Ancore

Hi!
I have an issue with the car I just bought. When decelerating from higher speed, on its way the transmission does the "hick-up" on 4th gear in particular.
My mechanic tells it could be solenoid issue, oil, valve body or worse - something mechanical.
I have an appointment on Jan 15th with the dealer, but not sure it's going to work out based on what other dealers were telling me over the phone.
So reprogramming (basically it's TCU) may help, but before they may need to clean the transmission, put new fluid, and then reset the adaptation. If that won't help, my mechanic suggested to take a look inside (particular solenoid may be the culprit).

This issue can be reproduced when there are higher RPMs (by higher I mean higher then 2K which very low I think). But here's a thing. On a Sport mode it doesn't do it when dropping gears, but still rough shifting to 2 or 3 sometimes in this mode. I'm driving in Auto. So I'm puzzled, but based on the symptoms it looks like the Adaptation reset should help.
The previous owner told this is the ECO mode where the highest possible gear is used to save on a fuel consumption. I turned it off and nothing changed. I found

the video

showing the steps to reset the adaptation but I'm not sure this does anything at all.

Please advise what to start with:
- reprogramming
- washing the transmission and put new fluid
- replacing solenoid(s)
- check the clutch, other parts that can be the cause (don't know what are they yet)
- else...

CaliBenzDriver

Whatch this quick video a
It is located in this popular thread.

How are engine solenoid and tranny related?
When the engine can no perform normally the tranny can hardly ever shift at the right time which causes it to bang from poor timings.

Ancore

I didn't say anything about poor performance of the engine. Solenoids are part of the transmission control and responsible for shifting certain gears, thus there are a few of them. Engine works great, but the transmission behaves oddly with 4th gear only - when sharply applying brakes or when accelerating the and the gear needs to be lowered from 5 (or 6 or 7th) to 4th and increase RPMs.

CaliBenzDriver

you're right about the tranny solenoids shifting all the internal clutches. That's where it happens but why it's done wrong leads to the fix.

Go see your local dealer to get fresh adaptations redone and an ATF service for perfect viscosity.


Then consider your gearbox TCU is acting comfused courtesy of the engine ECU....

Ancore

Got back from the dealer today. They were able to replicate the issue, but their diagnostics see no other code than P029900, which is low boost pressure of turbocharger (driver's side). Guided test showed Y77/1 valve does not have correct duty cycle. Vacuum tested waste gates and found the left side leaks down.
Offered solution - replace the left turbo, which is almost $4K, including everything. No TCU codes or anything related to the tranny. They explain that there's not enough air to provide smooth shifting and this is the culprit. I can buy this when I'm accelerating and it shifts down from 5th to 4th to have more power, but when I'm decelerating and it goes from 7th to 4th I'm wondering how it happens at all.

I doubt this is going to help, but who knows... The only concern now is the price. I saw some used turbos from junk yards are around $500 (up to 70K miles on them), plus labor. So thinking of doing that in a few weeks maybe. Any suggestions or thoughts?

L&N Caravan

A couple of thoughts:

- possible to just drive in sport mode?….have to ask

- did you actually see the exchanged tranny fluid? Coloring AND particle count AND smell can transfer a lot of information. May inform a sell decision.

- tranny shift (down, up) are regulated by fluid flow thru valve body, fluid pressure, fluid temp, tranny tcm, engine ecm. With that said, you’ll need to understand the difference b/w the 2 driving modes - sport & comfort - the fluid routing/electronic system controls (which specific solenoids, sensors, connectors) and tease out what is different, and then bring that to the attention of a solid tranny person who specializes in these 7 speed transmissions. My experience is dealers are not a good solution to sorting out at-first-seemingly-complicated-issues such as what you’re presenting. They will throw parts & your wallet at it based upon their prescribed repair heuristic regimen; read a lot of money with most likely not a lot of result.

- sell the car unless you got a fabulous deal.

Let us know what you learn pls.

Ancore

I'll check the Sport mode later in a few day. It's very cold in MI, the road is not the best for the tricks now.

I remember switching once to the sport mode and shifting up and down (auto) was OK, higher RPMs of course and higher speed obviously. For example, I'm getting 7th speed at 40m/h in comfort mode vs 3rd speed at 40m/h in Sport mode. On that day I was testing the car in Sport mode it felt a bit rough shifting too, but on other gears; don't recall which ones but when shifting up. So now, I'm assuming the turbo was not doing its best job probably.
I cannot find the right guy who is a PRO with MB transmissions. Everyone says they have never dealt with that.

Hypothetically, if I change the fluid, would that help? I think the previous owner never done that.

chassis

Dealer bait and switch.

MB boxes are generally garbage. Shifting clunks in and out of 3rd and 4th gears are hallmark MB box characteristics. Nothing can be done, that’s how MB designed it. Keep the trans oil changed and hope for the best, that’s all that can be done.

Ancore

Thanks for the advice.

L&N Caravan

Most important point if nothing else: frequently changing tranny fluid will increase dramatically the life of your tranny, period. MB or other doesn’t matter. I realize MB states that 722 tranny good until 100k miles for tranny fluid change - that’s very deceptive b/c they want to sell more trannys and especially the labor for it as that is their recurring revenue which literally keeps the lights on in their showrooms. I know it’s a ripoff but I use only the MB fluid and filter. I don’t have the ability to analyze viscosity, shear, and flow differences b/w MB vs other tranny fluids at various temps so I eat it on this one. I tow 2 trailers with mine, one medium and other heavy (20’ loaded with crap). Change fluid every year regardless of mileage driven. Current odom at 206k miles. It’s straightforward.

I have an n of 1 rebuilding these 722.9 7GTron trannys, though years on other variants. Having said that, they are sophisticated and robust having to move 2.75 tons of car at a good pace. I’ve used FCP parts through the years on our 4 MB’s and they have a nice overview guide that may guide your decision making and progress stepping through this issue:

https://blog.fcpeuro.com/the-definit...c-transmission

conversely, this tranny rebuilt & supplied by MB should run around $4900 w/ exchange (San Jose CA for reference) should you decide to go that route part # 166-270-80-01-8. Removing tranny a good bit of work though doable with a lot of photos and some videos support during removal phase in combination with a tranny jack and 2 balancing straps.

Ancore

I think changing the tranny fluid should not be the rocket science and I should be able to do it nearly at any respective shop.