Strange one.

[kwk]

Since buying my 2001 230k CLK about 5 months ago, it has always had an issue that, when cruising on a motorway/dual carriageway, at about the legal limit, the auto box, for no reason would change down from 5th to 4th gear. I could normally select 5th manually after a mile or two but, if left to it's own devices, it would remain in 4th. If I selected cruise control, it would instantly go back into 5th. If I used cruise control for long distances, the problem did not occur.
Various sources including this forum and the local Indy suggested that the problem was actually the maf. A local auto transmission specialist also suggested the maf and I was about to change this for a genuine part.
I have always run the car on the standard 16" wheels with 205-55-16 Khumo tyres. Today I changed to 17" wheels with 225-45-17 Pirelli tyres. I have just taken the car on a 60 mile motorway run and, no gearbox issue. I have done nothing other than change the wheels. I know that the previous owner used 17's but put the 16's on to sell the car. He said that he had no problems with the 17's.
I am not counting my chickens just yet, but has anybody got any ideas?

 

[Grey Area]

I'm completely speculating here but, these gearboxes are supposed to be kind of adaptive to a given persons driving style in terms of their shift patterns and acceleration, or so I've heard. So I guess it's theoretically possible that the smaller wheels (provided that the overall diameter of the wheels and tyres were different to the 17s) somehow made a difference to the gearing... --my god that's vague --

If you want to be sure, put the smaller wheels back on, see if the problem comes back, if it does, then disconnect the battery for an hour or so (this resets the gearbox apparently) re-connect, then drive it again and see if it still does it.

 

[DrNick]

I'd still suspect the MAF. If the MAF is giving slightly off readings then the engine would be down on torque and the gearbox may well shift down to compensate. Different sized wheels may give a slightly lower total gear ratio and this may be just enough for the gearbox to hold the gear. If the MAF is slowly failing then you will eventually get downshifts with the alternative wheels as well.
Does it need much encouragement to shift down when accelerating?

 

[kwk]

No, it's fine shifting down and all the other gearbox functions work correctly. Everyone I have spoken to has said that this is not a gearbox issue but some other sensor, the favourite being the maf for the reasons that you have just given. I will keep an eye on it and report back with developments.
The car was standing idle in the previous owners garage for over a year before I bought it.

 

[Grey Area]

Different sized wheels may give a slightly lower total gear ratio and this may be just enough for the gearbox to hold the gear.

Much more comprehensible than my ramblings. Thank you.

 

[Lyserman]

The different diameters between the two wheel sizes given by the OP is only a change of 2mm or 0.32%. Well within the recommended maximum difference of 2.5%. So that should have had no effect.

 

[rifiki]

Have just changed the wheels on my CLK from 17 to 16 inch, much better ride and no gearbox problems. The rolling distance is about the same.

 

[renault12ts]

It's mere coincidence...changing the wheel size will have had no affect on the gear box.

 

[DrNick]

I agree that is shouldn't have, but it does seem to have made a difference though! Its implied that the "fault" moves with the wheels so it can't be assumed that its a coincidence. (OP could swap wheels back and see, if he could be bothered). Something associated with the wheels is causing the issue. Granted the diameter difference is small, but that difference could just be on the threshold that causes the downshift. The other main difference is the tyre itself which will have an effect on friction and this may impact on the torque requirements at a given speed. The jury seems to be out on whether a wider tyre is better or not for economy purposes as there is so much variation in other parameters between different tyres. It could just be that this particular make/size of tyre at this inflation pressure has more rolling resistance than the originals, and so triggers the downshift.
The OP could try overinflating the tyres temporarily and see if the problem gets less. Not sure if its conclusive, but it tells us something if the problem goes away.

 

[kwk]

I know it's an old thread but I never posted a final update.
The car is now sold. I used it every day and covered thousands of miles with absolutely no problems. The gearbox problem never returned and I was not going to tempt fate by fitting the old 16" wheels.
If the new owner is on here ( I sold it about 8 months ago ), you've got an excellent car.