CVT Noise

[MickyP64]

This may seem like a really dumb question, and probably is!

Can anyone tell me if the belt inside a CVT transmission will turn if you have the car in ‘Park’ and you gently rev the engine?

I have a faint whining sound when driving at low speeds (up to 20/30 mph). It’s not loud and I’ve assumed it’s coming from the gearbox. So I’ve Googled the symptom and read so much conflicting opinion that I now don’t know what to think. I’ve read varying opinions that range from - it’s quite common for a CVT to make this kind of noise right through to it being potential gearbox failure.

So I put the car in park and revved the engine gently to see if the noise was still there when the car wasn’t moving – and it was. I’m now wondering if it’s the gearbox or something else in the engine bay that could be making this noise.

I’m not overly worried, the car seems to drive well and the sound hasn’t got any worse…so far…

 

[grober]

I believe that box has a torque converter meaning its never totally "disconnected" from the engine. These boxes do whine a bit evidently. http://www.mbclub.co.uk/forums/engine/113116-b-class-cvt-transmission-failure-4.html

 

[MickyP64]

Thanks Grober,

I recognise you from my failed ‘getting Mercedes Benz to replace my rusty doors experiment’

I get the feeling this is just a quirky feature of a CVT gearbox, nobody seems to have a definitive answer – I read somewhere that it was the noise made by the planetary gears (possible I suppose).

The car tends to sound a bit like and electric milk float or makes the kind of whine that you get if you reverse at speed in a manual car, (though low level if you get the idea). I’ve got used to it and I’m not going to worry myself over it unless something catastrophic happens.

I’m curious since I can’t seem to find any document or thread that points to a conclusive reason for it – plenty of people with a similar problem/question though…not just MB related…

 

[grober]

It might pay to check the gearbox service history and whether the fluid has ever been changed- or that its level is correct. - not sure how that is done to be honest- it may be "sealed for life" but fluid loss can occur.
ps other sources for transmitted noise in that area can come from the CV [ constant velocity] joints in the drive shafts but not obviously when the car is stationary.

 

[MickyP64]

The car has full MB service history and I’ve had a breakdown from my local dealer of all parts replaced on the car before I had ownership…the one interesting item that has been replaced a couple of times is an oil seal which I believe is related to where the electrical connections enter the valve body…I get the impression this is a known weak point for leaks (might be worth a quick check).

This car has suffered the dreaded speed sensor failure since I’ve owned it, and has had a brand new valve body fitted as a result, luckily for me the cost was picked up under the warranty supplied by the dodgy dealer I bought it from. I had the work undertaken by a very reputable indie so I know that the seals and transmission fluid were changed about seven months ago. I believe that the transmission fluid for the CVT has to be strictly the correct MB specified stuff and changed every 35 000 miles, which appears to have been done according to the service history.

I revved the car in park mode in order to eliminate those items (CV joint etc.) that might make a noise when the car is moving…