GLA gearbox/drivetrain 'shunting' - what did they do?

[sidewaze samm]

My car is a GLA 250 4matic auto, 2014, 55k miles.

Over the past two months or so I've noticed symptoms that are getting worse (or I'm becoming more aware of...). When driving at a constant slow/moderate speed (say, under 25mph) I feel and hear what I can only describe as 'shunting' in the drivetrain. It seems to come from the centre of the car to the rear somewhere - not from the front. The noise/feel is subtle and disappears in the increased road noise at higher speeds. It is linked to road speed and at constant speed is fairly regular, but not totally. It just feels like there's a tiny amount of slack in the drivetrain that is being taken up and then relaxed again, even at steady throttle opening on a flat road.

So now the odd thing...
Four days ago, I went in to MB for MOT and mentioned this to them. I asked whether someone could just drive round the block with me, to advise whether there is an issue or whether "they all do that". (I've had the car for nine months now).
My car goes off to the workshop for MOT, and returns two hours later. I'm told the technician took it for a test drive and could see no problem. "Sorry sir, but we're much too busy now for the technician to go out in the car again with you..."
So, somewhat bemused, I drive home thinking "the car will be due a service before Christmas when the warranty runs out, so I'll take it somewhere else".

But on the journey home and in the week since, there has been no repeat of this shunting issue, None at all.

So anyone got any ideas on what they did to my car ? I just do not believe it's a simple coincidence.

cheers

 

[malcolm E53 AMG]

Sounds propshaft/driveshaft related the tech may have found one of the couplings slightly loose an easy fix 5 min job

 

[John Laidlaw]

Do you normally do a lot of stop start town type driving?
Suspect the tech gave it some beans on the open road..
However, check with them (or your paperwork/service history) whether the ATF has been changed or due to be.....

 

[sidewaze samm]

Sounds propshaft/driveshaft related the tech may have found one of the couplings slightly loose an easy fix 5 min job

That was an initial thought - but I was told very clearly that the tech took it for a spin and found nothing wrong. If something was done (bear in mind the car was having its MOT) then why not mention it?
Conspiracy theory - is there a related issue that MB would rather people didn't know about?

 

[sidewaze samm]

Do you normally do a lot of stop start town type driving?
Suspect the tech gave it some beans on the open road..
However, check with them (or your paperwork/service history) whether the ATF has been changed or due to be.....

Yes I spend a fair bit of time on local, rush-hour driving. But I'm also known for using a fair bit of the 200bhp, and regularly use motorways too...
Isn't the gearbox supposedly a sealed for life affair?

 

[ajlsl600]

bizarre was thinking something to do with the TC . but if it went away ? anyway i would confirm yr visit ,yr request ref this issue in writing and request an in writing reply. send it recorded delivery. as you say warranty soon out and if no proper record you leave yrself open to "no one said anything, we dont know anything about the matter "

 

[sidewaze samm]

bizarre was thinking something to do with the TC . but if it went away ? anyway i would confirm yr visit ,yr request ref this issue in writing and request an in writing reply. send it recorded delivery. as you say warranty soon out and if no proper record you leave yrself open to "no one said anything, we dont know anything about the matter "

My thoughts also. I've now booked the car in again, as the engine management light has come on now...

 

[LostKiwi]

Isn't the gearbox supposedly a sealed for life affair?

MB gave up on that theory years ago.
Depending on the box it may be 37500 miles between services or more. None are sealed for life.
All that meant in the old days was "Sealed till it dies", usually at great expense to the final owner.

 

[ajlsl600]

MB gave up on that theory years ago.
Depending on the box it may be 37500 miles between services or more. None are sealed for life.
All that meant in the old days was "Sealed till it dies", usually at great expense to the final owner.

doubt that policy bothered benz much as averages would have them all out of warranty.

 

[John Laidlaw]

As above sealed for life was a flawed policy, needs ATF changes like every metal mechanical part, simple !

 

[sidewaze samm]

Update -
So, I've finally got the car booked in for service and for this vibration to be looked at. Again.
This time I went for a drive with the tech - and he agreed with me... a subtle, irregular, speed-related vibration from the rear.
Left the car with them overnight and have just received a call telling me the cause is my front tyres - cracking slightly in the tread (not bad though, as it's only just been through MOT), and I need new front tyres! Current tread depth around 4mm+ with even wear.

Really!??

I'm struggling to believe the small cracks in the tread will cause any issues at the moment at all, let alone a 'chunking' from the back end (nothing present in steering or at front end at all).

Any opinions?

 

[malcolm E53 AMG]

Are the wheel fitments staggered if not swap the tyres front to back and see what difference it makes.

From your description of the fault it could be a 4 wheel drive issue

The vehicle usually drives from the front wheels and the rear axle electronic clutch only operates to provide 4 wheel drive when conditions dictate, it may be this clutch operating that you can feel especially if it’s faulty and the conditions don’t require it

 

[Wighty]

Sounds like you will be out of pocket for 2 front tyres when you next take the tech for a drive round the block . To find the same problem .
Keep the 2 tyres when they change them ?

 

[Blobcat]

Very much doubt it’s due to small cracks in the front tyres.

 

[Botus]

I can imagine its the same issue as as other models, a result of fantastic (ly bad) engineering when they created the RHD versions

the GLC is well known for shredding its tyres and making the owner look like an embarrassed fool. It becomes more prominent as the weather gets colder. They mask with a set of free all season tyres. Its to hide the fact the steering geometry on a lock is nothing like it needs to be (or indeed is on LHD cars)

 

[Botus]

https://forums.mercedesclub.org.uk/index.php?threads/temperature-related-juddering-on-glc.157957/

https://forums.mercedesclub.org.uk/...re-skipping-letter-headed-redacted-jpg.33375/

 

[sidewaze samm]

So, after a long break, an update:

I didn't believe the tyres were to blame and was not prepared to spend almost £200 per tyre replacing pretty new tyres, only to discover that this had nothing to do with it at all. So I did nothing.

The car continued to drive with the same 'shunting/knocking'.

18 months later, the front tyres were getting a little low on tread so I swapped them for more MOE tyres. No difference experienced.
Another 6 months later and I thought I'd bite the bullet and replace the rear tyres, just half-worn.
When I drove away from the tyre depot, the 'shunting' had stopped! I thought it must have been the tyres all along. But no! After a day or so it returned.

I then returned to the MB dealer, and repeated the history of the issue, adding that rear seat passengers could feel the 'shunting/knocking' beneath them, and that it seemed to me as if the car couldn't decide whether to be in FWD or AWD mode. Also I commented that their previous investigation when they swapped wheels and when I later had new rear tyres fitted, appeared to have one thing in common - raising of the car chassis without the suspension being supported resulted in temporary cessation of the symptoms.

So far they've had my car for nearly a week, and in that time have identified that the noise was being caused by the fuel tank (!!??) which they replaced. But no change (I'm not surprised).
And now I'm informed they have identified the rear diff as the culprit (and I'm not surprised by that either - as that is essentially where I was pointing them to by my full description when I left the car with them!)

At the moment I'm waiting to hear about the warranty claim and when they can get a replacement part, and when I can expect my car back. If it doesn't go well, expect fireworks.

 

[Steve@Avantgarde]

So I am pretty sure I have had this issue before on the B class and the fault was the shock absorbers. The faults described were a hardish ride and a resonace from the rear but not a knock. We found the rear tyres to be severly stepped on the inside edge and shaped like 50p pieces.

I believe there was a technical document for rear shock replacement causing resonance that I read at the time which caused us to go down the route of replacing the shocks and tyres. When the shocks came off the car they had next to no rebound. The car never came back so ill assume we fixed it.

I wouldnt be surprised if its something similar with yours given one of the effects you have is tyre wear...

 

[sidewaze samm]

I wouldnt be surprised if its something similar with yours given one of the effects you have is tyre wear...

Think you've misread my posts - my only references to tyres are that originally the dealer claimed it must be tyres causing the issue, and that I was reluctant to change tyres as they were worn so little. When I eventually had the half-worn rear tyres replaced recently, I made sure the tyre depot inspected them - their opinion was that both tyres were equally and evenly worn and that there appeared nothing wrong with them. They were retained to send back to Goodyear for inspection/refund should they prove to be the cause of my issue - which has now been ascertained they're clearly not.

The ride/handling is not compromised, there is just a 'shunting' thunk that feels as if slack in the drivetrain is being taken up then released. Really quite noticeable when driving up hill at low speed.

I'm not familiar with the intricacies of MB's part-time AWD system, but my layman's opinion has always been that the fault is connected with the prop shaft/diff or related ancilliaries, whether mechanical or electrical.

 

[Botus]

it could be a similar issue to the GLC, the engineers won't have changed,

if they could do it on those they'll do it again. I suspect the reason you are noticing is because the weather is cold - on the GLC its entirely due to the faulty suspension geometry the engineers built it with for RHD markets. It causes the tyres to hop and scrub - the official fix was a set of softer compound winter tyres that made less noise as the suspension abused the tyres when using significant amounts of steering lock

if its doing it on tarmac, drive on to soft grass, if it instantly stops, its highly likely the way they built it - if its still there, something else needs to be checked