Would any of you guy out there know why my 1982 300sd has such a loud downshift? When I am slowing down to stop or turn, each downshift makes a clunking noise. The upshift is hard and quick. My mechanic said that was good, but I am a little concerned about the downshift. I am clueless about this other than some mechanic who I only went to once and did not know what he was doing - was obvious - told me the valve body on my transmission was getting worn out. I just had the fluid changed and put in a can of BG transmission conditioner and new filter. No change in shifting patterns. Would appreciate suggestions.

I would put my money on your vacuum control valve on top of your IP. If everything else is working properly and its just the last downshift into first- that may well be it. Your transmission is looking for 10-15 hg of vacuum on that last downshift, and if its not there, it will clunk. This valve may be adjusted, or you may need a new one.

Of course, you could have a leak somewhere else, your vacuum pump may be bad, or you may have horribly bad flex discs- so you need to eliminate those as problems

Rick

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)

Can you enlighten me on what and where the flex discs are? I assume the IP is the injector pump. I am a little new at this but catching on fast. Forgive the stupidity. I am a learner.

PS - it does the loud downshift in each gear, not just the final downshift into first. I assume it is a three speed transmission - not sure, but I know I hear three loud downshifts - clunk, clunk, clunk.

First off, the flex disks are the big rubber donuts on each end of the driveshaft.

I was willing to bet you have a vacume line off the white or cream color plastic thing on top of the rear of the Injection Pump.That varies the vacume the transmission sees, its right in front of the oil filter. One goes in top one goes on the side, and the one that tucks in behind it is what shuts the car off.

Make sure all 3 are hooked up and not just 2.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

i'm betting vacuum adjustment as well.

you should read following article

http://www.babcox.com/editorial/ic/ic20242.htm

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

I read the article by Steve Brotherton which I had seen before, but had not realized that my switchover valve that he identifies in the article in figure 8 which is mounted on the drivers side fire wall has no vacuum line attached. It has been capped off and the line removed. Does anyone know where the line from this switchover valve is supposed to go to??? This may be the problem.

The switchover valve shown if fig. 8 is for overboost protection.(pressure not vacuum) It should connect to a banjo fitting on the rear of the intake manifold and to the boost aneroid (alda) on the injection pump. This may have been bypassed by someone (mine was) thinking they would get more performance I guess. But bypass of the switchover valve removes a protection device. Also, ensure these lines are clear, not obstructed, for the IP to get proper enrichment.

The downshift problem you mentioned definitely sounds like vacuum leaks. Also, your upshifts should be firm but not harsh. Do you have a mityvac? Do a search for "vacuum diagrams" if you don't have one available. Problem areas are usually the 3-way fittings and flex portions of the vacuum system. If these are soft/mushy or hard/brittle they need replacement.(inexpensive, easier to replace than to isolate the defective one.) I bet that once you replace the flex portions and get the vacuum system in order, you will be pleased with the results.

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Sam

84 300SD 350K+ miles ( Blue Belle )

No, don't have a mityvac which I am guessing is a vacuum tester. Know where to find one? Also I just spoke to a mechanic in Raliegh, NC who was a referral by a local where I live who is driving a 300D with 500k miles. The mechanic is Billy at Ultimate Automotive. He repeated almost verbatim everything that was already said here. He said to diagnose the problem would cost $75. I suppose that might be worth it. Let me know what you think.

Before you hand over the $75, at least do a visual inspection of your vacuum lines, hoses and fittings. A leak anywhere in the main vacuum system could affect the transmission. I had the same problem in a 300D. Although not applicable to your vehicle, my problem turned out to be a leaking door lock actuator. The transmission shifted great when the doors were locked, rough when they were unlocked.

Vacuum testers are available at many parts stores, also Sears. $25-$50.

(300SD (W126) has a separate vacuum pump located in the trunk next to the spare tire for the door lock system.)

If you do not have a mightyvac, not a problem for now. Do a search and get a copy of the vacuum diagram. The section for brakes, trans. etc, vacuum control is really not that complex. This is to ensure that everything is, at least, connected properly. If everything seems to be attached, I would recommend replacement of the flexible links and 3-way connectors. ($15 & 5 minutes tops!). 90% of your vacuum problems will be these flexible connectors.
Then if you have continued problems and want to have the satisfaction of fixing it yourself, get a mightyvac (most auto parts stores and jcwhitney carry them for $40).
($75 is not unreasonable, but it will probably only take a good mechanic 15 min. to fix) DIY, save a little and learn a little.

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Sam

84 300SD 350K+ miles ( Blue Belle )

http://www.peachparts.com/shopforum/showthread.php?t=105297

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85 300CD
83 300TD
78 240D (daughter)

everyone should bookmark that post phantoms
thanks.

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1984 300D Turbo - 231k....totalled 11/30/07 RIP