Carsy

We have just completed a 16 hour round trip to Brisbane, Queensland.

When the gearbox warms to operating temperature a high pitch whine occurs.

It sounds as if it could be some sort of resonance.

I am sure it is from the gearbox as the sound changes a little when the gear box is shifting gears.

The operation of the gearbox is perfect.

I stopped the car for a short period & on restarting there was no sound present.

The car has done 49,000 km & is out of warranty.

I noticed on this trip the fuel consumption was abnormally low ie 4.7 litres /100 km. ie 60 mpg Imperial or 50 MPG US. Whether this has anything to do with the annoying sound I do not know. I have changed tyres to Michelin HP Primacy so they could be the reason for the improved consumption.

I would be very grateful of some advice & to know if anyone else has experienced this noise with the 5 speed.

Thank you.

Glyn M Ruck

Oh dear JC I hope it does not sound like this:

https://mbworld.org/forums/c-class-w...-oil-pump.html

Not common on 722.6

I have one theory as to your problem & why it went away on a reboot. I think your Torque Converter clutches stuck fully engaged. Likely just TCU brain fade.

Carsy

Hello Glyn, Thank you for the advice & sound clip.

No, that noise is far too loud & rough/coarse & I do not hear it at idle & at standstill. It is more of a fine harmonic type of sound.

That fuel consumption of 4.7 l/100 km has never been anywhere nearly achieved before. Under the conditions yesterday I would have expected about 5.3 L /100 km. That is about a 11 % improvement which is crazy . Its is not just the instrumentation ,I can comfirm that I have nearly half a tank of fuel left after 800 km!!

I noise is not heard on 50 - 100 km trips ,but starts about 2 hours into a long trip.

Your TC clutches sticking idea is an interesting thought. I understood the clutches locked in when under way in top gear to provide better fuel economy. We did notice that the engine did labour a little more than usual in the 60 to 80 km speed range, indicating that the " slip" was not there.

If it is a TCU brain fade , then it did it twice on the way up & then again on the return journey.

I would be very pleased if you could explain to me about this sticking clutch possibility. I wonder whether it is a mechanical or electrical problem ?.

Thanking you,

JC

Glyn M Ruck

JC It could be either electronic or mechanical. Step one would be to get them to reflash the TCU & redo the adaptions with Star. Pretty simple. I might tell you that these cars do have a natural harmonic at 80 to 90 Km/h depending on the car. The NVH is not perfect. It is very slight though. Most people don't notice it & with a radio on even at low volume you would never pick it up. If the C (closed position) ever intrudes into the parameter B (slipping) in any gear you transmit vibration. Especially with a diesel.

Carsy

Thanks Glyn,

The sound is there at & above the 80 /90 km harmonic.

It does quieten down above 120 km/hr.

I cannot help thinking there is a connection with the decreased fuel consumption.

Interesting list of variables.I am wondering whether oil temp is a factor which may trigger the noise. . I have fibreglass fly wire across the grill to prevent insects & grass seeds entering the radiator /heat exchangers which may inhibit some air flow . The large fan should make up for this.The engine temp never deviates .Attachment below. The day was not hot ..24'C.

I appreciate your help,

JC.

PS. Good luck with the World Cup Rugby match tomorrow !!! It could be tight !!

Glyn M Ruck

Indeed it could be tight. Good luck troubleshooting. The fact that it comes & goes tends to right off a mechanical gremlin (I hope)

Glyn M Ruck

Well we messed up 11 - 9 Congrats!

Carsy

The Boks were superior for 80 minutes. Can't believe the score line. Just dogged defence & luck won it !! This weekend will be a nail biter too. !!

I took the car to the local MB mechanic today & we test drove it. No ATM noise present at all . I checked for codes with my scanner ... NIL. He did not consider re flashing worth while at this stage. I was not charged !

He suggested to monitor it & bring it in when it is whining.

I bought the Fuchs 4 134 transmission oil today . Minimum size 20 litres for A$386.Enough to last me 5 changes !! should see me & the car out !!

Thanks for you help.

Glyn M Ruck

Well - the captain & the coach have resigned. Pleased your box is silent. The 722.6 is bullet proof. The only failure on early 722.6 transmissions was the 1st to second gear one way clutch in taxi service at huge mileage. Just too many high torque changes loaded. Long ago fixed which is why AMG used it for so long. With all the extra power you could not break it.