Broken transmission parking lock
 

The parking lock is broken on my latest purchase - a 1979 450SEL. While idling in Park after getting to work today, I noticed a periodic clank coming from the transmission area. Then I noticed I was rolling backwards.

How involved is fixing that? It seems like a simple thing and the transmission functions perfectly otherwise.

Fortunately the parking break is in fantastic shape - I'll bet the PO replaced it recently.

This is a transmission out job so I'd make sure that the shifter linkage is working correctly first. I reckon you are more likely to have trouble with the shifter linkage than a part inside the transmission.

If the linkage is OK..
Then removal of the rear cover on the trans is required.
MOST likely not broken. But an alignment issue with the pawl.

Heres a picture of the pawl and associated gear in my 300D's 722.418:

http://i.imgur.com/pIVeIl.jpg

Not really sure what I'm looking for here - can you explain a little bit? I've never done anything with a transmission except for look at it blankly from underneath and think "yep, there's the transmission."

I notice in your signature a note about cruise control. Got any advice for one that seems completely dead (stalk doesn't seem to do anything when pushed in any direction)?

This might help:
http://i.imgur.com/Rz9I1l.jpg

The pawl is engaged when you put the lever in to P. It locks into a notch on the parking gear (my term, not sure what actually is) The parking gear also has a worm gear on it that drives the speedometer cable (124,201,123).

I have some diagnostics you can run though, most of the time its the cruise control amplifier:
Diagnostics:
http://www.peachparts.com/shopforum/2949128-post5.html
How The System Works and Common Problems:
http://www.peachparts.com/shopforum/2996327-post14.html

The Parking Pawl on the 722.00_series trans is mounted just slightly different than shown.

But you must remove the rear cover to get at that problem part.
Will require a slotted socket for the rear flange & also a wide straight blade like a drag link socket for the governor spring cover.

You can see a few pictures of a 722.1 transmission in this thread - this is the four speed version of your three speed 722.0

http://www.peachparts.com/shopforum/diesel-discussion/304903-722-118-automatic-transmission-rebuild-monster-diy.html

The parking mechanism is shown on the first page - though the pictures aren't really made for your problem!

I have read that people try and remove the nut on the output shaft of the transmission by engaging the transmission in park and then having a go at it. This might be a bit of problem if you need to do this. I reckon this kind of makes it a transmission out job as it'll be a bit of problem stopping the output shaft from turning when it is fitted to the car unless there's a way of stopping that shaft from turning with the transmission fitted to the vehicle...

...I don't know if it is possible - perhaps someone here has done it?

Anyway - nuts and covers and all the rest of it shouldn't be considered until you are sure that the linkage isn't working properly. Disconnect the shifter linkage at the transmission - turn the selector and check to see if the park pawl engages when it isn't connected to the rest of the linkage.

I have a 1971 250C that was diagnosed by a Mercedes mechanic with a broken pawl for the same symptoms you are experiencing. I replaced the Shifter to Transmission linkage bushings, one at each end of the rod, and fixed the problem. So..........make absolutely sure that the linkage is correctly adjusted.