back, it revealed a broken insulator but not a broken conductor. On those I just cut in the middle and soldered with heat shrink tubing. I must have done 10 wires by the end. Only a few left are seemingly not failed.

Here are some shots of that.

So I'm actually glad this happened. It's funny. I had a premonition that the power trunk or something would stop working because of broken wires on my vacation. Well it should last a while now. If it happens again I will have to extend the wires because they really get bent at a sharp angle when the trunk is closed. Dumb design. Should be more slack in the harness.

I did use good shrink tube, then I taped it, then split loom. So you can see that in the last couple of shots. Not too shabby. I am hoping that my premium shrunk tube with glue will act as a strain relief as it all bends. So I fixed the "bulb" and overall improved the reliability for free, with this low tech fix. Yeah it took some time and was a pain to solder bent over. But that's ok.

Re: 04 E55 AMG: Third brake light out?

Posted: Thu Jun 10, 2021 9:11 am
by kevm14
So I still cringe when I open and close the trunk because I can't unsee the stupid design that flexes the harness.

Then this conversation happened: ... ost8354202

I think the plan is to pull off the left trunk liner AGAIN and see if I can provide some relief to the harness by strategically removing some zip ties that make the thing too rigid and cause flex to happen severely, and in a single place. With some of the zipties removed, the harness is more free and won't have such severe and concentrated flex. Then it might actually last.

I'm thinking that I would remove one or two on the trunk support and also maybe one at the other end where it goes into the car side.

Re: 04 E55 AMG: Third brake light out?

Posted: Thu Jun 10, 2021 1:55 pm
by kevm14
I inspected it. Did not see any zip ties. What I ended up doing was removing a 2" section of split loom, from a longer piece that I installed after my previous repair. I guess the theory is the split loom was going to add a strain relief function but I think it was just too stiff and seemed to cause the harness to bend past 90 degrees in the same area. So with removing it the theory is the harness will flex more evenly and not concentrating in one part of it. At this point that's as good as I think I can do. Like I said originally, if something breaks again, I will have to consider adding some length to the harness to further decrease the bend radius, as the trunk closes. Fixed forever?