Bought a '99 ML430 with a bad transmission so I could educate myself on fault diagnosis and rebuild techniques; instead I got schooled on how many parts in that propulsion unit have been updated and are no longer useable when putting it back together again. The transmission itself tossed the entire planetary gearset (all three -- love auction cars, everyone has to drive it to make sure it's bad), and I originally thought it was the typical mercedes planetary failure where the center bushing that links the front shaft with the back shaft wears excessively and fails, pinching the bearing surface and shattering the bearing, causing catastrophic failure as the individual hardened steel needle bearings are fed through the planetaries (the update replaces the bronze bushing with a roller bearing, requiring the purchase of the front shaft and tail shaft assembly.

The sungear assembly is updated (main sungear almost always fails, and requires purchase of entire sungear assembly); the rear planetaries are updated; the bearings in much of the transmission are updated (requiring replacement of MUCH more expensive componants). But I'm a trooper, and even though I could have bought a completely rebuilt tranny for the price I paid for the parts ($2700 wholesale), I learned quite a bit. For example, there are not many good sources for used parts, because so many things were updated, and WHEN these transmissions fail, they like to toss the planetary gears. Also, the failure in earlier transmissions is due to to poor designing on Mercedes part (later transmissions are much more durable); the earlier transmissions ('00 and earlier 722.6) require fluid changes, and given the weakness of the center bushing, about 50-60K on the fluid changes seems about right (given that the tranny weakness is on a transmission already past that service period on an older vehicle anyway, I don't like my odds, particularly on the higher-output engines). I'm sure the newer tranny's are more durable (I can attest to that... mine is updated, and the transmission design IS more durable).

Which brings me to why the transmission failed in the first place... the front sungear to rear planetary bearing failed (the bearing was updated as well, but still has a weakness in the planetary design with that bearing, IMO). Bearing failed and little needle bearings greeted the planetary; they did NOT get along at all. That torrington still needs designed IMHO (I'm full of it tonight). I'll update (no pun intended) as I get time, but runs great now.

Mechanically, the internal difference in the transmission regarding the torque output I believe is such: lower torque outputs have a 3-wheel planetary drive, and higher output tranny's utilize a 4-wheel gearset.

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