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DIY: Rear Door Lock Actuator Repair - '07 335i
Published by Built My Way
DIY: Rear Door Lock Actuator Repair - '07 335i

Well ladies and gents, I just saved myself a load of money by doing a repair job on my rear right side door lock actuator. The best part is, it didnít cost me a cent and I know what to do now if/when the other door actuators go, including the fuel filler door. All it took was a little of my time.

I had taken the car in to my Indy for an MVI but when I went to bring it home, the doors wouldnít open. I left it with the garage and after some investigative work they put a new fuse in at no charge and all seemed to work fine. When I finally got home, I tried all of the doors again and noticed that the rear passenger side still wasnít unlocking but at least the other doors worked. I figured it had to be either an issue with wiring in the door or a problem with the actuator itself. Some quick online research convinced me to go right to the heart of the matter and pull the actuator to get a good look.

While searching, I found a few posts about how people are getting trapped in their cars and are forced to crawl out a window and how blown fuses are being replaced by those having higher amperage ratings, potentially compromising the system wiring. What really gets me is the frequency of locking system failures and the amount of money that I can only imagine people are doling out on replacement parts and labour. Sorry BMW but Iím just not the ďpay to playĒ type.

This DIY is more of a paragraph style account of my experience with the door actuator but it'll still help guide you through. Letís get started.

Lock Actuator Removal:
There is really nothing out there in the way of information for rear door lock actuator removal to speak of but thanks to e90post member tracerit, his DIY for the Ďfront door lock actuator replacementí post really helped.

See http://www.e90post.com/forums/showthread.php?t=477507

All in all, everything came apart quite well. The tool kit for trim removal was a Godsend. A few nice things about doing the rear lock is that I didnít have to disconnect the battery nor unhook the glass from the window regulator. I kept the window rolled up the whole time which allowed for more than enough access to the actuator. Also, since the handle is keyless there is no need to undo the 4mm Allen bolt on the edge of the door or go near any part of the handle for that matter.

Actuator Housing Disassemble:
Now with the actuator in hand, my first response was that this thing is a sealed unit and cannot be opened without destroying the housing. The riveted strike plate is what momentarily fooled me but looking closely, there are five Torx20 screws; four around the plastic perimeter and one going through that same metal plating. I removed the 5 screws and noticed that the housing started to separate into what looked like two pieces.

I started pulling at the plastic tabs that hold the pieces together and began gently prying them apart. All of a sudden I could see that now there were three pieces to the unit. The third piece was plastic which had the triangular shaped strike plate attached. The end of the triangle was easily pried off a plastic locking nub. I tried to slide the third piece straight out as it only wanted to go in one direction but it seemed to be jammed on one end so I used a bit of force (maybe too much) and then it yielded and luckily it separated without any damage or parts flying everywhere. Whew! I was a bit more cautious when separating the two other pieces. Talk about a Chinese puzzle!

Point of Failure and Actuator Repair:
I set aside the strike plate section of housing and focused my attention on separating the other two pieces. As expected, one of the halves was filled chock a block with gears, linkages and springs and the other was just a mating cover. Two toy size 12v DC motors accompanied the workings. A visual inspection showed no obvious damage to any of the exposed parts. The condition of the plastic gears looked fine. All linkages seemed to be in their place and all springs ready for action. The greased brass worm gears on the motor drive shafts showed no signs of wearing and I was able to rotate them using my thumb with little or no resistance from the motors.

Carefully, I pried one of the motors loose leaving two small +/- electrical prongs exposed in the housing base. I hooked the motor up to a 9v battery and put it through a series of stop and start tests without any problem. The other motor was a different story. It would stop alright but would only start intermittently. The motor would only work again by rotating the worm gear with my finger.

I then separated the metal motor casing from the plastic end piece which houses the tiny brushes by straightening the two crimped metal side tabs. Inside, the brushes were in good condition but the commutator (the rotating part that the brushes touch) was black with carbon. There was also a half melted tiny nylon flat wear washer separating the rotor from the end of the plastic motor housing.

With the motor now apart, I took the time to clean the commutator. I sealed off the magnet and windings with a layer of masking tape, leaving only the commutator exposed. I used very fine steel wool to remove the carbon build up, paying special attention to the lateral grooves. The tape kept the steel wool filaments away from the magnet. After the cleaning, the motor was reassembled and tested again. It started flawlessly every time. I even cleaned up the other motor while I had everything apart.

Iíve come to the conclusion that proper electrical contact between the commutator and the brushes had become impeded to the point where the motor could no longer function when energized. Carbon build up on the commutator was interfering with the work of the brushes. The layer had become such that the rotor when re-energized, could no longer overcome the resistance of the residue and caused an over-current situation that consumed the 15A fuse. Intermittent failure of the motor is explained when the actuator function resumes after putting in a new 15A fuse.

Replacement of a used 15A fuse with a new higher rated one is likely to prolong the life of a dying motor as the resistance is more easily overcome. The problem here is that inevitable motor failure from further carbon build up could do severe damage to the locking system wiring. Higher rated fuses could now render wiring as the weak point in the electrical system.

Of course my conclusions are only based on one actuator but I am confident that many of the BMW and even other car brand locking system failures could be traced back to the actuator motors.

Iíve included some pictures at the end of this DIY of the open actuator to not only get a feel of what to expect but to know what order and where some of the pieces go just in case a loaded spring decides to go rogue, unleashing its power on unsuspecting neighbouring components, sending them off in all directions.

Removing the Door Handle/Pull and Handle Backing Plate (Optional):
Before reinstalling the lock actuator, I decided to take a detour and remove the entire door handle/pull workings just to see how it all fit together and it gave me a chance to clean and wax the painted surface area and to properly lubricate the sometimes Ďstickyí spring mechanism on the handle/pull backing plate. Removal of the handle and backing plate is unnecessary when doing the lock actuator fix but Iíve included a short procedure in case you decide you want to take this route.

I followed traceritís DIY and removed the 4mm Allen bolt located on the doorís edge under the black plastic circular cover. I removed the loose end block piece of the door handle, being careful not to damage the courtesy light and then pulled the end of the handle closest to the doorís edge out. The other end of the handle rotated and pulled right out. A Torx-20 screwdriver was used to loosen the exterior screw holding the backing plate to the door pull area. I slid the door handle backing plate off and fed it out through the door. Putting the handle back is somewhat tricky. Dry fit the handle/pull and the backing plate together first to understand how it goes together. Reverse the order of steps to assemble.

Here are pictures of the door handle assembly.

Backing plate (interior side)

Backing plate (exterior side)

Door handle/pull

Pretty Pictures of Inside the Actuator:
Note: These pictures only show re-placement of the components that are more likely to become dislodged during handling of the open actuator.

Order of component installation Ė left to right and top to bottom

The motors can be installed or removed at anytime during the procedure. The problem motor was the one on the right.

Yellow groove cam gear

Locking rod module with spring in place

Dry fitting the locking rod module without loading the spring to ensure that the pin on the underside lines up with the yellow groove cam gear

Installing the locking rod module by first putting the spring into place

Setting the long plastic linkage in position as shown

Loading the large spring against the plastic tab as shown

Aligning the small black slotted linkage in the channel.

Setting the second black linkage on top of the slotted one

Setting the third black linkage on the previously loaded spring and rotating it CCW over the threaded pillar, pushing it down into place. It should stay put against the pillar but just be careful not to bump it.

Putting the mating cover back on using four Torx-20 screws

Sliding the third housing piece into place making sure that it lines up properly

Attach fifth Torx20 screw as shown (lower right)

Well thatís it. Everything went back together nicely and the locking mechanism works like a charm. I plan on doing the remaining door lock actuators over time as preventive maintenance. I read somewhere that you donít have to detach the window from the regulator during front door lock actuator removal procedure. Simply unbolting the bottom of one of the window guides will provide the space needed to get the actuator out. I guess Iíll find out soon enough.

One last thing to consider: If instead, you decide that you want to replace the actuator then maybe give a thought about selling the old one. Motors donít last forever and I reckon that these in particular will become a difficult item to find in the future.

Thanks for dropping by!

Built My Way

PS: See my DIYs for the 335i Water Pump/T-Stat replacement and the Hard Brake Pedal issue.
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