Driveline clunk/slop and harsh shifts - Mercedes-Benz Forum
 
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post #1 of 10 (permalink) Old 06-10-2019, 10:37 AM Thread Starter
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Driveline clunk/slop and harsh shifts

Alright, I'm out of ideas on this one, anyone have ideas where to look next?

Clunk in Rear: Changed out the rear diff last week and this improved 10 fold but on quick transitions off the throttle I'm still getting a light pop or clunk in the rear end. Old diff was very much toast, something rattling around inside at times and there was a substantial amount of freeplay. New one was very tight, no freeplay between the axles and driveshaft. Spun very smooth, couldn't find any issues. Axles were replaced as well. I've also had the front flex disc replaced, replaced the rear thrust arms and there appears to be no wear on the rear flex disc. What am I missing?

Harsh Shifts: I'll admit I haven't done much diagnosis here as I was hopefully this was a byproduct of the bad diff, but I'm not that lucky. The car and does shift very smooth about 50% of the time. This would lead me to believe the modulator has adequate pressure? Fluids clean and seems to be filled to the correct amount, I am going to go ahead and change the fluid next week just to see, but I'm hesitant thats the issue. When it shifts hard it is really rough on the driveline and want to get this fixed asap.
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post #2 of 10 (permalink) Old 06-11-2019, 05:43 PM
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That diff must be spent. I noticed a little clunk in mine after swapping to 5spd, but with the auto it didn't make any noise that I could notice.

There are a number of tutorials on adjusting the trans that can be found here and elsewhere online. The trans is supposed to shift at 3k when the car is cold, and it does shift a bit rougher unless you're super ginger with the throttle in anticipation of the shifts.

Proceed with caution. >>>
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post #3 of 10 (permalink) Old 06-12-2019, 08:00 AM Thread Starter
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That diff must be spent. I noticed a little clunk in mine after swapping to 5spd, but with the auto it didn't make any noise that I could notice.

There are a number of tutorials on adjusting the trans that can be found here and elsewhere online. The trans is supposed to shift at 3k when the car is cold, and it does shift a bit rougher unless you're super ginger with the throttle in anticipation of the shifts.
Yeah diff was apparently worse than I thought. Bowden cable seems to be adjusted well, shifts right at 3k on normal driving. I'm going to change the fluid next weekend along with some wheel bearings and check out the motor/trans mounts as well, but the noise is clearly in the rear so I'm not hopefully any of that will fix it.
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post #4 of 10 (permalink) Old 06-12-2019, 09:01 AM
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Bowden cable affects shift timing; vacuum modulator affects shift quality. Quick search yielded this very comprehensive link for adjusting the modulator. I've seen others that are more concise.

https://www.benzworld.org/forums/w20...l-replace.html

3k shifting is a little high for normal driving, IMO, but perhaps we have different ideas of what normal driving is.

In my limited experience, the diff would either have to be totally fucked to clunk during forward gear shifts. With an auto the diff should stay loaded the entire time.

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post #5 of 10 (permalink) Old 06-12-2019, 09:30 AM Thread Starter
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Bowden cable affects shift timing; vacuum modulator affects shift quality. Quick search yielded this very comprehensive link for adjusting the modulator. I've seen others that are more concise.

https://www.benzworld.org/forums/w20...l-replace.html

3k shifting is a little high for normal driving, IMO, but perhaps we have different ideas of what normal driving is.

In my limited experience, the diff would either have to be totally fucked to clunk during forward gear shifts. With an auto the diff should stay loaded the entire time.
Wondering if theres some freeplay with my trans in that case. I'm 95% this diff is fine.

Re: Modulator, I guess my question here is that my car is capable of smooth shifts. If the modulator was toast wouldn't all the shifts be harsh?
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post #6 of 10 (permalink) Old 06-12-2019, 09:37 AM
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Re: Modulator, I guess my question here is that my car is capable of smooth shifts. If the modulator was toast wouldn't all the shifts be harsh?
Which shifts are harsh and when? If it's only the lower gears perhaps because the higher gears are closer and RPM drop is less you're not feeling the harshness. I would try adjusting the modulator before anything else.

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post #7 of 10 (permalink) Old 06-12-2019, 09:47 AM Thread Starter
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Which shifts are harsh and when? If it's only the lower gears perhaps because the higher gears are closer and RPM drop is less you're not feeling the harshness. I would try adjusting the modulator before anything else.
Haven't been able to find much consistency. Some days its nice and smooth cold and hot and other days its harsh one some and not others. I will say if I force it to start in 1st the subsequent shifts are almost always harsh, even beyond 1-2. If I start in second the 2-3 shift has lets say a 50% chance of harshness. If I start in 1st the 2-3 shift has 90+% chance of being harsh.
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post #8 of 10 (permalink) Old 06-12-2019, 09:50 AM Thread Starter
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Also from my understanding a modulator shouldn't cause issue in any WOT shifts as there is no vacuum, at thats what I read. My shifts at WOT are definitely harsh at times.
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post #9 of 10 (permalink) Old 06-12-2019, 10:04 AM
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WOT shifts should be a bit harsh otherwise I'd suspect a slipping trans

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post #10 of 10 (permalink) Old 06-19-2019, 07:22 AM Thread Starter
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WOT I guess make sense.

I did pay a bit more attention yesterday to the shifts. I really have no experience with automatics but it feels like the car is holding the clutch and gas at the rpm and a second later dumping it into gear and immediately letting off the clutch and gas. I know there isnt really a clutch, but its the closest thing I could describe to what its doing. Again, other times its a super quick soft shift.
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