Cruise control won't select 9th gear...common?

JasonHookey

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Hi All,

I am absolutely livid with Mercedes (Harrogate mainly) right now and wanted to see if my problem is common before I explode!

So I have a 2017 C250d 4matic, 9-speed gearbox. Lovely car, driver assistance package.....sweet. Got is as I live 140 miles from my office (yep, each way, don't ask) and wanted something nice but reasonably economic for the commute.

It was all going great until recently. Last service, they updated drive train software and engine management. Since this, when in automatic and cruise control at 70mph it won't go into 9th gear!!! My local dealership (Harrogate MB ) did some tests and said car was fine (although did almost no testing at 70mph) and that the car went into 9th at 76mph...which i pointed out was illegal!

So I did some further testing and it's weird. So using paddle shift I can manually go into 9th at 70mph and if I manually down-select to 8th the car RECOMMENDS I go into 9th! As soon as it drops back to automatic this up-shift recommendation goes and I stick in 8th. My local dealer said this was 'different software' using 'different sensors' but that seems strange to me????

They then tried to tell me it was the same economy - they'd done a test using a pump measure of fuel in, a drive (not motorway at 70mph I add) and then filled it up and calculated it. Since this came out at 48mph they were happy - me pointing out that at a 56mph average it WILL do 48mpg and my problem was at 70mph didn't land with them and they then told me the reading on the dash is not appropriate for measuring fuel consumption. I get it's an average and it's not 100% accurate but if I do the same drive, at the same time in comparable conditions 100's of times and since it now won't select 9th it's showing 5mpg lower, calibrated or not that large data set show's there is a comparative problem.

So to me it was simple - if the car, at 70mph is revving at 1800rpm for 2hrs in 8th on the motorway but I manually put it 9th and it drops to 1100rpm, this will burn the same fuel will it?? They are trying to fob me off and it's really getting to me.

The final thing is that apart from maybe 1 other person and me, this isn't a common problem to them.......so I want to test this.......

Please, if you have a similar 9th gear issue, can you please reply to this threat telling me what your issues are? I have no idea if I'll get anywhere but it possibly will (a) make me feel a bit better and (b) help me prove to Mercedes they have a real problem here.

Really long post for a 1st one but my blood is boiling here and I want to know if I'm nuts or I'm right and there are lots of us in the same boat!

Thank you for any advice or confirmation you've got the same problem.

Jason
 

greenmanslk

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1100rpm is barely above tick over so I am not surprised it wont engage 9th gear.

What did the car do before the update?
 
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mersum1es

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You won't see 9th (same thing 7G highest gear is 6, 5G 4). It is not forced gear but highest allowed gear unless you push kick down. Otherwise car uses lower gears than seen in display if torque cannot be realized with selected highest gear.
 

artyman

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A drop from 1800rpm to 1100rpm between 8th and 9th seems a big drop to me. I have an old 5 speed box and the drop between 4th and 5th isn't 700rpm at 70mph, so I would have thought that the gear ratios on a 9 speed box would be relatively close together, with just a few hundred rpm between each gear.

I suspect this 'update' is all to do with the emission scandal.
 

markjay

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I don't think that the dealer's response that the transmission will shift into top gear at 76mph is inappropriate in any way, i.e. if that's how MB designed the car then that's how it is.

That said, if I understand correctly then your complaint is that it wasn't like that before the software update was applied?
 

markjay

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EU Directive 98/69/EC says:

'“Defeat device” means any element of design which senses temperature, vehicle speed, engine RPM, transmission
gear, manifold vacuum or any other parameter for the purpose of activating, modulating, delaying or deactivating
the operation of any part of the emission control system, that reduces the effectiveness of the emission control
system under conditions which may reasonably be expected to be encountered in normal vehicle operation and use.'

I don't quite see how shifting-up early can be seen as a 'defeat device'....?
 

leef44

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Around 1800 rpm at 70 mph sounds about right to me. However with 9 speed diesels, perhaps they are happy to go lower, cruising on the motorway.

But then what do I know, my Suzuki has 5 gears and my SLK has 7. That's enough for me:confused:
 

Smart320

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And my SLK also has 7 gears and I think it's plenty, as my wife's 54 plate petrol Rav4 Auto only has 4 gears. Mind you it does only about 25 to the gallon at 90 mph on the motorway ( but very nice and smooth it is too ! )
 

TeddyRuxpin

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I don't think that the dealer's response that the transmission will shift into top gear at 76mph is inappropriate in any way, i.e. if that's how MB designed the car then that's how it is.

That said, if I understand correctly then your complaint is that it wasn't like that before the software update was applied?
Yeah, it's the same issue as on my 7G car, since the emissions update. I don't understand how running higher revs at this speed helps NOx emissions, but I am not an engineer.
 

Mikesmith29

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Do we know for certain that this update is to do with the NOx?
All my paperwork says is powertrain update, could be a gearbox update.
I have the C300 hybrid so it may behave differently.
 

markjay

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The suggested connection to Dieselgate is because it seems that only Diesel cars get this update.
 

BTB 500

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So to me it was simple - if the car, at 70mph is revving at 1800rpm for 2hrs in 8th on the motorway but I manually put it 9th and it drops to 1100rpm, this will burn the same fuel will it??
I don't understand how running higher revs at this speed helps NOx emissions, but I am not an engineer.
I'm not an engineer either but I suspect the combination of rpm and throttle needed in 9th gear could actually be less efficient at that speed. All down to the engine's power & torque curves I think. A bit like going up an incline in top gear (low rpm) with your foot flat on the floor vs dropping a gear and backing off the power.
 

TeddyRuxpin

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Sure, I can’t speak for 9G cars, but for my 7G car, the revs are higher and it’s definitely less economical. Same with a 7G+ car

However on another forum a user has said that their car with same engine and 9G box as the OPs, goes into ninth gear at 65 miles an hour. The owner has not had the update applied to their car.

Furthermore it ‘wasn’t like that before’ and the cars are reporting that they’re less economical.
 

grober

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You have to think "NOx " above all. This involves the EGR valve, which involves recirculating exhaust gases back into the manifold to lower combustion gas temperature = lower NOx without adversely affecting driveability. To this end typical figures appear to be between 5 and 35% EGR. There are operating thresholds as to when the process kicks in and RPM may well be one of them. A higher gear at a lower RPM may require -for lack of a better word more engine "grunt" to generate more torque which may adversely affect fuelling/combustion generating more NOx. In engineering terms it might be described as the relationship of degree of throttle opening to engine rpm to generate sufficient engine torque in a particular gear. Sort of leading and trailing throttle phenomenon ???
Remember this is a "fudge " to avoid breaking NOx emission regulations
 

TeddyRuxpin

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On full throttle, EGR is not used. It's used a lot more at idle and low RPM. So for example, if you map out the EGR via software (via a tuning company), you won't get anymore BHP at full acceleration. I actually called up some tuning companies to double check that.

However, I have no idea if the EGR to throttle relationship is roughly linear or not - so if you're at idle you're using more EGR, and then the higher the RPM, the less EGR until it's not used. Or, perhaps it's based on the throttle.

But it's still not intuitively obvious to me why increasing the speed/RPM when the final gear ratio is selected - and bear in mind it is only in this gear, not when going from 4th to 5th - would help reduce NOx.

...and it may be the case that we'll never know.
 

John N

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As far as I am aware my car hasn't had any software update, but I have always had to Manually Select 9th Gear under 74 MPH!

It has been improving since the first Servicing at 15,500 miles, but I put that down to the Engine "Loosening up" Unless I am on a road that is completely flat or on a slight slope I have to "Paddle" to get 9th at 70MPH and that only happens if the Engine is running fully warm!
 

TeddyRuxpin

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As far as I am aware my car hasn't had any software update, but I have always had to Manually Select 9th Gear under 74 MPH!

It has been improving since the first Servicing at 15,500 miles, but I put that down to the Engine "Loosening up" Unless I am on a road that is completely flat or on a slight slope I have to "Paddle" to get 9th at 70MPH and that only happens if the Engine is running fully warm!
Hi John, what year is your car, and how long ago did it last go into a MB garage for a service? They probably applied the update without saying anything, they've been rolling out these updates for a while, whenever a car is taken in for a service. That's what they did on my dad's car, for example, they didn't mention it.

Of course, most people don't care, don't notice fuel consumption or RPMs, and have no idea what RPMs their cars should be at for a particular speed... or what gear they're in, in an automatic.
 

Rorywquin

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Mine had a sw update last April. Still goes into 9th in C & CV.
 

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