Harsh thud or clunk when shifting from Park or N to Drive or Reverse
 

Hi all,

Been reading through some threads related to clunks when shifting into D or R as well as threads on clunks while in gear and slowing or accelerating. I've troubleshot my vac system pretty well and fixed my transmission shifting issues with the Superior spring kit as well, but I still have not solved this darn thud / clunk issue when shifting from P or N into D or R. Here's some stats:

1987 300D Turbo
245,000 miles

A few Vac Stats
- 20-21" HG from the new vac pump I installed 2 months ago. This reading is after the little plastic filter.

- From the Vac amplifier (aka blue flying saucer), I have 15" going from the VA to the transmission modulator. Modulator cap is intact and was replaced over a year ago as it was torn. On that note, I tapped the line from the VA to the TM and ran a vac gauge to the cabin. At idle, the vac reading was 15". Previously on my old vac pump it was 12" HG. When it's at idle and I shift into D or R, it produces a harsh clunk but the HG doesn't move (which I think is normal).

- When I park on a slight incline and put it into P with my foot on the brake, as soon as i release my foot from the brake, the car will either roll forward or backward. This is the replacement differential I was sold by Enrique at MB Motors, btw.

Regardless, I've had this damn clunk with both vac pumps, but I don't think its a vac issue. My car shifts well, though it does shift a little mushy. As I understand it from Brian Carlton's advice, high HG to the vac modulator produces mushy shifts

I want to run a few things with the group here to see if I may be on the right track to check:

- Is the HG from the vac amp too high? When I moderately floor it, the HG goes from 15" to about 4" before shifting. I haven't floored it yet to see if it goes all the way to 0---should it? I know how to adjust the vac on the VA but I spent alot of time dialing it in to make it smoothly shift, so I'd like to leave this as a last step.

- Would a worn pinion in this replacement differential be a culprit? How to check w/o opening?

- Would a worn flex disc(s) be a culprit?

- Transmission fluid level? I'll check tonight.

Any advice would be greatly appreciated.

Thanks,
Bob

You can do a visual inspection of the flex discs and motor mounts to see if they're worn out. cracks in the rubber mean tey're done, put it in park and grab the drive shaft and twist it back and forth while looking at flex discs to see if there's any play. try to turn the driveshaft carefully to see if the companion flange turns alot before the axles move. you may be able to feel the clearance between the pinion and ring gear. pry on the motor and trans w/ large prybar to see if the mounts are cracked (careful not to dent the pans). these items can clunk when worn out.

69,

Thanks for your reply. I'll check the flex discs this weekend, thanks for the tips. as for the motor and trans mounts, I replaced them over a year ago. I'll take a look, but my feeling is that they're probably still good.

Bob

You may want to check out my thread here:
http://www.peachparts.com/shopforum/diesel-discussion/276284-park-drive-jolt.html
I am having similar issues, though the clunk isn't apparent when i shift into reverse, only drive.

How about with the e-brake? If the wheels are locked, there should be no clunking, unless it is in the drivetrain itself. Check the half shafts, rear diff, flex disks, center suppor bushing and bearing, and the u-joint.

rr,

Good tip on the parking brake test. I need to adjust that too, thanks for reminding me.

No prob. If you raise the car up, and do the e-brake test, have some one shift from park to Reverse, to Drive and see what moves if you can't get the clunk to happen by hand.

Checked vacuum modulator?
 

I came accross a 300D several years ago with a very pronounced clunk when shifting. Turned out the owner had made a few "adjustments" to the vacuum modulator. When I got a gauge on the transmission, it was immediately obvious the working pressure was way too high. Backing off on the vacuum modulator adjustment solved the problem.

Gary

Vacuum will not fix the clunk when stopped and shifting from Park in to Reverse or Drive.

It would be true when driving and the tranny is shifting through 1-2-3-4.

I agree with checking the flex discs. Typically the front one will give out first. My 190E had a completely dead flex disc but installed you could not tell. Once it was removed, it was clear. The symptoms on it were most noticed when you were accelerating a bit rapidly and then let off quickly. You could here the slack being taken up with a loud thunk.

Another thing to check is your differentil mount bushings. You can see this by looking at where the differential bolts to the subframe with two bolts on the rear of the subframe. On mine, the drivers side was most worn, sinve that is the direction the drive shaft moves.

Per your suggestion/find, I checked the vac going to the trans modulator. it said 15hg at idle. I drove it around and at WOT, the vac was around 3-4 hg. It should be close to 0hg as I've read.

So I checked the hg I last recorded about a year ago on the old vac pump and it was 12. I recall that I has a very small thump (not a hard clunk), so I dropped it to 12.5 and now my clunk is not as noticeable. I plan on dropping it down to 12 and then by tiny increments until there is a good balance between optimal shift quality and eliminating the clunk. Because I dropped the vac, the vac is closer to 0hg now.

Note: i am adjusting the vac hg at the blue flying saucer.

i do plan to check the flex disks, etc as suggested above as well. But thanks guys, I believe this solved 90% of the problem.

Wow - This is an old thread, but Title explains just what I have.

Recently, whenever I put car in reverse to back out of garage I head a clunk. It seems to have got worse. I checked it once out and found that there is also a clunk when moving to Drive.

I jacked the front of the car up and had a look. Rear wheels on ground. Flex Disks look sound. I will have help tomorrow, so will watch while moving from N to R & D. Maybe U-joint? Otherwise, maybe axles, but they were replaced not long ago. Also noise seems to be more toward front of car.

If I want to measure shift vacuum, where does gauge connect?

Any other suggestions?

No lift, so I have rear wheels on ground. Presumably same as using e-brake.

I watched while helper moved shifter from N to R and N to D. The rear flex disk rotates about 1 bolt head width. Not sure if this is normal lash on a 38 yr old car. This was with foot on brake. With foot off the brake, clunk hardly noticeable.

Flex disks are good, but the transmission mount has cracks. I checked it by jacking slowly under the "ears" just ahead of the mount. I found that the tranny could be lifted and lowered without anything else moving! OK, needs replacing. Not sure that is cause of clunk though.

Jacked rear up and repeated the N-R-N-D-N shifts. Loud clunk at rear hubs but likely misleading because I had one wheel off which left disk loose. Bolted disk to hub and tried again. Still seems to be some slop - maybe in outer CV joint? The axles are quite new, but are cheap Chinese. Bolts holding hub to axle were tight.

Not sure what else to check? Maybe e-brake shoes?

Otherwise will change the tranny mount and see if that helps.

Clunk is usually slop.
Either the diff gears are worn, or the trans yoke locknut is shot.

It can be slop in the axles, but not common.
The 87 axles use grease, and that grease doesn't protect the steel inside the boot.
I've seen the shafts rusted nearly all the way through.

I seldom contradict VS but would say the slop in the cv joints is not uncommon in high mileage cars. I recommend checking it.

The axles are only 2 years old, but like many, are of the cheap China variety.

I was concerned that the outer splines might be loose. They were fine, but at same time, I did check as best I could for slop in the axles. But only by sight. Did not notice anything.

Even before we installed the axles, I did notice a small amount of lash in the diff. But after 460k km, I didn't think it was excessive.

As best as I could check, the flex joints and u-joint were tight.

AS mentioned earlier, I did notice that I could jack the tranny up quite a bit (maybe an inch or more) without anything else moving. So first step will be to install a new transmission mount (the motor mounts were new a year or two ago)

If I still have slop and clunk after that, I need to get a shop or friend with lift to get it up where we can see what is going on.

Thanks for input from you both.

I'm going with diff also. U-Joints or CV joints generally will cause a driveline vibration.

Just a point of interest, I changed the differential fluid on a new to me '85 300SD and I swear it was filled with ATF. It poured out like water. (upon further review, an '85 isn't new to anyone ;>) )

I have the new transmission mount. Been hot and humid here, but I have got as far as jacking the car up, putting on stands and figuring out what wrenches I need. Hopefully get at it next rainy or cool day :) Luckily have other cars.

Finally got the mount changed (after some URO issues covered in another thread). But the clunk is still there. Louder going from drive to reverse, but does it both ways.

I need to get it up on hoist. Don't trust most shops for diagnosis on these old cars, and we don't have any MB indies locally.

Another area to check would be the differential mounts. Typically there are 3 mounts, 1 fore and 2 aft. Hard to see to evaluate. You could place a jack under the diff and assess how much slop it has. Should be fairly stiff if the mounts are still good.

We had the rear mount loose when we changed the axles. It is still in good condition. The front two mounts are what we also call subframe mounts. Can't really be inspected, but nothing apparently wrong there. I often jack using diff, so will keep an eye.

Yeah, thw 124 diff is mounted for rotational stability.
Jacking up the car from it can damage the side bushings.
So the diff can rotate a bit when put into gear.

You can see the two rear diff bushes by looking at the rear suspension cage. There are 2x half dollar sized circular openings in the subframe where the bushes reside. These bushes have two voids in the rubber. Look at the condition of those carefully. They rarely get changed as the diff has to be dropped out of the cage to replace them. I did mine because the rubber was gone, and it clunked going into gear, and at times it felt like the rear was steering itself, mostly on acceleration.

As vstech mentioned, jacking the diff stresses these bushings.

My car is an '85 W123 300D. Does it have those openings?

Not sure what a half dollar size is! Here it would be two quarters or a Loonie cut in half ;)

Sorry was on mobile app and did not see your car w123/avatar, as the tread started with a w124 that's what I thought we were discussing, my bad.

As to whether the w123 has something similar? Possibly, if the diff is suspended in a cage/subframe. It's been a few decades since I crawled under a w123, hopefully other can chime in to clarify.

Best of luck on determining/correcting the source of the clunk.

Half dollar about the size of a silver maple tho worth less :D

Spent more time under car today. I measured the lash in the diff. I found I could move the inner CV joint flange circumference about 1/4" without moving the drive shaft flange or the other axle. Roughly 5deg?

I tried listening with a stethoscope while rocking the jacked up rear wheels. Max noise actually came from driveshaft flange area, but that flex joint looks good, so must be transmitted from the inner diff shaft. I checked oil level and added a little.

I had car jacked up and on jack stands, but some weight still on diff. I tried starting the car and got a kind of alarming noise from under the car - tranny end. I put the car back down on wheels and the noise was still there when I first put it in reverse, but after changing gears back & forth it went away. Clunk of course still there :(

At 83, I am more or less at end of road doing this sort of stuff. I think I will see if someone would like to buy the car. Sad to do - we have owned it for 33 years. It has good body, only 75k miles on rebuilt engine and many recent upgrades to brakes suspension etc. No A/C. Runs very well.

So what could I ask for this car given someone would need to likely do some drivetrain work? Car is in Eastern Ontario, Canada.

Just wanted to acknowledge your noteworthy milestones. "Kudos" to you and "Congrats", as well. It is truly amazing that, at the age of 83, you're still "gettin her up on jackstands" and "crawlin' around underneath"!! Very impressive feats, indeed! I'm "only" halfway through my sixties, and can definitely envision the "end of road" for my mechanical pursuits that involve the garage floor...not "happy thoughts", by any means.

I can also imagine it's hard letting your car go. 33 years of ownership is also an amazing milestone. I'm sure that car feels as if it's a part of you, now. What a great record, though. The number of years you've owned her is almost unbelievable and a true testament to these incredible machines.

Here's to you, sir. Cheers and my hat's off!:) Best of luck in whatever pursuits you choose (maybe keep the Benz just a little longer;))

I always read your posts and have learned a lot from what you've added to many threads....aftermarket parts from a certain 3 letter supplier not withstanding:eek:. Hope you'll continue to do so far into the future!

Appreciate the sentiments!

You are right about the 300D - It has that comfortable feeling, like a worn in pair of shoes :)

While diagnosing the 300D noise, I have moved to our 98 W210 (owned for 22 years) and my wife is driving the 2019 Outback (she likes to have A/C!). The 350SL (also owned for 33+ years) doesn't get driven much but still looks and runs just great.

I finally got back at 300D again after ignoring it for a while.

Today, I checked lash/slop in the only 2 1/2 yr old (5100 miles) annular axle Cv joints. Did this by first jacking and supporting rear of car so axles were in normal drive position.

-Removed wheels and bolted brake rotors to hubs.
- Marked position of inner flange vs diff so I could watch to ensure the inner end of the axles did not move.
- Rotated the rotors back and forth and determined tat, yes, there was slop!
- Marked the edge of the rotor at one point, then slowly rotated until diff end axle flange just moved. Repeated this several times and marked the extremes on edge of the rotor.

What I measured, was that the rotors are about 274mm diameter (137mm radius) and that I had about 7.5mm movement at the periphery. A little Trig using calculator said arctan(7.5/137) = 3.13 deg. So ~3deg of slop in the two Cv joint (mostly inner one)
(Repeated above for other side and got comparable results.)

I tried a similar test on bench using my original axles and found they had zero slop. A video on merc-sauce said that is how they should be.

Still don't know if this is the main cause of the clunk I hear, but I guess it is one possibility.

Next, I plan on getting car up a bit higher and re-checking the driveline components best I can.

Some say it could be a vacuum modulator issue - I will check that vacuum line to tranny is intact and cable connection is in place and not over-tight. Somehow I doubt these would be a factor.

Failing all of this, I will try and get a small shop to put it up on hoist, but doubt they will see more than I can from under car.

I am sure a competent MB shop could pinpoint the problem, but we don't have one anywhere near.

Hello Graham,
I am having the same issue on my 82' 300D with 350k miles on it. I just put a rebuilt transmission in, so I am finally ready to adress this clunk. I belive mine is worse than yours though, because it also clunks when downshifting as I brake to a stop. Makes a very nice luxury car feel old and tired.

On the chance that it will help you, here's what I have figured out:
All my flex discs, engine mounts, transmission mount, and diff mount have been replaced within the last year or 2. The axles are also remanufactured OEM axles replaced 2 years ago. So, my entire drivetrain as been addressed at this point, aside from the diff and rear brearings.

I have also made sure the transmission is in perfect tune, and all vacuum to it is perfect.

I am fortunate to also own a 240D with 196k miles. So, I compared how much rotational slop the two cars had if I rotated a wheel or the driveshaft by hand with the rear wheels off the ground. The difference is very obvious.

All of this has lead me to believe that my differential is simply worn out. I plan to replace it with a tight lower mileage unit in the next month or two. If you'd like, I can update you with the results.

Best of luck! 33 years of ownership is great. I for one would love to see you keep it, but I am biased.

Thanks for the input imgolden!

Today I jacked the car as high as I could and spent some time under car checking the driveline components more carefully. The flex disks look fine and I could not determine any play between the front and back parts of the drive shaft. The U-Joint looked fine. The mid shaft bearing is hard to check and although it doesn't look that great, I could not move the shaft laterally. I also checked the rear diff mount by jacking under diff to see if car and diff both moved about same amount. They did. The rear sub-frame mounts don't look great, but they don't look like they have moved.

So all that was left was the diff itself and the axles.

I still suspected the new Chinese axles. I did a test and found that there was 3 deg of play between the diff end of axle and the wheel hub. About same on both sides. My old original axles have no play.

I put rear wheels up on ramps and also put the emergency brake on. Then with transmission in N, I rotated the drive shaft at the diff end by hand. There is quite a bit of play and the axles moved when I did that. Some of the play would be within the diff (I didn't measure that), but with wheels on ramps, the axles shouldn't be able to move at all. I made this poor video of this . Bottom left corner you can see me rotating rear flex joint flange back & forth. Right side, you can see the axle moving. Clicking sound while doing this.

I don't know if this slop in diff/axles is the actual cause of the clunk (It's actually more of a metallic click!).

I did find that one vacuum connection was disconnected. Thought that might be the cause, but it was not :(

I agree that diff will no doubt be worn after 285k miles, but this clunking came on sort of suddenly. Car has just been parked since I first heard it.

You can get an idea of what's going on inside the diff by dumping the oil and having a look at the magnetic drain plug. If it's got bad wear the magnet will be covered by a large ball of steel bits. The oil might be cloudy, too.

Don't recall having a magnetic drain plug on my car or seeing any steel bits when changing the oil.;) Maybe those magnetic plugs are aftermarket add-ons? Probably a good idea.

The oil in my diff has only been in there for 5100 miles.

Guess I take it for granted that there's a magnet in there, in the center of the inside of the plug. Maybe there is and you've missed it because it wasn't covered by a lot of steel (a healthy diff won't shed a lot). Maybe M-B didn't use them and I'm crossing memories with VW and Toyota.

You could remove the fill plug and check the oil out. Twist up and dip a bit of white paper towel into the oil to see what it looks like. Drive it a around the block or give the wheels a good spin by hand before doing that to stir things up. If you see grey in the oil that would likely be steel.

I'm curious to see what your trouble is.

Had my wife shift gears with foot on brake this morning while I watched and listened. Objective was to locate source of the loud metallic clunk.

Well, it definitely comes from the rear. What I really noticed, was how much the back end of the car lifted when put into reverse. I am familiar with the slight squatting we get when putting into drive, but hadn't noticed how much the rear end lifts in reverse.

If I carefully go from D to N and then to R, I get more of a thud. Presumably as lash in gears and axles gets taken up. But if I move direct from D to R, then rear lifts quickly and there is more of a metallic sound.

Thinking the metallic sound may be due to wear in rear suspension parts? Now need to find out how to check these?

I have a 72 350SL that has essentially the same drive train design. I should compare it. I did check the amount of squat and rear end lift it has when shifting between D & R. It shifts into these with just slight thud and the anount of squat/lift is much less.

Today I measured the 300D's total lash in the combination of axle + diff. Did this by jacking just one wheel with other on ground. Transmission in park so drive shaft could hardly move. Then rotated tire and measured total lash between wheel and driveshaft.

The total amounts were 7.5deg and 7.2deg. Previously I had measured the lash in just the "new" axles. They were both close to 3 deg.

Result is that the diff has total lash of about 4.35deg. This doesn't seem excessive although the combined ~7.35deg is probably enough to cause a clunk?

In this thread owner, like you, compared his/her 300D with a 240 that had low lash. The numbers were 16deg for the worn 300D and about 3.5deg for the 240D. Post says they guessed 80% was in diff, rest axles.

My next research will be on the diff mount (that "looks" OK) and the trailing arm bushings. My tach is not working, so will also address that and make sure idle rpms are not too high.

Then maybe back to vacuum modulator. I need to learn how to determine if it needs replacing.

Quick update in case you didn't see my many other threads on this issue :o

Yesterday, I completed the new axle install. Diff mount at same time, although it didn't look like it had been moving.

Haven't driven car much yet, but no clunk going into D or R. Just a hardly noticeable sound about same as my 107 and 210. Time will tell, but it looks like the axles were cause of the clunk. There is no doubt wear in the diff after 275k miles, but it doesn't cause that metallic clunk.

I am going to change the ATF & filter because this is well overdue (80k miles). Recommended brands of ATF getting harder to find around here.

Next will be to find someone who will take over and enjoy the car :)