Over the last few weeks, my transmission has been quite jerky. When slowly coming to a full stop, the car will sometimes jerk into a stop. Then when I'm slowly driving in traffic (slightly pressing the brake pedal to control the speed), the car will also jerk. Here's a video of when I come to a stop and the jerking back and forth starting - https://imgur.com/a/JkFFWnR

I do have an appointment with the service department in a few weeks even though I just completed the B service, but was still wondering if anyone else had a similar issue and if so, what it turned out to be?

 

So it happen, when u slightly moving pressing brake on traffic light and the whole car vibrate, I dont know about the transmission but it could be the rotors started getting bad, I would suggest you to check the brake pads and rotors first

 

Forgot to mention that I replaced both front and rear brake pads within the last 3 months.

The car doesn't vibrate, it jerks back and forth. When I'm parking in reverse without pressing the brake, it also happens, so I don't think it's related to the brakes/rotors

 

How long have you had the car for?
You've never noticed this before I assume, what setting are you in C/S/S+?
The trans in the GT's are pretty bulletproof but take some getting used to. They also have adaptive learning where they "remember" that you were gunning it a few minutes ago and "think" you are still in that frame of mind, so to say.
Disconnect your battery for 10 seconds and reconnect, go for a drive in C mode see if stops the jerking you felt.


Edit: I looked at your video and did see that it went to 1st, then 2nd gear then back to 1st so you might have an issue with the speed sensor possibly but a scan of the OBD system should show a fault if there is a fault with the Transmission Module.

 

Had the car for almost a year and this is a relatively new issue. Happens in both C and S+. I'll try the disconnect tomorrow and will report back, thanks.

 

That makes sense, but how could you tell it went to 1st, 2nd and back to 1st gear?

 

Disregard, was looking at the Media Screen and that was the HP number i saw when i glanced.
Def. would try the battery disconnect first to reset the adapative learning, if that doesnt do it get it diagnosed, youve had the car for awhile now and know how it should be.
Good luck.

 

Ah yes, I put the HP monitor on the display to show it was jumping up and down without me pressing the gas or letting go of the brake pedal.

 

I keep watching the video clip for any clues so apologies for so many replies but looking at the RPM's I can see a fluctuation in there when you are stopped and that should not be happening.
The engine builds power and its all there, nothing feels out of place when you are getting on it?
Def keep us posted this has me baffled a little.

 

No worries at all, I appreciate the time and effort from you looking into it. I've spent hours researching different MB forums for similar scenarios and have gotten absolutely nowhere.

I know when the engine is idle - meaning the gas and brake pedals aren't pressed - fuel still flows into the engine which allows the car to inch forward. In the video, I had my foot halfway down on the brake pedal, so it should've been creeping very slowly but instead it was jerking like that. I guess it could even be a brake sensor issue since I got my brakes and rotors swapped recently, but if so, why is it only happening in 1st gear...

Tomorrow before I disconnect the battery, I'll connect my cheap OBD reader on it too and see if any codes come up, but I'm not holding my breath.

 

Hi,

Thought I would weigh in here as a couple of phrases in your initial post really caught my attention.

Driving a DCT is much different than a slushbox trans with a torque converter. A DCT is a manual transmission with a computer doing the clutch work and the gear choices. It uses several inputs to decide, not only where to be (what gear to be in) but anticipates what gear to shift in next. It uses:
Accelerator position %
Brake (pedal) pressure
Wheel speed/RPM

Let me try to describe a couple of scenarios where the driving habits you describe in your first post is tricking the DCT.

If you're cruising at a moderate speed 35mph and the trans is in, let's say, 5th gear and you decelerate, the computer is anticipating you are probably coming to a stop so it tees up a 4th gear downshift. As soon as RPM/wheel speed meet the criteria the computer executes and selects 4th gear. If, on the other hand, you go back to throttle, the computer needs to quickly switch back to an upshift anticipation to 6th. That isn't that noticeable given 4th and 6th are on the same clutchpack.

Now let's come to a stop. If you decelerate (let off the accelerator) and start applying the brake, the computer will anticipate the downshifts getting ready with the next lower gear. Using some brake pressure affirms that decision so it will begin to downshift. As more brake pressure is applied with no accelerator the computer continues with downshift gears. However when you decide to switch back to accelerator BEFORE coming to a stop, the computer has to react in the opposite direction it was anticipating.

Now lets get down to the situation you describe, (slightly pressing the brake pedal to control the speed) here the computer is teeing up a downshift, then by letting off the brake pressure, even going to a neutral, no throttle, now the computer realizes you're changing your mind and it must change to anticipate an acceleration and an upshift. If you're just creeping along (as in rush hour traffic) the DCT is switching back and forth between clutchpacks and with some engine torque involved this will be felt as a clunking as it disengages one clutch and engages the other one. Does that make sense?

The two worst things you can do with a DCT is -
When slowly coming to a full stop,
(slightly pressing the brake pedal to control the speed)

As both of these driving habits are common in stop and go traffic, but not very healthy for a DCT. Even when not in stop and go, I stress to folks who are new to a DCT to change their braking habits. It is much better with a DCT to coast longer then brake a little more aggressively coming to a stop, compared to "soft pedaling" it which is very normal and common with slushboxes and torque converters. Slowly coming to a stop with your foot dragging on the brake, eventually the clutch that is engaged has to start slipping to prevent engine stall. This extra dragging/slipping really wears on the clutches. However, when you brake more aggressively and reduce the time/distance the brake is engaged, the engine is doing more of the gradual braking but the clutches are staying fully engaged, then finally break loose which is much healthier for them.

To make matters worse, the DCT with adaptive learning is now setting gear changes based on lower RPM/wheel speeds with braking interspersed between lower gears. It is doing what is has to do to be in the correct gears, but with some engine torque at a lower RPM and the computer switching between clutchpacks back and forth is giving the clunking. I am almost certain about this. You could still have a tran problem or might have created one.

So, my advice would be - change driving habits. If you have to use a DCT in rush hour driving, especially constantly, expect this will become normal, and it is very hard on the clutches.

Clearing adaptive learn in the DCT can be helpful to getting you back on a clean track, however, disconnecting the battery won't clear these adaptations in a MB. If you're in the dealer already as you suggested, the tech can clear adaptations using Xentry.

Hope any of this makes some sense and is helpful to you,

Acta

 

Were you able to hook up the OBD reader.
I keep going back to your RPM's fluctuating while you are fully stopped. I dont think trans is the problem.

 

I can add that I had this exact issue and I added about a half a quart of trans fluid to the transmission and the jerking completely stopped.

 

Thanks for the well detailed response. It's quite eye-opening for me, and I definitely learned something new.


I did hook the OBD reader up and there were zero codes, as expected. I haven't had a chance to disconnect the battery but will be trying that next along with checking the transmission oil. Hard to believe MB didn't check the transmission oil during the Service B a few months ago

 

Thanks for the suggestion. Will be checking that asap.

 

Man i hope you get this figured out as it now has me perplexed too. I thought for sure there would be a code just because the way your RPM's were jumping but that could very well be the trans doing it.
While its hard to believe that MB Tech didnt check oil as he should have obviously done, it has happened that they dont if alll looks good and no leaks. Dumb i know.
Keep us updated thru your trials as I am desperate to find out what the fix is going to be. These trannies are weird as hell, at least from me as I never had a MCT before this car but of course you get used to them and how they act.

Good luck.

 

Hi newengineerhere:

Just a couple of points:

The DCT is a special service unit. It is serviced at a 4 year/40,000 mile interval and it does not fall under the service A/B intervals. So dealer would not be checking the fluid level in the DCT. Also, the DCT has a very special, very involved process to attain the correct fluid level. It is a "dynamic" fluid check, not static. The engine has to be running, the car in gear and the fluid at a precise temperature. If you're going to try this yourself, please know this; otherwise, you'll get the fluid level wrong. There is a sticky that has all the possible info on the DCT. It is long but has what you'll need if you want to check the DCT fluid level.

DIYers Doing the Transaxle Oil service - CAUTION - MBWorld.org Forums

On disconnecting the battery, not sure why you're doing this, but if it is to attempt to delete adaptive learn in the DCT it cannot be done this way. Adaptive learned shift points are stored in NVM, and not VM, so disconnecting the battery won't accomplish this. Instead, you have to intervene with a deep scan tool like Xentry, or a "step" process to delete learned shift points. Much like the oil service interval, which is also in NVM, it too won't change by disconnecting the battery.

Hope this helps,

Acta

 

Service A or B always includes all fluid checks. But I was at Benz yesterday picking up my wife's G wagen I asked my SA and he laughed at the question.
The techs are required to check all fluids on any vehicle they are performing A or B service. That includes any kind of transmission fluid whether they are servicing it or not.

I have read your detailed and extremely helpful guide on changing the fluid in the GT's DCT. I have yet to tackle that on mine but it looks like a lot of "fun".

As far as my recommending the old battery pull is that I was told by a tuner and also my AMG tech that services my cars that sometimes that will help and that it does put the ECM/TCM into a relearn process.
But they are human as well and may be wrong.

 

Hi,

Trust me, no way.
Not in this world.

Best regards

Stenzel

 

Hi guys,
@newengineerhere
@BBS63

I've been meaning to get back to this thread regarding the fluid level checks under service A, service B schedules for the GT platform with the DCT. I thought the best way to show that is to use the checklist the tech uses on each of these services. It is incorrect to expect that all fluids in the car are checked every service interval. In the case of the DCT, it falls under the "Z" service that has specific time/mileage intervals, so the tech isn't going to touch it or check fluid level.

In the AB columns the fluids that get checked are - brake fluid, PS, low temp circuit coolant levels.

Then look at the Z column -
Service 3 - Engine oil/filter change at every service interval (both A and B) so the oil level would be checked every time.
Service 20 - This is the DCT service interval. It is a special service unit that is not going to get touched (fluid level check) until 4 years, roughly 40,000 miles.

Hope this helps. I know it can be frustrating if you think you're paying for a service that didn't get done. But, trust me on the DCT, the time/effort JUST to check the fluid level will easily add two + labor hours (at dealer rates) to your A/B service bills. Most of us would choke at that bill!

Best,

Acta

 

Have dealer do a clutch adaptation to start…..

 

Thank you everyone for taking the time to chime in with useful information!

Thought I'd share a quick update after taking the car to a MB service center - they couldn't replicate the issue and the AMG tech is stating everything is fine. They're checking the fluid level this weekend and I should have the car back in a few days. Sucks that they couldn't use the video to diagnose the problem.

They did find something else that's not really related. A few weeks back I had an independent shop that claimed to specialize in MB AMG and other exotic cars change my rear rotors and brakes. It turns out they applied the brake pads incorrectly on one of the wheels and put the inner pad on the outer side of the rotor. I'm pretty mad about it but glad they caught it.

 

How does it idle - does it hop at all like it is with the video? It's dropping pretty low too - 500 RPM feels like some anti-stall input might be happening too. You might have a vacuum leak somewhere. Although, you would think that the MB mechanic would catch that.

 

??? I've never seen nor heard of left/right (inside/outside) pads in AMGs ... what vehicle is this? Is this just something I've not yet encountered (or missed)?

 

Whilst the pads themselves are identical, the backing plates may well have an inside and an outside due to which side the brake wear sensor is supposed to be located. I can’t remember from when I changed the pads on my GTR, but iirc the sensor attaches to the inner pad via a specific cutout in the backing plate, which makes sense as the inner pad tends to wear at a quicker rate than the outer.