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E320CDI 722.6 cold transmission issues solved

10926 Views 1 Reply 2 Participants Last post by  cvincent
I have a 2005 E320CDI with just over 100k miles. This fall when temperatures started to cool (Im in the NYC area), the transmission would exhibit unusual qualities when cold: bucking and groaning in low gears. The bucking seemed like a transmission shift. The groaning would occur when I was decelerating at low speed. After about 5 or 10 minutes of driving everything went back to normal. Flushing the transmission fluid did not remedy the problem.

I did a lot of reading on the topic, 722.6 tranny issues. I found no shortage of dead-end posts, problems noted with no resulting solution, so I thought I'd post my experiences here.

I took the car to my dealer (a good one by the way). There were no codes thrown. The dealer gave me a loaner and spent 5 days evaluating the issue. In the end, they concluded (as had I) there were issues with the TCC or torque converter clutch function.

Bucking: This seemed like an issue of harsh shifting for 1 to 2 and 2 to 3. But upon closer scrutiny, the bucking was not a gear shift at all. For example, in 1st gear at a stop, I used the shift lever to keep the car in 1st. Proceed from the stop and drive normally (abeit in 1st gear). Well before the shift point, where a shift to 2nd would occur (yes I had the trans in Sport mode), the bucking occurred predictably with each try. I concluded this was the TCC grabbing in sudden fashion rather than a gradual fashion. I could feel it and I could see it in the tachometer.

Driving in comfort mode did not matter. The bucking still occurred.

Groaning: Similarly, when decelerating to a stop, I would sometimes hear/feel a groaning from the drivetrain. Hard to describe. But almost seemed like keeping the clutch engaged too long in a manual transmission. The diesel motor has a lot of low end torque. This was the TC clutch shuddering when it should have been disengaged.

Back to the dealer. The shop foreman studied the issue for several days. Remember it could only be observed when stone cold. He turned the TCC off with the Star system and confirmed it was in that TCC function. He said "Officially, based on the car's age, my advice is to get a new torque converter, new valve body, and new tranny." My reply was "..and your unofficial advice is?......" He said "unofficially, I'd have that valve body replaced or rebuilt and if there is crud in the filter screen in the TCC circuit, I'd replace or have the torque converter rebuilt."

There is a lot written in the forums about the TCC circuit as well as on Sonnax's website. There are two valves that deal with TCC control in the valve body. There is (I believe) one related solenoid. The clutch pack in the torque converter has issues with draining which can cause faulty release. Newer Sonnax clutch packs remedy that problem.

I didn't have time to take an iterative approach to solving this and I knew that the problems would compound if I did not deal with it soon. I had the TC and valve body rebuilt.

Could I have gotten away with simply replacing the solenoid on the valve body? Possibly, this is doable within a DIY. But not for me in the middle of winter. Also the solenoids in these gear boxes are not vulnerable points of failure. Could I have gotten away with just a valve body rebuild? Possibly. But since the groaning had subsided a fair amount over two months, I figured that the TC clutch had worn down considerably. I bit the bullet and had that done too. Both valves flunked the vacuum test indicating wear. Remember the TCC function activates at every shift (at least in the CDI and AMG models I'm told) so it is vulnerable to wear.

After getting the car back, it's all back to normal. (albeit $1900 later) I found a competent transmission guy that was familiar and able to discuss the 722.6 issues intelligently. As a precaution he replaced the conductor plate. All other solenoids and shift valves tested fine. One other item that showed excessive wear was a stator bushing. That was replaced as well. He said that this cold tranny problem occurring later in life is not that unusual to the 722.6.

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