Hello,
I have a 1981 300SD with 230k miles. I'm troubleshooting a "Thunk" noise during the 3rd - 4th gear change. I've done some reading on these forums but my situation seems different from most I've read. The noise seems to come more from the rear than directly under the shift lever, and its a harsh change you can feel as well as hear. It's not terrible by any means and most of my friends don't notice, but The 1 - 2 and 2 - 3 shifts are absolutely flawless, so the engineer in me is not satisfied. The transmission has a lag of right around a second going from P to D or R when cold, and faster when warm.
The thunk noise does not always occur during first few minutes of driving. It seems to be a greater noise when accelerating slowly rather than quickly.

From reading, I think my next step is to check the 3/2 vacuum valves on top of the valve cover. I replaced the rubber connector piece there (old was brittle and broken), and eliminating that vacuum leak seemed to help the transmission in all ways. I do have a BB in the EGR control line.
From there, I think I will use a vacuum gauge and check the modulator, perhaps replace with the upgraded green version.

I wanted to see if anyone else has experienced this, because as I said the noise almost seems like it's coming from towards the rear but I see no way the driveshaft or rear end can make such a noise with that particular gear change only.

Thanks in advance the for help!

Also, forgot to mention I replace transmission fluid and filter ever 20k and haven't seen anything but nice red fluid come back out. The B2 piston was replaced by P.O.
Everything else vacuum related on the car works perfectly.

Hmm... I'm wondering if it might be an axle rather than the transmission. I know you mentioned only noticing the "thunk" from 3rd-4th but the drive train is turning at higher speeds then. Are you noticing anything when decelerating?

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Sam

84 300SD 350K+ miles ( Blue Belle )

The slight hesitation in shifting from park to reverse seems to be normal. I have an 81SD and it takes half a second most of the time for it to shift and catch. When I start it in the garage and shift to reverse, it will sit there for just a fraction of a second and then I can feel it go into reverse. After I back out and shift to drive, it will also hesitate just a fraction of a second. I can feel it in my bones when it catches. Not hard but just noticeable.

The upshift in the tranny is sans vacuum. So I do not think the thunk when it shifts to fourth is from the vacuum modulator. All the vac does is give you a smooth downshift, not upshift.

No problems to speak of when decelerating. You can feel the 2 - 1 downshift but no noise or clunking. Makes sense about the vacuum for downshifting - when my vacuum check valve failed and temporarily lost vacuum, the 2 - 1 downshift was pretty bad.

A friend mentioned he thought it may be the driveshaft carrier bearing. How this starting to wear would manifest itself only in this problem, I'm not sure. No other vibrations or noises from axle or drivetrain.

SD Blue, would an axle problem manifest itself in other ways? Is there a good way to check?

Also from reading it seems I should adjust the bowden cable to slightly raise the shift points. Gear changes occur right around 2,000 rpm - I'm in 4th by 25 mph. The car has always felt quick enough to me, but lately been reading perhaps the shift points should be a bit higher?

I'd start off by checking the Bowden cable and the accelerator linkages; and the fluid level.

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

My 300SD does it too bur at a half throttle. Under full power, smooth as glass.
Tried every adjustment there is on an automatic transmission, all with no better results.
The problem turned out to be gear slack in the diff. I was told I could set up the pinion gear a bit closer to eliminate the slack but the gears would wine in the new position. So I live with it and drive a little harder I don't hear a
3 - 4 thud under the back seat.

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1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

Ok Thank you for the help guys this really is a great forum.

List seems to be -

-Rear Diff Fluid change & inspection. I use Redline synthetic in my other vehicles manual transmissions and transfer cases, it works wonders. (Haven't done this yet since owning the car...I know, i know ..)

-Bowden cable adjustment.

My harsh shifts have always been VCV related. Less vacuum = harsher firmer shifts. FSM calls for 10 mm spacer for vac to drop to 0. I made a gauge by drilling a 1/8" hole in a nickle. 2 nickles glued together would be an upgrade.

I would also inspect rotational parts before opening diff. Is drive shaft, center bearing, flex disk, axle shafts.

Send me an email & I'll give a dropbox link to some trans manuals.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

As far as the axle is concerned, I can only tell you what I experienced on mine. There was some clunking but not consistently as others have mentioned here. The other thing I saw was a strange offset in the cv joint when the rear was jacked up (Z-shaped vs a direct bend). Not soon after, the boot cracked and leaked considerably.

About the only way I can think of to check would be raise the rear and watch the CV-joint, while someone turns the rear wheels back and forth. I'm not sure how evident a problem will be without any wheel loading. Everything I've seen online only talks about how to check them once the axle is removed.

Maybe someone, like VSTECH, who reboots many of these axles can tell you more.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Well my SD is now my daily driver while I work on installing a 617 into my Pickup.
So, I have had time to sort thru this transmission issue. I checked the driveline and rear end for anything that could cause an issue and found nothing. So I decided to start from square one with the transmission even though the 3 - 4 shift was the only one acting up. Initially I found both the 3-way vacuum connectors looked great, but upon very close inspection had cracks near the union. I also found EGR 3/2 valve leaking. I also replaced both connections to the VCV. This helped all shifts greatly, but still had a noticeable clunk from 3 - 4 upshift. So I tested the VCV, the setting was off by a very wide margin (too tight by ~1/3 turn). So I adjusted per spec (thanks Junkman!) and adjusted linkage to spec.
Now, not only is 3 - 4 shift smooth at all throttle settings, every shift has been improved and downshifts are not even noticeable - and it was a very easy job. It's always the simple things!
Thanks for the advice guys!

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-1981 300SD 242k, stock - sold @ 250k
-1994 Toyota Pickup 4x4 R150F 5 Speed, 5VZ V6 swap.
-2004 Volvo V70-R 2.5T

Well after driving the car for a while after tuning VSV, I think I will have to live with either a 3-4 flare when cold, or a slight 3-4 shift clunk when warm. Can't seem to tune it in between. Any comments on which situation is better for the life of the transmission?

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-1981 300SD 242k, stock - sold @ 250k
-1994 Toyota Pickup 4x4 R150F 5 Speed, 5VZ V6 swap.
-2004 Volvo V70-R 2.5T