Originally Posted by G. P. Burdell
I finally found the time to rebuild my car's trunk wire harness. I say that I rebuilt
the harness because, once I was done, there wasn't much left of the old harness. Here's my writeup for anyone wanting to do the same.
This work took me an entire day from start to finish. I will readily admit that I'm not the fastest DIYer; I frequently stop and take the time to document the process with written notes and plenty of photographs for future reference. On this task, I checked and re-checked my work several times to make sure I spliced the right wires to each other. These checks involved numbering the wires and testing for continuity, both in the car and on the bench.
The temporary repair I made about a month ago held just fine, but I had to make an inline splice right in the area of the harness where the original ground wire had broken. The splice would eventually have broken, or it would have chafed against the other wires in the harness (one of which was looking rather kinked already) and caused more damage. Furthermore, in order to find the broken wire, I had cut into the existing corrugated EPDM rubber cover that protects the harness and seals out water. I wasn't able to locate a complete replacement harness in the BMW parts catalog, so I decided to replace the entire wire bundle that passes from the body to the trunk lid.
I AM NOT A BMW TECHNICIAN.
I am sharing my experience based on the repair I made to my E46 sedan. Depending on the year and configuration of your vehicle, your car's wiring may vary. I will not be responsible for any damage or injury that results from following anything I have posted in this thread. Verify part numbers before you purchase your own materials. It is your responsibility to also verify technical information with a trusted source, such as BMW TIS, your copy of the Bentley Publishers repair manual, or a reputable mechanic. If you are not comfortable with doing this work on your own car, I recommend that you seek the assistance of an experienced friend or your mechanic. Use appropriate safety measures and have proper ventilation when working with heat shrink materials inside the trunk. As always, you are welcome to do things differently and discuss your method here.
Most of these products, except for the ones with BMW part numbers, are available from vendors such as McMaster-Carr.
- Wire. I used irradiated PVC (XLPVC) insulated wire in 18- and 20-gauge sizes.
- 18-to-22-gauge splices. I chose butt splices that are insulated with adhesive-lined heat shrink for the best moisture resistance. Others may choose to solder and heat shrink.
- Zip ties
- Wire markers. I have a book of self-adhesive, preprinted Tyvek wire markers that I used to number the wires sequentially, from 1 to 16.
- Electrical tape. Not for wrapping the entire harness, but used in small quantities for bundling wires together at certain points.
- Fabric tape. A 15-meter (45-foot) roll is BMW P/N 61 13 6 908 716. Dealer list is $17.72. The factory harness, along with most other interior wire harnesses in the car, is wrapped in a fabric tape that's similar to this tape.
- Rubber covering. The replacement for the cover for a sedan is BMW P/N 61 13 8 366 627. Dealer list price as of the date of this posting is $6.38.
- Painter's tape
- Common sense
- Bentley manual
- Safety glasses
- Disposable gloves
- Wire cutter & stripper tool
- Crimping tools
- Ratchet and 10mm socket
- Plastic pry tool
- Phillips head screwdriver
- Heat gun
The basic steps involved were:
- Disconnect battery. Disassemble trunk lid and right side of trunk.
- Identify and number wires and connectors.
- Pull old harness out of trunk lid and into trunk.
- Test for continuity to confirm wire assignments.
- Cut old wire harness out of car.
- Build new harness on the bench from pigtails of existing connectors.
- Test continuity of new harness on the bench.
- Reinstall new harness into trunk and trunk lid.
- Splice new harness into existing wires in trunk.
- Reconnect battery and test trunk lights, license plate lights, and central locking functions.
- Put trunk back together.
- This is not a job to rush through. I had to get to work the next morning, and splicing the wrong wires together could have shorted out a component, blown fuses, and resulted in a lot of cussing.
- Before starting the work, I wrote down and numbered the wires, then researched the Bentley manual electrical diagrams to make sure that I knew the size of each wire and the appropriate size of the wire I would splice in. The wire sizes in the BMW literature (and hence the Bentley manual) are in metric units, so I did a Google search to find a table that lists metric and equivalent SAE and AWG wire sizes. SAE and AWG wire sizes are not the same; AWG wire sizes are slightly larger than SAE.
- I bought a few different colors of 18- and 20-gauge wire and planned the colors I would use for each splice. For example, I replaced all of the brown ground wires with black wire. It's not completely necessary, but it helped me to identify the wires in the rebuilt harness a little more easily.
- The existing wire harness is wrapped in a fabric tape whose adhesive has broken down with age and exposure to heat. Just touching the tape left my hands feeling sticky. For the more involved work with the old harness, I wore disposable gloves to keep the goop off of my hands.
- Before pulling the old harness from the trunk lid, I tied a piece of string to the last connector on the harness. I pulled the old harness out of the trunk lid and untied the string only after it had entered the trunk. This piece of string was my pull cord that would help me to feed the rebuilt harness into the trunk lid.
- Very Important Thing No. 1: Before cutting the old harness out of the car, I checked each wire for continuity. I picked a portion of the old harness that wouldn't remain in the car and stripped the insulation off of each wire, one by one, to check continuity and number the wires in the trunk. The good news is that the majority of the wires are unique in terms of their insulation and stripe colors, and the grounds that don't go back to the LCM or the General Module all go to a single ground point (X498) inside the trunk.
- I rebuilt the harness on the bench. I put strips of painter's tape down on the bench and traced the old harness onto the tape with a marker. I marked where each branch came off of the main harness and noted which connector/device was on that branch. Next, I marked the locations of the splices I would make for each branch. I cut the old connectors, each with lots of the original wire still attached, off of the harness and discarded the old harness. I cleaned the old adhesive off of the pigtails and spliced the new wires to these pigtails, one by one, according to the "map" I had drawn on the tape on the bench.
- After checking continuity for a second time, I wrapped the completed harness in fabric tape. In addition to protecting the harness from chafing against the sharp edges of the trunk lid as I reinstalled it, wrapping the new harness in this tape helps to prevent it from rattling around in there. The old harness didn't have a lot of tape wrapping the portion where the wires tend to kink and break. I wrapped this area completely in an effort to give the wires more support.
- Very Important Thing No. 2: The hole where the wires enter the trunk lid isn't very large. Splicing all 16 wires at one point along the harness and then feeding it back into the trunk would not have worked, because the resulting bundle would not have been able to fit through that hole. I staggered the splices so that only three or four passed through the hole at a given time.
- I left plenty of excess wire on the trunk side of the rebuilt harness so that I would have lots of slack inside the trunk. More slack meant that I could make splices more easily without bringing the heat gun too close to paint, carpet, or plastic inside the trunk. I wound up with about a foot of slack inside the trunk.
- 16 wires x 2 splices per wire x 2 crimps per splice = 64 crimps. A ratcheting crimp tool saved me from some serious hand fatigue.
- Very Important Thing No. 3: Before making splices in the trunk, I made sure to put the new rubber cover on the rebuilt harness! It would have been very bad if I had forgotten to put it on there before making all of those splices.
- The inside of the trunk lid is full of sharp surfaces. I didn't wear gloves while feeding the rebuilt harness into the trunk lid, and my fingers had more than a few tiny cuts by the time I was finished. Disposable gloves might help prevent this from happening.
- I had a fire extinguisher close at hand when applying heat to the splices in the trunk, just in case. The Tyvek wire markers will not survive exposure to a heat gun. When using a heat gun, it helps to have the nozzle attachment that's made for use with heat shrink.
- After finishing all of the splices and testing the trunk lid devices, I tied back the bundle with zip ties to prevent strain on the splices.
Whew! That was a lot of work, but now I have all new wires passing from the body to the trunk lid. We'll see how long the new bundle and its tougher-than-regular-PVC insulation lasts. BMW could make this repair a lot easier and more convenient if they would produce and sell an affordable replacement harness like they did for the E36.