94 W124 E220 Auto - Jerky gear changes

[jeffreyli86]

Hello all,
Having just solved the exhaust problem, my beloved W124 has developed a jerky gear change. This is especially noticeable on part throttle, gear change from 1st to 2nd, and when the car has not fully warmed up. It does it on the change from 2nd to 3rd too but less dramatic.
I've changed the various mounts and other rubber shock absorbing components in the drivetrain and have narrowed it down to the gearbox itself.
When the car has fully warmed up (say after 10mins of easy driving), the gearbox changes gear flawlessly.

Another problem, which may or may not be related, is that my car used to "soft kick down" flawlessly on full throttle but NOT engaging the kick down switch. For example, you are cruising at 35mph and the box is in 4th gear, on full throttle WITHOUT engaging the kick down switch it would change into 3rd, but once you engage the kickdown switch it would engage 2nd.
Now, it would refuse to change down in the same circumstances unless the rpm in 4th gear is below 2000rpm.. Otherwise you have to engage the kickdown switch to get it to change down.

Does anyone know what is up with my car?

Cheers,
Jeff

 

[television]

First check the vacuum system on the box, pipes come off or split

http://www.detali.ru/cat/oem_mb2.as...M=717.413&CT=GA&cat=09S&SID=27&SGR=195&SGN=09

 

[jeffreyli86]

First check the vacuum system on the box, pipes come off or split

http://www.detali.ru/cat/oem_mb2.as...M=717.413&CT=GA&cat=09S&SID=27&SGR=195&SGN=09

Thank you for the quick reply. I thought it would be vacuum related at first however, the gear changes when the box has fully warmed up is totally flawless!! Would a vacuum leak cause that?

Also, I just remembered, with the A/C on, the gear changes are more jerky too... Any ideas?

 

[television]

Thank you for the quick reply. I thought it would be vacuum related at first however, the gear changes when the box has fully warmed up is totally flawless!! Would a vacuum leak cause that?

Also, I just remembered, with the A/C on, the gear changes are more jerky too... Any ideas?

Then do a fluid change along with the filter, its worth doing, every one that has done it comments on the smoother changes.

It still could be vacuum related, when the engine is at cold idle the vacuum is not as high, it a matter of checking what you can really, the air con also shares some vacuum pipes this is all of it


The links open after BM half way down

http://www.detali.ru/cat/oem_mb1.as...960&GA=722.428&GM=717.413&CT=F&Cat=15T&SID=80

 

[jeffreyli86]

Then do a fluid change along with the filter, its worth doing, every one that has done it comments on the smoother changes.

It still could be vacuum related, when the engine is at cold idle the vacuum is not as high, it a matter of checking what you can really, the air con also shares some vacuum pipes this is all of it


The links open after BM half way down

http://www.detali.ru/cat/oem_mb1.as...960&GA=722.428&GM=717.413&CT=F&Cat=15T&SID=80

Cheers. Fluid change has been done some 2000miles ago. I religiously change the ATF every 5000miles or so.
I will check the vacuum pipes.
My mechanic mentioned that the valve body may have gunk in it.. Could this be a cause?

Thx a bunch.

 

[jeffreyli86]

The links open after BM half way down

http://www.detali.ru/cat/oem_mb1.as...960&GA=722.428&GM=717.413&CT=F&Cat=15T&SID=80

Thank you for the link. Does that diagram show all the vacuum lines there are to the car? If so, I would check all of them and change if necessary.

 

[television]

Thank you for the link. Does that diagram show all the vacuum lines there are to the car? If so, I would check all of them and change if necessary.

OK here is the vacuum box


http://www.detali.ru/cat/oem_mb2.as...M=717.413&CT=GA&cat=09S&SID=27&SGR=045&SGN=03

 

[Number_Cruncher]

The other thing that springs to mind, after checking the vacuum pipes, but perhaps before an oil and filter change, would be to check and adjust the settings of the throttle cable and the control pressure (sometimes reffered to as Bowden cable, or kickdown cable) settings.

If the engine now sits differently on it's new mountings, it possible this needs re-adjusting.

 

[television]

The other thing that springs to mind, after checking the vacuum pipes, but perhaps before an oil and filter change, would be to check and adjust the settings of the throttle cable and the control pressure (sometimes reffered to as Bowden cable, or kickdown cable) settings.

If the engine now sits differently on it's new mountings, it possible this needs re-adjusting.

That is a good point often over looked as one never knows what has been twiddled with over the years

 

[jeffreyli86]

I have actually been fiddling with the bowden cable the past few weeks (ever since the problem was getting annoying)..
Tightening the bowden cable will just make the box change up later (higher in the rev range) however it still refuses to shift down on full throttle but kick down is fine..
I have been careful not to fiddle with the modulator valve just in case softening the gear changes will cause my brake bands to wear out prematurely (this might be a misunderstanding by me)..

Will check the vacuum tomorrow..
On a separate note though, if I decide to change the modulator valve, would the garage have to measure the vacuum carefully or should they just adjust it according to the shifting "hardness"?

Cheers

 

[television]

There are no adjustments on this box,,on earlier car (much earlier) the is(or was)

 

[Number_Cruncher]

>>would the garage have to measure the vacuum carefully or should they just adjust it according to the shifting "hardness"?

No, they wouldn't have to adjust anything to do with the vacuum, but, they would have to set the modulator pressure (by adjusting the Tee handle on the valve). To do this properly, you need to temporarily attach a pressure gauge onto one of the gearbox test ports, and drive with the gauge in the car via a long pipe.

Don't fiddle with the modulator valve unless you've eliminated everything else, and don't change the modulator valve unless you're sure that it's faulty. There's not too much in them to go wrong.

There is a link between bowden cable settings and the firmness of shift, although it's not as strong as via the modulator. I would advise against "fiddling" with the bowden cable, and I would return it to the recommended adjustment.