Auto box problem on a W202

[Tensofbenz]

I have a W202, C180 with a four speed 772421 gearbox, the problem it has, is that it will not go into any forward gears, reverse is fine. when attempting to engage a forward gear, the revs drop slightly so something is happening.
Before i pull the whole thing out and find a good spare box, is there anything that i can do to check it while still on the car? i have checked the oil level and it is clear and does not smell burnt.
Any likely causes that are fixable while in the car?

 

[grober]

http://www.w124performance.com/docs/mb/other/tranny_722_Mitchell.pdf

 

[Tensofbenz]

Thanks, that has told me that it is worth a try to swap the valve body of a good used gearbox. The rest is a bit too technical for me to try.

 

[Tensofbenz]

Is the Valve body removable on the car? i can't find any good links to doing this job on line anywhere

 

[grober]

Jammed kickdown switch? just disconnect to check.

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[Tensofbenz]

I wish it was that simple, but worth a try.
Has anyone removed their valve body while on the car?

 

[grober]

Here's a video about an in-situ removal of the valve body on the related 722.3 transmission. I expect the 722.4 is very similar. I would council against venturing into the innards of an autobox without the right experience. A clean working environment is also very difficult to maintain in such circumstances.
You might spend a lot of time and money for little reward. Autoboxes are one of these areas best left to an autotransmission specialist. Even Mercedes dealers will farm out this work.

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[Tensofbenz]

That is helpful, thanks.
I have no intention of pulling the box apart myself, but do think it is worth a shot at swapping the valve body for a known good one. Estimates for taking out my gearbox and re fitting are for about 12 hours, meaning two things. It is obviously not an easy DIY job and labour costs for a pro will outstrip the value of the car.
Just need to find a good used part now.

 

[grober]

Evidently the symptom of no forward gears but still reverse on a 722.xxx box can mean a B2 [brake band 2] piston [SERVO] failure. https://www.benzworld.org/forums/at...mission-does-not-engage-any-gear-b2piston.pdf Unfortunately on many MB models this is not possible in situ due to lack of clearance.
THE B2 piston rebuild is dealt with on this video at about 2hrs28minutes in.

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[grober]

For folks that are unfamiliar with what a brake band / servo piston does in an automatic transmission there's a good visual explanation from 22 minutes in here. Basically it locks part of the sun gear carrier assemblies from rotating by braking it- this is necessary to transmit drive- failure of the servo/piston means this can't happen.

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[Tensofbenz]

Wow, how do you find all this information, i've been trawling the net for ages

 

[grober]

Doesn't help you much with your problem unfortunately. Failure to activate Brake band 2 could be down to several things, B2 piston failure, brake band adjustment/wear, lack of hydraulic pressure, a blocked hydraulic gallery or poor hydraulic pump performance. The fact that the gearbox works in reverse when piston B2/ BB2 is not used but not in the forward gears would point in that direction the more so because its a known weakness in that transmission?

 

[grober]

Here is a pictorial "how to" on B2 piston replacement on a W126. Note clearance for this is usually only possible on the larger bodied W126 model.
How to Replace Your Oil Cooler Lines

 

[Tensofbenz]

Hmm, i think you are right, i might just break the thing, no sense wasting time and money on something thats not worth much in the first place.
thanks for all you help

 

[grober]

Another B2 article simply for completeness for others with a similar problem. It does state the B2 piston assembly was updated in later models to afford reliability but who knows?
The (infamous) B2 Piston

 

[Tensofbenz]

Update.
Gearbox swapped, all running well again.
Not a job that i enjoyed very much, scrabbling around underneath, would be a fairly straightforward job on a lift i think.
Lessons; Check that the O ring is in place before fitting dip stick tube ( what a dip stick)
Check that every single part has been swapped back onto new box BEFORE fitting.
I might have a look at that B2 piston on the old box if i get chance.