Hi all.

I am the current owner of a 1998 ML320 with about 275,000 miles.

The transmission has started making a rapid clicking noise (actually more of a 'clacking') when in park or neutral. It feels fine in drive, but binds and jerks terribly when driving in reverse, with a feeling like a CV joint is binding-up. The clacking noise increases with RPM.

My assumption is the parking pawl is somehow contacting the parking gear, but I'm not familiar with the 722.6 transmission (or many other automatics, for that matter). Does anyone have diagrams of this transmission that would help me locate it?

I dropped the transmission fluid this morning and it looks fine - no metal shavings or particles.

I bought this car with other issues and have enjoyed sorting through them (engine mounts, rear suspension, door locks, etc). I like taking it off-road, my wife likes driving it around town, and I am on a mission to keep it alive.

Any advice would be greatly appreciated!

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you have the older version of the 722.6 which has a failure point where the front planetary gearset loses alignment due to a worn bushing which then causes it to bounce and make noise.

its best to stop driving it and get it rebuilt. Otherwise it wont be too far that you have a transmission filled with metal grit.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Stop driving the car otherwise you will be walking in short order.

Find another transmission, needle bearings went through the planetary gear set and you will find broken gears.

The K2 bushing has worn allowing the sun and ring gear to run off center, this wears the aluminum housing, causes excessive end play allowing needles from the thrust bearing to fall out and jam the gears.

Another contributing issue. Early ( pre 99? ) thrust washers tended to spin the race further contributing to aluminum wear. Later units have thrust washers with ears to prevent spin and a needle K2 bearing. Late needle bearings can be fit to the early housings with some machine work. EDIT but not the K2 bearing as the shafts are different

You can swap the entire planetary gear set and output shaft between early / late but not individual parts.

Be aware, 4 and 6 cyl ratios are different from high power 6 , 8 and 12 cyl.

Very early in K2 bushing failure there will be an intermittent buzz / siren type of noise due to the planetary running off center. My 97 C280 suddenly started to do this at 164,000 miles. There was no evidence of metal in the pan but I rebuilt it at a preventive measure.

These transmissions are not difficult to take apart. Also have a look on car-part.com This searches participating salvage yards.

You might like this site https://www.motortrendondemand.com/show/dirt-every-day/42026322/ ( older shows are free, newer ones are pay )

I dug around in the oil again and found nine rollers from a needle bearing. As you suggested, something has let go.

I pulled the gearbox and am going to buy a replacement. Thanks for that link.

Really appreciate your help!

Now for the miserable job of putting it all back together again...

Your welcome.

Few salvage yards understand e mail so calling is the best bet.

Another question, if you wouldn't mind...

The part number from the side of my gearbox is 1632710201.

Can I use any other type of box without running into ECU issues? Perhaps something from a later car that doesn't have the K2 bushing issue?

I have spotty info on late 90's valve bodies. I'm showing a split in late 2000, see attachment.

The upper half of the trans has more or less clutch plates based on engine power, this shouldn't be an issue but I haven't mixed and matched.

There is a separate trans controller for these, is isn't part of the engine computer. In theory you could use the TCM from the donor car but electronic programming being what it is, I can't say what will work with what.

When these trans were introduced in 96, there were quite a few of updates and valve body along the way. If you use the valve body from current car on another trans you should be safe.

Places for info , ATSG is pretty much the place for aftermarket trans info, they get info from real shops and don't just reprint a factory manual.

$ 30 rebuild / general info manual

https://www.atsg.us/atsg/import-manuals-1/mercedes-722-6-nag-1.html


$ 125 in depth manual , says it has detailed info + CD

https://www.atsg.us/atsg/import-manuals-1/mercedes-722-6-dvd-video-companion-manual.html

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Thanks for this. I guess it's best I stick to a transmission of the same part number (and hold my breath for the next bushing failure... !).

These trans are not difficult to take apart, practice on the old one.

You could strip down the replacement just enough to change the K2 bushing and go from there. For parts you will need a K2 busing , a " paper and rubber kit " and maybe the output shaft seal if the 4 wd is different than the 2 wd.

These trans are built from the bell housing rearward rather than main case forward. In general you would unbolt the bell housing, place entire unit bell down, remove output flange then pull the main case off.

Next you would separate the input / output shafts and pull clutch drums / ring gears off the input shaft stack until you can change the bushing. You will also need to take the B1 housing / pump off the bell to change the gasket.

Given the bushing went X miles to failure, the new bushing should go another X miles.