Good Evening All;

A search of past posts fails to turn up one with the same issue, so I need to ask for a consensus on the cause of my non-operational passenger side front door lock... Vehicle is a 1985 300TD.

This is not a lock cylinder which will not turn; rather the key inserts just fine and the cylinder turns without issue. The problem is that turning the key/cylinder does NOT release the door locking mechanism. The only way to lock/unlock the door is via the locking knob on the inside of the door.

The cylinder did work without incident, then suddenly no longer did, as if the connecting rod is no longer engaged.

I pulled the door panel, but it is difficult to see up into that area of the door to get an understanding of how the rotation of the lock cylinder manipulates the door locking mechanism.

What is the likely cause of failure, and the best fix?

Thanks,
Dave

Glad you got into the door panel.

I recommend go back in there and remove the entire door handle mechanism to investigate further.

True it is almost impossible to see up in there.
Wish I had a picture to illustrate, but it sounds like the connection between the back of key tumbler and lock has come apart.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

Try a digital camera. I've found it helps in tight places. Stick in the camera from a couple of angles and look at the pictures.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html

I am having what seems to be a similar issue on my 1987 190D, on both front door locks; you can put the key in the lock and turn it, and even wiggle the lock knob thing on the inside a bit, but not enough that they door will actually unlock. Which is a problem, as it means that I can't lock my car.

I only bought this car about a month ago, so I don't know how/when this started happening. Apparently in my car, there is a vacuum pump in the back, under the backseat, which controls the doorlocks, so I'm going to take a look at that and possibly replace it with one I pulled from a junkyard 190E (which has donated several other parts to my car, including a tie rod, horn, fusebox cover, lugbolts, transmission fluid dipstick, and heater pump).

Hopefully that will work; otherwise I'll probably be pulling off door panels and wiggling around digital cameras at odd angles as well!

Dave there is a plastic cone shaped piece that is part of the doorlock mechanism that brakes. It it was transfers the rotating key/lock to the actually door lock mechanism inside the door. The older models have a metal piece while the newer models have a plastic one. The piece brakes and doesn't transfer the rotating motion to the door lock. My wagon has done the same thing. A junkyard is the only place I've ever found them. But I've never tried very hard to find a new one

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