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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

Hey all! I just saved myself a bit of money by repairing one of my door lock actuators. It didnít cost me a cent and all it took was a little of my time.

I had a situation where all four of my doors wouldnít unlock. The only way into the vehicle was to use the integrated key from the fob. Once inside, I tried pressing the central locking system button on the center console but no luck. After some online searching, I discovered that the systemís 15A fuse which is located at the back of the glove box had blown. A new fuse was installed and this time oddly enough, all but the rear passenger door unlocked. The other doors worked fine repeatedly. I ruled out a possible short in the door wiring because the new fuse would have blown. Something within the lock actuator must have gotten jammed but not in a way that would trip the circuit.

Ultimately, I managed to take the actuator apart to discover that one of its two drive motors had seized. As you read on youíll learn what caused the motor to fail and how to make the repair by simply using a couple of household items.

This DIY is based on a 2007 335i e90 and please note that much of it is not the usual step by step set of instructions but is more of a personal account. Nonetheless it will help guide you through the process. Letís get started.

Lock Actuator Removal:
There was really nothing 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 was a great starting point.

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.

Well, you are now at a point where you have two options. You can either install a new actuator by reversing what you just did to remove the old one or continue on into the mysterious and wondrous realm of door lock actuator repair.

Actuator Housing Disassemble:
At first glance I thought that the actuator was a sealed unit and couldnít be opened without destroying the housing. The riveted metal strike plate fooled me momentarily. Looking more closely, I noticed the five Torx20 screws; four around the plastic perimeter and one going through the strike plate. Once removing the five screws, the housing began to separate into what appeared to be two pieces.

I pulled at the plastic tabs around the edge and could see some more separation going on. It was clear at this point that there were now three sections to the unit. The third had the riveted strike plate attached. I lifted the triangular end of the plate off a plastic locking nub and slid the piece straight out. It had jammed on one end so I used a bit of force (maybe too much) but it finally yielded and luckily 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 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 could only be started again by rotating the worm gear with my finger.

I then went on to separate 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 a patina or carbon. There was also a half melted tiny nylon flat wear washer separating the rotor from the end of the plastic motor housing.

So with the motor apart, I sealed off the magnet and windings with a layer of masking tape (sticky side out) leaving only the commutator exposed. Then using very fine steel wool, I removed the carbon build up, paying special attention to the lateral grooves. The purpose of the tape is to keep the steel wool filaments separated 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. Repair complete!

Information regarding commutator surface conditions can be found here:

I believe that electrical contact between the commutator and the brushes had diminished over time due to a build up of patina/carbon. Finally the rotor could no longer overcome the resistance from the layer of residue and the motor seized. This caused an over-current situation in the door locking circuit and blew the 15A fuse. I'm still not sure why the replacement fuse didn't blow.

In technical service bulletin SIB 511907 of June 2013, BMW recommends changing fuse #57 (up to 9/07 production) or fuse #73 (from 9/07 production) from a 15A to a 20A. A function test must then be performed and actuator(s) are to be replaced if locking and unlocking is sluggish or abnormally noisy. BMW blames silicon contamination or intermittent but acceptable amperage spikes. I really have my doubts about these explanations. My bets are on the motors.

Iíve included some photos at the end of this DIY of the open actuator to not only get a feel of what to expect but to know in what order the pieces go just in case a loaded spring decides to go rogue.

These actuators are failing far too early in the life of the vehicle IMO. Originally I blamed the overloading of the motor on the sheer work required to move the locking mechanism components. A further search leads me to believe that the buildup of black patina has resulted over time from the effects of the presence of a damp electrical environment within the motor. Unusually high air moisture content coming in contact with the copper commutator and carbon brushes will produce these results. This makes sense as I live in a region where relative humidity is high.

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 aid in the re-placement of 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 follower 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

Positioning the small black slotted linkage.

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.

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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By wisstringa82 on 11-06-2014, 04:02 PM
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Average 53%
Good DIY

Thanks Man.
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By NBTBRV8 on 04-25-2015, 05:14 AM
I bought a new door lock for mine, even though it was only replaced about 4 years ago.

One thing to note is that the metal housing that the two torxs screw go in to through the door aren't threaded in the housing. So you need a 6mm x 1 tap to cut threads in them.
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