W211 E320CDI door lock "bounce"
 

My door lock had a common problem, the door lock would bounce while locking the door. It unlocked fine. I did a search, and found that a spring inside the door lock actuator was probably broken. I ordered a spring kit, it came with the two springs, a pair of forceps, and bolts to replace the drilled out rivets. Process: After removing the door panel, I drilled out the two rivets holding the lower part of the door track. There is a T-20 screw behind the weatherstrip holding on the lock. Pull the lock out so the actuator assembly can be removed. You can either pull the connector from the lock, or pull the wire from the tabs on the actuator to give enough slack to work on the actuator outside the door shell. Unbolt the actuator with a T-30 (3 screws). It is a little difficult to manipulate the actuator out, but doable- you have to hold your mouth just right. Once the actuator is out, there is a screw holding the plastic cover on. Also, half a dozen tabs need to be pried so the cover will come off.
Here is a youtube video of the process: https://www.youtube.com/watch?v=gZryAPBYE7I&sns=em

Part 2
 

Once you have the actuator where it can be worked on, pull out the broken spring. Compare the broken spring with the new one to make sure you have the correct side (left and right are different). Use the forceps to install the spring. One end of the spring goes in the hole of the moveable part. You will see a tiny hole on top, that will probably have the broken part of the old spring. Putting the new spring in will push out the piece. The other end is a little tricky. It gets slipped into the slot just barely in view, not in the hole above. Assemble the actuator cover (don't forget the little screw), and manipulate the unit back in place. The white plastic shield needs to be moved towards the outside skin of the door so the actuator slips into place. Then you can connect the wire to the lock assembly, and insert it into position. Tighten the T-20 screw under the weatherstrip. Use the 5mm bolts, nuts and washers in place of the drilled out rivets. Install the door panel. Be sure to put on the door latch trim piece before testing. I got ahead of myself here, and it only unlocked, as the trim piece is necessary to completely actuate the switch. Actually, you can test it before installing the door panel by using a spring clamp to depress the switch next to the door strike.

Yup had the "sewing machine" door lock - I played hell putting the spring in an E500. Good lord, but cheaper than the actuator and you still have to take it out.