7G Gear "slips" when cold

[Eddy77]

Hello,

I've got a 2008 CLK 350 with the 7G tronic box fitted. Just recently I have noted that when starting and pulling away from cold, the gear box slips on the first change (and possibly the second) of the journey. By "slips" I mean that it seems to over-rev slightly on the change before selecting the next gear. Only does it on the very first change as I pull off the drive. And very occasionally the second.

After a couple of gear changes (i.e. by the time I reach the end of my road), it's all fine and it never does it again for the journey. Park up, go shopping, and it's fine on the return journey. (as I guess the box is still warm).

So it seems to be when the car has sat for a long time and is stone cold from starting. It often sits for two weeks between drives.

It doesn't really bother me as it does it once or twice and never again for the entire journey. But does anyone know if this is a common issue and what the problem is?

In all other ways the box is 100% perfect with zero judders, super smooth changes and no apparent faults.

Car is 9 years old / 48k miles.

It had the auto box oil and filters done at 37k miles but perhaps there's some new crud in there which is causing this glitch (it was done two years ago - car does few miles annually).

Maybe I should drop the oil and filters from the box at the next service....unless anyone knows of a specific fault that would cause this??

Any thoughts appreciated if this is something you have come across before!

Thanks!!

 

[whitenemesis]

When the 'box is cold it will hold the gears longer (higher revs before changing) more than usual as it attempts to warm the CATs more quickly. Perhaps this is what you're experiencing?

 

[PobodY]

I can't say that I've noticed mine doing the same, but I'd also suggest it's just trying to warm things up a little faster.

In my case it might simply be that I drive relatively sedately to the end of the road, stop at the intersection, then I might use a little more power (so I don't notice any change in the length of time it holds the gears).

 

[Eddy77]

Hmm....it's not so much that it holds the gears longer, it's that when it decides to change it sounds like someone slipping the clutch on a manual.....so the revs buzz up quite high for a second before dropping right back down to the next gear. From then on it changes in the usual way. It feels like it is temperature or oil viscocity related. After a couple of changes whatever it is that causes it settles down and all ok.

 

[Osgee]

Possible TC on its way out?

48k is it not due an TM Oil change ?

 

[CarlPea]

Completely random but my E63 W212 also does this (10,500 miles) but only if I give it a decent amount of throttle.

After about 200 yards it's fine.

 

[whitenemesis]

Possible TC on its way out?

48k is it not due an TM Oil change ?

"It had the auto box oil and filters done at 37k miles "

Perhaps it's marginal on atf level when cold? Have you had it checked recently?

 

[Eddy77]

Think I will mention it at next service and get the mechanic to check levels.....thanks all. As it is, it's no bother and a google reveals its a common issue with no obvious reason. Some say a box oil change helps, others recommend an ecu reflash (?!) but there seems no universal fix. Will monitor and report back with any updates!

 

[PobodY]

Out of interest, does it make any difference whether you're in "C" or "S" or is the behaviour the same in both?

 

[Eddy77]

Same in both so it's not a First to Second issue as car starts in Second when in C mode. Someone else on a US forum reckons a TCU reset can sort these kinds of issues out. Effectively blanking out the gearbox learning where it adapts to your driving style. To do that you switch to ignition position two (but don't start the car), hold accelerator right down so kickdown button is pressed. Hold for ten seconds. Then switch ignition back to off. But keep foot on accelerator and do not remove key. Then take foot off accelerator and leave for 2 minutes without operating anything. Don't open doors or do anything. Just sit stil. Then start car and gearbox should be reset. Going to try it as worst case is it does nothing!

 

[Yugguy]

On a general note my 5g holds the gears longer when cold which can be annoying as I like to keep the revs down when cold.

 

[55AMGsuperchare]

MY cls55 does this too and I recently changed the oil too.

 

[whitenemesis]

Th 5G box will start off in 1st when cold, even in C mode, not sure if the 7G is the same.

AFAIK and was told by an indie, the TCM can only be 'reset' using STAR. This puts the TCM back to factory, all adaption is lost. The car must then be driven in a very prescribed manner (holding each gear up and down the box, something like that) to restart the the learning from scratch.

I thought the above described procedure, using the throttle actually resets the throttle pots?

 

[Eddy77]

So, a quick update. I was at home this morning and needed to pop out. Decided to get the Merc out of the garage and try this throttle reset.

Followed the instructions and it definitely has changed the car.

The gear changes are definitely less slushy - you feel a satisfying change rather than what had become a VERY slushy change through the gears in recent months.

AS to my original problem, the car did it once on the change up from 2 to 3 BUT it was less of an over-rev than before and it only did it once. To be honest it was so slight that unless you are listening for it, you'd never pick it up. I don't think a passenger would have thought "Ooh, what was the car doing then".

And from then on silky smooth changes, but definitely the car felt keener.

So, not a complete solution, but definitely something that is worthwhile doing now and again as I think the car can learn some odd change patterns if you mix up sporty and sedate driving.

I don't know what the procedure with the throttle does, but I am adamant it has improved the change quality overall and diminished, but not eradicated, the odd over-rev on the first change of the day.

Try it - what can you lose?!

Thanks to all for their comments. I'm just going to leave it now - if it gets worse again or becomes a constant change issue I will report back.

Interesting to hear that quite a few of us have similar issues though.

 

[Eddy77]

Here's a link which explains the gearbox reset if anyone fancies trying it:

https://www.bing.com/videos/search?...8E33D89B57B855F3A5C18E33D89B57B85&FORM=VRDGAR

 

[anderoo]

I have the same engine and transmission as you and do notice it holds the gears significantly longer when cold, especially first to second. But that sounds a bit different to what you're describing.

Incidentally, I've tried that reset procedure a few times and honestly can't tell any difference once it's been done. Am I alone...?

 

[Crustyclam]

My ML had an issue in the past when cold, a jerking action would occur with the transmission when slowing / acelerating at a low speed, but only when cold. Once warm perfect. Had a software update that resolved the jerking action, but whilst cold seems to hold gear, and rev just before changing.

 

[markjay]

A TCU software update might not be a bad idea?

 

[Codger49]

As Markjay states, I would check with your indy whether a gearbox software update is available for your car.

My 7G box received a software update in 2015 and was noticeably improved - pretty much imperceptible changes thereafter.

 

[ube63]

Sorry about resurrecting this thread, I have started seeing the same symptoms as the OP recently. When outside temperature is really cold, starting in second gear in comfort transmission setting, engine flares up to 2000 RPM for about 1-2 seconds before the car moves. It only does this in the first 100 yards or so then all gear changes are good.

Does anyone know what the cause for this?