Key Does Not Operate Driver's Door Lock
 

To lock and unlock the 190E any door other than the driver's door will accomplish the task. Any of these other locks will also lock and unlock the driver's door. But the driver's door lock will not lock or unlock anything.

It does not appear to be a vacuum problem because I can activate the driver's door lock from any other car locking mechanism.

If I open the interior panel of the driver's door, what might I look for to diagnose the cause of the dysfunction?

This is a common problem. There is a small shaft at the end of the lock cylinder that engages the linkage inside the door. The shaft is made of a poor grade of die cast alloy. It breaks, and it cannot be repaired. The bad news is that you have to replace the entire lock cylinder assembly. The good news is that it is an incredibly easy job and does not involve removing the door panel. With just a bit of work, you can swap all the old tumbler plates (nomenclature?) into the new cylinder and keep your old key. I think you can order a cylinder keyed to your VIN, but I'm not certain of that.

That is good news Jon. Since the passenger side front door is never used to lock the car... maybe I can simply switch the two front door cylinders.

Its my wife's car and she only uses the driver's side and trunk key ways. If this idea will work the next question is: How do I remove the lock cylinders?

I don't think you can swap the locks. The cylinder is attached to the small black trim piece on the door handle, and I don't think it can be rotated 180 degrees. I may be wrong. Anyway, to get the lock out, there's a small round plastic cap on the side of the door (the part of the door that's hidden when it is closed). The cap is perpendicular to the lock cylinder. Remove the cap, and there is access to a small socket head setscrew, maybe a 4mm hex. (Could be 3.) Remove that screw, put the key in the lock and use the key to pull the whole cylinder out of the door. There was another thread on this very recently, in the last week or two. Search for it, I think there were even some pictures.

OK John. I did the procedure as you described and the cylinders came out quite easily.

The super duper good news is that the front door cylinders are interchangeable. Simply turn the whole cylinder assembly 180 degrees (just a flick of the wrist) and reinsert on opposite side of vehicle.

Procedure is: insert key into cylinder, remove black rubber access cap at end of door, loosen exposed #3 set screw, turn key in cylinder and slowly wiggle out the cylinder assembly. Then remove white plastic wire guide from the assembly to expose black wire connector. Then, from sides of black plastic, gently pry plastic clasp off the metal lock assembly.

With flash light and a pair of needle nose plyers remove broken tip that remains inside the door of the broken assembly.

Reassemble and reinstall cylinders on opposite sides of vehicle.