Trunk wiring issues

[CarterH]

Hey all, new to the forum but I've been browsing for a while. Since I bought the car there's been an issue with the wiring in the back where if I open the trunk, I get a check control message saying LIC PLATE LIGHT. I've noticed that it doesn't appear when closed but if I open the trunk while the car is running I get the message.

Now I'm not the most knowledgeable for electrical issues but I suspect it's some exposed wires touching since when I open the trunk beyond a certain point, the interior trunk lights (the ones attached to the trunk, not the one further towards the back) suddenly dim and then the license plate lights come on but they're also very dim. I suspect their wires are crossing and exchanging power which is causing the check control saying the license plate lights are out.

Are there any DIYs for fixing this? I went to two auto electricians and both quoted almost $400 for fixing it I've done some speaker wiring and a little electrical wiring in the past but obviously I'm no electrician. Should I leave it to the pros or is this able to be done by me?

[m735is]

First- take your keys out of the car before starting this job. I failed to and ended up touching wires together and locked mine in the car. Luckily I have the ski pass through and a fishing pole.

If you've done speaker wiring you should be fine. You'll have to remove the trunk hinge plastic cover and most likely splice in some good wiring to fix the worn/cracked wires. Just do one at a time and make sure they have just enough slack and yet don't get caught in anything when you open/close the trunk.

[Kingpin025]

This is a very common issue in both the sedan and wagon. We replaced 2-3' of the loom just to be safe in both of my E34s. It's amazing how far reaching the issues can end up being over these very small sections of wires. Phantom battery drain, intermittent speedo and tach failures and lighting failure alerts just to name a few. I've had those crazy busted up hinge wires cause all of those things.

[CarterH]

First- take your keys out of the car before starting this job. I failed to and ended up touching wires together and locked mine in the car. Luckily I have the ski pass through and a fishing pole.

If you've done speaker wiring you should be fine. You'll have to remove the trunk hinge plastic cover and most likely splice in some good wiring to fix the worn/cracked wires. Just do one at a time and make sure they have just enough slack and yet don't get caught in anything when you open/close the trunk.

I forgot one of those wires would be the locking mechanism. Is it safe to be doing all that with the battery connected though? My other wonders are the wire colors, sizes etc since I don't really know what's in there. Audio was easy for me since there are gauge measurements but I'm not sure electrical wires are the same.

This is a very common issue in both the sedan and wagon. We replaced 2-3' of the loom just to be safe in both of my E34s. It's amazing how far reaching the issues can end up being over these very small sections of wires. Phantom battery drain, intermittent speedo and tach failures and lighting failure alerts just to name a few. I've had those crazy busted up hinge wires cause all of those things.

That's pretty concerning then. My guess is the crossing wires are very close to the trunk hinge since the lights only malfunction once I lift the trunk beyond a certain point.

Anyone got some other important information and tips? If I go through with doing it myself I'll probably make a DIY for other members since I couldn't find one for the E34.

[Tre0Tre34]

Its really not too bad of a fix. The worst was finding a comfortable position to work on the wires since you must lay on your back inside the trunk.

I suppose i got lucky and after unwrapping the factory loom tape, found that my harness was broken all in one place. Someone had zip tied another wire to the harness causing the breakage over time.

Since the damage was localized to one area, I did not add any extra lengths of wire. Just like a car stereo harness, you must match up the colors of each wire and solder them back together. I slipped a length of heat-shrink tubing over one end of the broken wire before soldering back together. Then slid the Heat shrink over my repaired joint to be sure everything was properly insulated from its neighbors. Once i was done repairing the wires (bout 90% were damaged) i wrapped the whole bundle in electric tape since i didnt have any of the fancy fabric tape they use from the factory.

Performed this repair about 5 years ago and everything is still working properly.

Fyi, a butane powered soldering iron was very useful as space is kinda tight once you get in there.

[CarterH]

Just like a car stereo harness, you must match up the colors of each wire and solder them back together. I slipped a length of heat-shrink tubing over one end of the broken wire before soldering back together. Then slid the Heat shrink over my repaired joint to be sure everything was properly insulated from its neighbors. Once i was done repairing the wires (bout 90% were damaged) i wrapped the whole bundle in electric tape since i didnt have any of the fancy fabric tape they use from the factory.

I was thinking of doing that but the auto elec said he'd probably crimp the wires since soldering could come apart. I have to say though my first though was also soldering, which I might still do. I should disconnect the battery though correct? Considering that the lights turn on when the trunk is opened and they're what I'll be disconnecting and reconnecting. I don't want to blow any fuses.

I think I've found where it's breaking though, there seems to be a metal tab that the wires catch on towards the back which I guess is a tab to keep the wires out of the way of luggage. So I'll unwrap the partially unwrapped tape on that loom and have a look at it.

[fo3]

I was thinking of doing that but the auto elec said he'd probably crimp the wires since soldering could come apart. I have to say though my first though was also soldering, which I might still do. I should disconnect the battery though correct? Considering that the lights turn on when the trunk is opened and they're what I'll be disconnecting and reconnecting. I don't want to blow any fuses.

I think I've found where it's breaking though, there seems to be a metal tab that the wires catch on towards the back which I guess is a tab to keep the wires out of the way of luggage. So I'll unwrap the partially unwrapped tape on that loom and have a look at it.

Auto and aircraft electricians hate soldering because in theory vibrations could make the solder joint crack and then there would be a bad connection. They prefer crimps and plugs.

That being said, I used to install car audio with solder and never had a problem, it's more a problem in engine bays where there is more vibration and of course lots of thermal expansion and contraction.
Also a DIY crimp job is often horrible too unless you have proper $$$ crimpers and quality connectors

[CarterH]

I think if it starts to come to using really high quality material, then I might leave it to the auto elec. Since if costs get high then at least it would be good stuff in good hands. I haven't really had a problem with soldering either but these wires would probably be moving and bending a lot. I'll still take a look at the wiring in the morning and see how it looks though.

I also need to figure out what bulbs the interior trunk lid lights are since the manual doesn't seem to say except for the bigger one towards the back.

[Tre0Tre34]

Definitely disconnect the battery (i tend to just do the neg - lead) any time you are working on electrical systems.

I dont recall this metal tab of which you speak. Was there a nice plastic cover for the wiring as it runs along the trunk arm? Could probably locate one in the junkyard pretty quick.

Also, if crimps are used, they will probably be a bit bulky once properly wrapped and insulated, and will no longer fit into its cover.

I have had ZERO issues with my soldering fix to a 90% broken wiring harness. This was also without using any additional lengths of wire... And over 5 years ago

[CarterH]

Definitely disconnect the battery (i tend to just do the neg - lead) any time you are working on electrical systems.

I dont recall this metal tab of which you speak.

trunk loom.jpg

This tab. It's very small but my guess is that it's the point where the wires are crossing each other, but I'll pop off the plastic cover too. In the future I might get some soft adhesive foam and wrap it around that metal tab so there's less rubbing.

[Tre0Tre34]

I can see that causing problems. Maybe you are missing one of the plastic cable ties designed to hold the harness away from the tab? Weather's kinda shitty today, but ill see if i can get in the trunk for a look. I remember there being at least one or two mounted ties in the area to hold the harness.

Yep. Remove the plastic cover as well. You will probably want to unwrap several inches of the harness surrounding the damage in order to give you room and slack to do your soldering. Also gives you an opportunity to inspect for any other breakage

[Dash01]

I'm following this thread with interest, as my central locking is inoperative for the cabin doors, but the trunk lock still works. In other words, insert and turn the key in the driver's door and only the trunk makes a noise and locks, not the cabin doors. Repeat with door key and the trunk unlocks. I get various messages on the OBC such as failed license plate lights, even though the license plate lights do work. However, when opening the trunk, the license plate lights are dim until the trunk is mostly open, then the license plate lights become brighter. I crawled into the trunk and can see exposed (stripped or chafed) copper wire in the trunk hinge wire loom. Further, there is moisture around the relays/modules under the back seat. I reflowed the solder in the relay module and the control module next to it under the back seat, but got no change in locking.

So, it appears that the central locking switch in the driver door lock assy. works well enough to work the trunk lock, but no other lock. And the left side trunk hinge wiring loom is chafed.

I've read that there is a "wire splice" (whatever that means) under the back seat, and that moisture (leaking sunroof drain?) can cause problems with central locking. That said, I don't see a "wire splice" under the seat, so I'm stumped.

Thoughts?

[bluptgm3]

BMW sells a repair loom for the E36, I'm sure they do for the E34. Just a thought …


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[CarterH]

I can see that causing problems. Maybe you are missing one of the plastic cable ties designed to hold the harness away from the tab? Weather's kinda shitty today, but ill see if i can get in the trunk for a look. I remember there being at least one or two mounted ties in the area to hold the harness.

Thanks, I'd really appreciate that. If I'm missing a cover or something when I'll have a look in a wreck for a new one or buy a replacement part.

I've read that there is a "wire splice" (whatever that means) under the back seat, and that moisture (leaking sunroof drain?) can cause problems with central locking. That said, I don't see a "wire splice" under the seat, so I'm stumped.

Thoughts?

I'm not quite sure either but a good place to start is by doing a good check of the trunk loom to see if there are any wires that are cut or crossing (which it sounds like there are). I did also have a central locking issue a few weeks ago where I didn't quite close the trunk properly and when I touched the drivers side door, the car locked itself just as I pulled the handle. Then of course after I started the car I had the LIC PLATE message again which meant I didn't close the trunk properly. So I'd call it a good guess the trunk loom could be to blame for your issue.

[Kingpin025]

I've used the same stereo shop do all of my work for years and they've been excellent! They've done both hinge looms in the touring, the hinge loom in the E34 M5 and the parking sensors in the E39 M5 after a squirrel ate the wires. Of course all the stereo installs (there have been many) and believe it or not the touring double sunroof repair. In the cases where the issue causes me to be twisted up in a ball I'd rather pay the professionals to handle it.

[CarterH]

Success! I had the wiring fixed by the auto electrician 2 days ago and the lights finally work as they should. I could hardly believe the amount of light that comes from those trunk lights, it's like having headlights aiming down into the trunk. I got to have a good look at the old wires and they were pretty ruined and it appeared the PO had done a bodge job of just twisting the wires and wrapping them in tape so in the end all of the wiring along the hinge arm had to be replaced which took a few hours. I'm honestly thrilled with the repair.

As much as this job can be DIY, after looking it might just be better to leave it to a pro.