My car is a 1985 300CD turbo, California, with transmission 722.416.
Mileage is 189,130 on the original transmission.

This past weekend, I did the transmission service on the vehicle:
-Drain pan and torque converter
-Replace filter
-Replace pan gasket
-Add new dex/merc fluid (6.5 quarts)

This was the sixth time that I have done the transmission service on the car during the years that I've owned it.

This time, there was a difference:
For the first time, I have discovered that there is a leak at the transmission.
Specifically, it appears that the leak is coming from around the transmission modulator that is located on the left (driver's) side of the transmission just above the pan flange.

My question:
At this mileage, is it common for the seal ring (014 997 11 48) that situates behind the modulator to shrink/disintegrate/fail and thus allow transmission fluid to leak? See attached diagram.
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If it is time to replace the seal ring, is it best to also replace the modulator? Any other parts in there that should be replaced? Does replacing the modulator change the transmission's shifting a bit? Does replacing the seal ring/modulator require draining the pan again? How awful a job is replacing the seal ring/modulator? And...How does one safely remove the old modulator and seal ring from the transmission housing?

Thanks for whatever advice anyone has to offer.

Attached Files

Diagram.pdf (94.7 KB, 99 views)

IMO The
Vacuum box seal ring
MB# 0149971148
(also called vacuum modulator seal ring), simply dies of old age.

Special Note: For several years I have encountered very serious (less than one year) durability issues using aftermarket Vacuum box seal rings.

At this point:
* The aftermarket part goes directly in the trash, never installed on a transmission.
* MB# 0149971148 comes from my local MB dealer...
* I keep at least six MB# 0149971148 in personal stock.

.

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It isn't necessary to change the modulator itself. However, check the little rubber cap for cracking or aging. If so, then you will need to replace it as well. If the modulator doesn't leak (vacuum), then it should be good to re-install.

Removing the modulator, without breaking the plastic pin, is a little tricky. It needs to be pulled straight out and evenly. I believe you might have better access if you remove the transmission mount and allow it to lower some. Better yet, buy the little plastic pin (#110 & #117 in your pic). IIRC, the pin is now 1-piece instead of the 2-piece shown in your picture. Double check this as it may be different with some transmissions. Having the number on your transmission would be a great idea when ordering.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

Part #110 is the part the breaks. #117 isn't an issue.

If you are going to the trouble of removing the modulator, replace it with a new one. Especially if it's original. I have never seen a used modulator that didn't have a severely rusted spring, even when they did still hold vacuum.

Thanks for the helpful information, fellas.

Referencing the ATSG Mercedes Techtran transmission manual, it appears that removal of the vacuum control unit is accomplished by removing the two allen screws securing the retaining plate that holds the vacuum control unit in place.

In addition to the failed seal ring, I am inclined to replace the vacuum control unit as it has been in place for 28 years as of this coming April. I will be certain to acquire the seal ring (014 997 11 48) from M-B.

Question 1: Is it necessary or a good idea to drain the transmission fluid prior to replacing the vacuum control unit and seal ring?

Question 2: Is there much adjustment of the adjusting key (the "T" shaped item under the vacuum control unit's rubber cap) necessary after the new vacuum control unit is in place?

It is not necessary.

Random variables make that question impossible to answer.

You are probably aware that your 1985 CA tranny is different from 1984 and federal models. I understand it is the same as 1986+ trannys. I found that out when changing the seal behind the torque converter when I swapped engines. I think the starter is also different. I know you gave the PN's, but I don't recall those. I think the change was due to the "trap oxidizer" on those cars, which might have contributed to my engine failing at 330K miles.

Thanks for the additional information and observations.

Yes, I am well aware that the California 1985 is quite distinctive under the hood, largely (entirely?) due to the trap oxidizer. I am *very* careful with ordering parts so as to ensure that I receive that which is exactly called for for that specific model and year.

It all makes the work on my car that much more interesting, right?! ;-)