There are three types of common failures on these door latch assemblies. Although the actual part varies between chassis the basic design and function is the same on almost all models from the early 1960's up to the mid 1990's. First, they can seize preventing you from opening the door. This usually happens on older cars that have sat outside for long periods without use. Secondly, the latch mechanism can fail and the door will not stay shut. There is a part inside the latch that breaks. Finally on some of the later 123 models they used a white plastic part inside to control the door lock function. When this plastic part fails you will not be able to lock or unlock the door with the key. You can not purchase this little plastic part separately.
Problem & Solution
Common among these chassis:
In all cases you will have to replace the entire latch with a good used assembly as pictured above. There are no repair parts available and trying to take it apart to fix the problem is very time consuming and seldom successful. We have no specific instructions at this time on removing the latch. You will have to remove the outside door handle and the inside panel. There are a number of screws that hold the latch to the door. Once removed you can try to "snake" the latch out from the door. On some models this can be like a chinnese puzzle and you will wonder how they ever got this thing in at the factory. It has be to twisted and turned just the right way to get it out.
If the door is seized shut, first try having someone work on pulling the inside handle back and forth while you get on the outside and pull REAL HARD on the door. If that does not free the door you may have to cut into and destroy the inside door panel to get to the latch to lubricate it with spray rust buster or Kroil oil. Then you will be looking for a replacement door panel.
This is another good case for lubrication. You can lubricate these latch assemblies from the outside if you remove the outer door handle. You can get to it even better by removing the door panels and lubricating all moving parts inside your door including the rods and levers. The latch is best lubricated with a quality spray lubricant using the long spray tube attached. See Kent's video on 126 door lock repair for a live demonstration.
If you need instructions for removing your door panel we offer multiple resources. For outside door handle removal see Kent's free YouTube video on the proper procedure for the W123 chassis. Other chassis are similar. We also cover a number of different types of lubricant to lube your door checks, hinges, and window regulators. See related products for more information.