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  #1  
Old 04-28-2010, 06:59 PM
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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

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  #2  
Old 04-28-2010, 08:25 PM
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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.
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  #3  
Old 04-28-2010, 08:37 PM
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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
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  #4  
Old 04-30-2010, 03:19 AM
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You may want to check out my thread here:
Park-to-Drive jolt
I am having similar issues, though the clunk isn't apparent when i shift into reverse, only drive.
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  #5  
Old 04-30-2010, 11:32 AM
rrgrassi's Avatar
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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.
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  #6  
Old 04-30-2010, 12:04 PM
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rr,

Good tip on the parking brake test. I need to adjust that too, thanks for reminding me.
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  #7  
Old 04-30-2010, 12:08 PM
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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.
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70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K
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  #8  
Old 04-30-2010, 01:49 PM
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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
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  #9  
Old 04-30-2010, 03:36 PM
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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.
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70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K
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  #10  
Old 04-30-2010, 03:45 PM
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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.
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  #11  
Old 05-14-2010, 05:17 PM
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Quote:
Originally Posted by Stinkindiesel View Post
...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
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.
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  #12  
Old 07-01-2023, 05:23 PM
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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?
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  #13  
Old 07-02-2023, 01:14 PM
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Quote:
Originally Posted by rrgrassi View Post
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 support bushing and bearing, and the u-joint.
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.
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  #14  
Old 07-03-2023, 01:57 PM
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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.
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  #15  
Old 07-05-2023, 04:40 PM
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I seldom contradict VS but would say the slop in the cv joints is not uncommon in high mileage cars. I recommend checking it.

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