W124 300D violent downshifts

roadhog

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I've been experiencing increasingly violent downshift from my gearbox of late particularly when coasting to a stop at slow speed just before arriving at a roundabout for instance. There's a definite thud being transmitted through the body of the car as (I assume) the box grabs a lower gear ready to move off again. This doesn't show as much (if at all) if I've selected 3 but happens every time I'm in D. Shifting down when accelerating below 60 mph is also very violent with the whole car squatting down before continuing at the same sedate pace, quite embarrassing.
I first noticed this after changing the engine mounts albeit with much lesser symptoms and this leads me to think it may not be the box but rear the subframe mounts, although I can't really see much wrong with them on a visual inspection. Ideas anyone?
 

television

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I would check the vacuum pipes that go to the box
 
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roadhog

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Will do Malcolm but the box and the vacuum pressures were checked when I had the recent scare with it and they were spot on apparently. Would a tired propshaft doughnut give symptoms like mine?
 

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Will do Malcolm but the box and the vacuum pressures were checked when I had the recent scare with it and they were spot on apparently. Would a tired propshaft doughnut give symptoms like mine?

I cannot see that they would or could would be better,,poor hard changes have mainly been associated with the vacuum system on these boxes could the modulating valve be at fault
 

brianbrian

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Put it over a pit and get some one to put it in drive then reverse to see what movement you have in various places of the car. Hope this helps
 

wireman

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If your driveshaft couplings are on their way out it will clonk on gear changes, at least mine did.

Check the fluid level and colour (red not brown) of the gearbox fluid, if its not been changed in the last 36,00 miles it needs it. Old fluid does not help your box to work correctly.
If the level is not spot on dont bother diagnosing it untill it has been corrected.

The rear subframe mounts can cause troubles, as Brian notes, put it somewhere you can lie underneath it and look at the subframe mounts etc as an assitant puts it from drive to reverse WITH THE PARKING BRAKE ENTHUSIASTICALY APPLIED, you will see any rubber bits which should be replaced moving or clonking, the front pair of subframe mounts are a good guess but dont forget to watch the diff mounts on the back of the subframe.

There are two control inputs from the engine to gearbox, the control cable and a manifold vacuum simulator on the fuel pump.

Adjust the control cable for no slack at resting throttle position after the slack has been taken out of the throttle linkages, ie with the throttle depressed suficiently to almost cause the engine speed to rise from tickover.

The vac parts to check are the rubber pipe couplings onto the gear box and all the pipes to the thing on the side of the fuel pump (manifold vac simulator), some where on this forum there is a post about cleaning the innards of this unit which helped me improve my gear change smoothness. My car was rough on power changes but OK at light throttle changes untill the simulators nozzle was cleaned.
 

Number_Cruncher

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Will do Malcolm but the box and the vacuum pressures were checked when I had the recent scare with it and they were spot on apparently. Would a tired propshaft doughnut give symptoms like mine?

If what you said in the top post about the problem going away when 3 is selected is also correct, then, this does tend to point to something in the gearbox** rather than an external cause.

Do you know what gearchange is causing the jolt? i.e., is it 4 - 3, 3 -2, or 2 -1, or a combination of them?

** I suspect a one way or overrun clutch has siezed up, but more info, thought, and testing required before that could be confirmed. Don't go taking the gearbox out at this stage!
 
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roadhog

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It definitely doesn't happen when slowing down from 3 so I guess it does point towards the box, perhaps amplified by tired bushes in the subframe. Not knowing the precise way the box works I can only guess at which gearchange is the problem. Accelerating away (well trying to) from below 60 mph gives a tremendous jolt so I guess that'll be 3 - 4. Using kickdown in 3rd seems a lot smoother by comparison. I had thought that when slowing down to a halt it was changing from 3 - 2 but it might well be 4 - 3 ?
There's some food for thought in the links provided by Malcolm and I'll investigate along those lines but not until I'm home next week. These things only happen when I'm miles away from home and I don't want to get stuck here for wont of a bit of plastic. The supply T piece will be a good start as a previous owner had repaired it with what looks like gum. I'll also give the vacuum controller/amplifier some attention.

I'll post up any results as and when but in the meantime I'd be grateful for any other suggestions. Cheers chaps. :)
 

Number_Cruncher

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I hadn't realised it was also playing up during upshifts. Then, check the vacuum system very carefully, making sure there are no leaks, and that the valve on the injector pump is working properly.
 
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roadhog

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I've just been out in it and changing UP from 2nd to 3rd is none too gentle. I'll hopefully know more next week.
 

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