PSA: Check Your Trunk Wire Harness

I knew this day would come.

Earlier this week, the trunk refused to open when I hit the interior trunk release or the button on the key. I could only open it manually by turning the key in the keyhole. The trunk interior lights didn't come on when I opened the trunk, and the trunk-open light in the instrument cluster was continually lit. It occasionally flickered over hard bumps.

I'd done this repair once already - several years ago, on my 1997 328i. (See signature for details.) When the trunk lid wires on that car frayed and started to cause short-circuits, the central locking system fuse kept blowing. My E46 didn't do anything that exciting, thankfully.

After some exploration today, I found that the ground wire for the trunk lock actuator was completely severed after years of bending and kinking. See the attached photo. All of the other wires in the bundle (16 total) are still intact, but they're bound to start fraying too. Since there doesn't seem to be an affordably priced E46 replacement harness available at BMW dealers, I'm going to splice all new wires into the part of the harness that passes from the body to the trunk lid. I already have a new rubber cover and splice connectors; I just need to purchase the wire. The wires in the trunk lid are all 18-gauge and smaller, and the wire I have on hand is just too big for 16 of them to pass through the rubber cover. For now, I've spliced a short length of new wire to replace the broken wire, and the trunk lock and trunk lights are working.

I will update this thread with observations and tips after I complete my repair. If you're having trouble with your license plate lights, reverse lights, or the trunk lock actuator, you might want to check for frayed wires.

Yow !!!! Homey don`t like no Crispy Critters !


As they say down in Miami:

"Dass focked opp, Mang !"

Originally Posted by Fast Bob

As they say down in Miami:

"Dass focked opp, Mang !"

Yes, it is! Actually, this damage is pretty mild compared to some of the pictures I've seen on other forums.

Does anyone have any thoughts on whether PTFE-insulated wire would fare better in this application than standard automotive PVC- or XLPE-insulated wire? As far as I can tell, the factory stuff is PVC. It seems that PTFE would be tougher and better than PVC at retaining its flexibility at low temperatures.

Originally Posted by G. P. Burdell

Yes, it is! Actually, this damage is pretty mild compared to some of the pictures I've seen on other forums.

Does anyone have any thoughts on whether PTFE-insulated wire would fare better in this application than standard automotive PVC- or XLPE-insulated wire? As far as I can tell, the factory stuff is PVC. It seems that PTFE would be tougher and better than PVC at retaining its flexibility at low temperatures.

GP,

I was a field service engineer for an air pollution equipment manufacturer many years ago, involved in inspection and repair supervision of equipment subjected to intense vibration. The short answer is yes, Teflon covered wire will offer better heat resistance, and enhanced slippage between wires to extend wear resistance. The key though is the amount of strands in the wire. The more wires, the higher the tolerable bend radius, and the wires will last much longer. Teflon will also tolerate much colder temps than PVC and thus resist insulation cracking much better. Once the insulation, which supports the wire, cracks, the conductor is not long for this world.

In E39, Bluebee has a great write up for trunk wire look.

Originally Posted by SJBimmer

The short answer is yes, Teflon covered wire will offer better heat resistance, and enhanced slippage between wires to extend wear resistance. The key though is the amount of strands in the wire. The more wires, the higher the tolerable bend radius, and the wires will last much longer. Teflon will also tolerate much colder temps than PVC and thus resist insulation cracking much better. Once the insulation, which supports the wire, cracks, the conductor is not long for this world.

Thanks for the reply. The PTFE insulation also appears to be thinner, which allows me to approximate the diameter of the existing wire bundle.

Will adhesive-lined heat shrink bond to the slick surface of PTFE insulation?

Originally Posted by dalekressin

In E39, Bluebee has a great write up for trunk wire look.

I found the thread; it's interesting. With the help of the wiring diagrams in my trusty Bentley manual, I've identified the wires in the harness. The splices I make will be inside the trunk lid and inside the trunk; there will be no splices in the bundle that passes from the body to the trunk lid. Splices in the bundle are likely to fail again or chafe the other wires in the bundle.

I'm contemplating ways to make it easier to inspect the wires in the future. I'll probably add a few extra inches of wire so that it will be easier to slide the rubber covering away from the portion of the bundle that bends every time I open and close the trunk. Maybe I'll put an inline connector inside the trunk so that I can disconnect the harness and pull the slack up and out of the body for inspection.

Originally Posted by G. P. Burdell

Thanks for the reply. The PTFE insulation also appears to be thinner, which allows me to approximate the diameter of the existing wire bundle.

Will adhesive-lined heat shrink bond to the slick surface of PTFE insulation?

I was gonna suggest using test lead cable, the kind we use in voltmeters and such. It is, indeed, Teflon insulated and is comprised of lots and lots of very thin and flexible wires, which is why test leads last a long time. Only trouble may be getting a variety of colors.

Nope, adhesive heat shrink will NOT stick to Teflon. So, you cut your heat shrink an inch longer or more so it stays in place mechanically and not adhesively. Plus, the longer overlap will make it harder for the moisture to seep in. I take it you will be soldering the new wires in, right? Be sure to use rosin core solder to avoid future corrosion.

You may also want to investigate using TapeUp! "rubber amalgamating tape" by Motormite. Made in England, I get it at AutoZone. You cut a piece a bit shorter than you need. Then peel off the vinyl protective backing and stretch the rubber tape 50% or more to activate and wind around whatever you want to protect with a 50% overlap. It will only stick to itself but will amalgamate (fuse) into a solid rubber cover in a day or two. Is incredibly flexible and longlasting.

Originally Posted by dalekressin

In E39, Bluebee has a great write up for trunk wire look.

- Trunklid open, check licplate light, and other trunk wiring loom woes (1) (2) (3)

Seems to happen mostly to blue bimmers!

Originally Posted by G. P. Burdell

I knew this day would come.

Earlier this week, the trunk refused to open when I hit the interior trunk release or the button on the key. I could only open it manually by turning the key in the keyhole. The trunk interior lights didn't come on when I opened the trunk, and the trunk-open light in the instrument cluster was continually lit. It occasionally flickered over hard bumps.

I'd done this repair once already - several years ago, on my 1997 328i. (See signature for details.) When the trunk lid wires on that car frayed and started to cause short-circuits, the central locking system fuse kept blowing. My E46 didn't do anything that exciting, thankfully.

After some exploration today, I found that the ground wire for the trunk lock actuator was completely severed after years of bending and kinking. See the attached photo. All of the other wires in the bundle (16 total) are still intact, but they're bound to start fraying too. Since there doesn't seem to be an affordably priced E46 replacement harness available at BMW dealers, I'm going to splice all new wires into the part of the harness that passes from the body to the trunk lid. I already have a new rubber cover and splice connectors; I just need to purchase the wire. The wires in the trunk lid are all 18-gauge and smaller, and the wire I have on hand is just too big for 16 of them to pass through the rubber cover. For now, I've spliced a short length of new wire to replace the broken wire, and the trunk lock and trunk lights are working.

I will update this thread with observations and tips after I complete my repair. If you're having trouble with your license plate lights, reverse lights, or the trunk lock actuator, you might want to check for frayed wires.

Good stuff to know!

GP

I agree with jcourcoul that it will not stick to teflon. Kind of makes my orig. recommendation a moot point. (facepalm) I do like jc's solution though with the rubber stretch tape. May be the only solution if you want to use the teflon coated wire. In any case, I don't think you will ever have to do this repair again. Good luck, and keep us updated on how it goes. I'm sure I'll be there soon too, as my Wife just loves to put the purse in the trunk on the vert.

Thanks for the tips, jcourcoul and SJBimmer. I've found a few varieties of stretch tape that I can get my hands on.

Originally Posted by G. P. Burdell

Thanks for the tips, jcourcoul and SJBimmer. I've found a few varieties of stretch tape that I can get my hands on.

Try to get one of the MILSPEC-approved variety....that`s top-of-the line stuff.

Originally Posted by Fast Bob

Try to get one of the MILSPEC-approved variety....that`s top-of-the line stuff.

I can't tell you what to use ... but I can tell you what NOT to use ... (ask me how I know) ...

And now I will bore everyone with talk of wire...

PTFE-Insulated Wire
In case anyone else with frayed trunk wiring is looking to use Teflon-insulated wire for this repair, I did some searching yesterday for ways to make waterproof splices that will stick to the surface of PTFE. It's possible to do so by chemically etching the PTFE surface first; after that's done, most conventional adhesives will work. Here's an example of an etching process, which seems pretty time-consuming.

Many sources also mention that the use of regular wire strippers on PTFE insulation is likely to nick the silver plating on the copper strands, but this page recommends an inexpensive Imperial brand stripper that you can get at McMaster-Carr.

XLPE-Insulated Wire
I mentioned XLPE (crosslinked polyethylene) insulation in an earlier post - it meets SAE standards for wiring in the engine bay (-51 deg. C to 125 deg. C). It's readily available from several sources, including KayJay Company and Pegasus Auto Racing.

If you're going to go with XLPE, you need to get the thinnest specified insulation (TXL) in order to make a bundle that passes through the rubber covering from the body to the trunk lid. The 18-gauge XLPE-insulated wire I have in the garage is apparently either GXL or SXL, the medium and heavy thicknesses. These wires make too big a bundle.

XLPVC-Insulated Wire
There's also irradiated PVC, sometimes abbreviated XLPVC. It's PVC that has been exposed to electron beam radiation, making it tougher, more heat-resistant, and more abrasion-resistant than regular PVC. This wire (-55 deg. C. to 80 deg. C) can't take engine bay heat like XLPE can, but the thickness of XLPVC insulation is comparable to that of PTFE insulation, which means I'll be able to keep the bundle diameter down. A local supplier has seven different colors of a mil-spec XLPVC wire available. It's about one-third the cost of Teflon wire, and it doesn't require chemical etching to get heat shrink adhesives to bond to it.

So it looks like I'll be going with the mil-spec XLPVC-insulated wire instead of Teflon. I have no doubt that Teflon would last longer in this application, but XLPVC would allow me to make waterproof splices much more easily and at lower cost.

GP

Looks like you really did your homework. Thanks for the definitive thread on wire loom repair.

Originally Posted by SJBimmer

GP

Looks like you really did your homework. Thanks for the definitive thread on wire loom repair.

Thanks for the compliment! I'm hoping that this thread will ultimately help others who want to do the job right the first time.

Originally Posted by G. P. Burdell

this thread will ultimately help others who want to do the job right the first time.

I agree.

Every thread should have the intention of pushing the ball of knowledge further and further toward the goal.

That's why we wrote this thread (which identifies EVERY wire and problem that can be caused if/when it breaks):
- E39 Electrical Problems Traced to Trunk Lid Harness Wire Chafing (DIY Diagnostic)

Do you plan on lengthening the loom and loom rubber snorkel itself?

Originally Posted by bluebee

Do you plan on lengthening the loom and loom rubber snorkel itself?

No. I have a brand new rubber covering waiting to be installed, and I would prefer not to compromise its water and dust resistance by cutting it. I don't see the benefit of lengthening it, as doing so would alter the way the harness bends - and perhaps introduce more opportunities for the wires to kink or chafe. There isn't much room for slack in the covering as I open and close the trunk.

Originally Posted by G. P. Burdell

I have a brand new rubber covering waiting to be installed

The current tribal knowledge assumption is that the wiring loom is a tad too short, hence the stretching pulls the insulation apart at a certain point, and eventually breaks the wires.

What did you end up doing?

Nice investigative work

Bumping up an old thread: http://realoem.com/bmw/showparts.do?...37&hg=61&fg=10 is #3 a replacement harness for this? Or is that something else?

Thankfully my wires are still good, but I'd really rather not go through the trouble of rewiring it myself when the time does come. Any suggestions as to what can be done to relieve the stress?

Originally Posted by TerraPhantm

Bumping up an old thread: http://realoem.com/bmw/showparts.do?...37&hg=61&fg=10 is #3 a replacement harness for this? Or is that something else?

I think that's the same part I asked our local dealer parts guy about, and he didn't think it was a replacement.

Originally Posted by TerraPhantm

Thankfully my wires are still good, but I'd really rather not go through the trouble of rewiring it myself when the time does come. Any suggestions as to what can be done to relieve the stress?

Short of never opening your trunk, I'm not sure that anything can be done to relieve the stress. The wires have to unfold and fold every time you open and close the trunk.

Since you can't relieve the stress, would it make sense just to wrap electrical tape over the wires to help reinforce them and help prevent rubbing?

Originally Posted by johnrando

Since you can't relieve the stress, would it make sense just to wrap electrical tape over the wires to help reinforce them and help prevent rubbing?

Rubbing doesn't seem to be the problem; it's the flexing that weakens the individual wires in the bundle.

Originally Posted by jcourcoul

That's why the maker uses fabric tape instead. Longer lasting and doesn't gum things up as bad as deteriorated vinyl tape.

The fabric tape makes quite a mess, too, when its adhesive breaks down. Taking the old harness out and marking the wires was a sticky PITA.

That's why the maker uses fabric tape instead. Longer lasting and doesn't gum things up as bad as deteriorated vinyl tape.

All old cars develop the 'bad trunk harness wiring' sindrome. No escape. In my old Fords, I've had lasting success by wrapping the harness in rubber self-amalgamating tape. After a day or two, it fuses into a seamless rubber 'tube' that closely encases and protects the wires. The UK-made MotorMite brand I find at AutoZone is reasonably thin and doesn't increase harness diameter too much. Don't know if a harness wrapped like that could be teased thru the trunk access holes and rubber hose in the Bimmers.