I reversed the upper and lower 3/2 way valves in a effort to either clear or implicate said valve(s) with respect to the clunking transmission. Transmission still clunks, but the clunk is now more pronounced at the 3-4 upshift and less obvious during downshift. (If coasting to a stop, the downshift clunk occurs just prior to coming to a stop. Or, it will occur if pedal pressure is reduced after the 2-3 upshift.) As best I can determine, the lower 3/2 valve receives source vacuum. It is closed at idle, and opens with slight accellerator pressure. This appears to route source vacuum to the upper valve, which opens at about half throttle, routing vacuum to the EGR valve. It also appears that both valves affect transmission vacuum. The role of the 3/2 valves with respect to the EGR valve is pretty clear. Not so obvious is their affect on transmission vacuum. Since the vacuum valve on the IP serves to reduce vacuum as the throttle in increased, what is the exact function of the 3/2 valves? Does the upper valve just exist only to operate the EGR? Or do both valves affect the transmission?
At the transmission modulator line, I have 15" at idle, 6" at half throttle, and 0" at full throttle. Should the next step be to R/R the 3/2 valves?

Both of the 2/3 valves control the EGR. If I recall, the upper valve closes the EGR at full throttle and the lower valve closes the EGR at idle (I may have those backwards). The EGR is supposed to be open at all other times once the engine is warmed up. The only way that the valves affect the transmission is if they leak, which reduces the vacuum signal to the modulator and causes clunking. If your EGR is disconnected, then these valves serve no purpose but to dump vacuum. You really don't need them. Try disconnecting the two lines that go to the 2/3 valves at their source, and I'll bet that you see a significant increase in vacuum being sent to the transmission. I'll bet that the clunks disappear, too! Not only that, but you won't have the EGR valve blowing soot into the engine.

__________________
1982 300D, black w/ palomino leather, 203K
2001 E320, champagne, 105K (Mom's)
1999 Dodge Dakota, squeaky & battered, 142K

Okay, thanks for the amplification. I already tried disconnecting the vacuum lines to the 3/2 valves. That resulted in the transmission shifting like there was no vacuum at all; more of a jolt than a clunk. But I will try that again. Maybe I had a leak but I don't think so. I will vacuum test the system before driving next time. I suppose the next step is to adjust the modulator pressure, which I initially set with a gauge.

I believe this has been covered.
The two 3/2 valves ALSO produce HIGH vacuum when switched at closed throttle to get rid of this exact problem, clunky downshifts. Read the MB grey books, it's in there. If they leak or such, it'll foul things up. Also see willrev's recent post(s). The dealer replaced his, after the first two done by and indi were defective. Fixed it right up...

__________________
One more Radar Lover gone...
1982 VW Caddy diesel 406K 1.9L AAZ
1994 E320 195K

I could always be wrong, and I've grown rather accustomed to the taste of crow over the years, but how could they produce high vacuum to the transmission? Other years may be different, but on my '82 300D (same year/model as Tangofox007) the 3/2 valves only change the signal going to the EGR. The EGR valve is on the output side of the switches, while the input lines tee into the two vacuum lines coming from the main vacuum line that goes from the vacuum pump to the brake booster. With both of the switches closed they are not using vacuum, and should behave as if their vacuum supply lines are blocked off.

One of the 3/2 switches tees into the same line that feeds the vacuum control valve on the IP. There should be about 23 "Hg coming from the main line where this line attaches. It seems that the only way this 2/3 valve could affect transmission vacuum is by reducing it, either through leaking or consumption. A leaky 3/2 valve will surely cause a clunk, which can be fixed by replacing the offending valve, but plugging the lines to the 3/2 valves will also fix the leak...for much less money . I could have gotten the same results by replacing the 3/2 valves, but by transmission shifts beautifully without them. The fact that diesels are emissions-exempt in Louisiana doesn't hurt either. The only thing in the system that actually PRODUCES vacuum is the vacuum pump. Everythinng els is a vacuum consumer, not producer, and can only reduce the vacuum signal.

Tangofox, it certainly sounds like you have a vacuum leak SOMEWHERE. Have you double-checked that the lines for the door locks and climate control hold vacuum? Perhaps you have a weak vacuum pump. Have you checked the vacuum in the main line that goes from the vacuum pump to the brake booster? You should have about 23 "Hg. Are you sure that when you plugged the lines to the 3/2 valves that your plugs weren't leaking? By the time you're done troubleshooting this problem, your Mityvac should feel like a part of your body .

__________________
1982 300D, black w/ palomino leather, 203K
2001 E320, champagne, 105K (Mom's)
1999 Dodge Dakota, squeaky & battered, 142K

Could you provide the title of the "MB grey books."

Yes, I agree, it has been covered. But whether it has been covered accurately is another story. So....I bought the "grey book." The grey book says that the switchover valves exist to operate the EGR valve. Period. They do not exist to operate the transmission. They are not there to eliminate clunky downshifts. They do not "produce" vacuum.
At idle, both valves are closed. With slight throttle take-up, the lower valve is actuated, routing vacuum to the EGR valve, causing it to open. At near full throttle, the upper valve is actuated, venting the vacuum and causing the EGR valve to close. There are three vacuum lines routed to the switchover valves. One is vacuum supply, one is a vent line, the third supplies vacuum to the EGR valve.

A more complete investigation revealed that one of my switchover valves had an intermittent leak. Bypassing the switchover valves substantially improved the transmission shift quality.

I would especially like to thank Cateaux for his insight, without which I would have no doubt dropped $75 for 3/2 valves that I absolutely did not need. The problem with my initial attempt to bypass the 3/2 valves was rooted in my belief that the the switchover valves had two vacuum supply lines. That led me to jump the supply line to the vent line, creating an even larger vacuum leak.

Well it sucks that's for sure. I just completed replacement of the two valves (my old ones were shot for sure) but the clunks remain.

Seems like the two of you (Cateaux and wolf_walker) are on top of this. My clunks seem to have reduced as observed from a brief 5 mile city only test drive I conducted after the replacements. However, I still get clunks quite a lot. In fact, my 2-3 shift is not only hard, but it seems like all my downshifts have the clunk with them. So, I guess my problem lies in the vacuum control valve on the injection pump. What is this thing anyway? And also, what does it involve to measure all those vacuum levels you mention? Do I need a mityvac or something? Sorry but I'm new to this vacuum business. I think it is about time I got into the vacuum business on this car since everything has some vacuum attached to it.

Edit: The car I'm currently dealing with is my 1984 300SD with 166K miles. My other 300SD shifts so smoothly and rarely experiences those clunks. I guess someone worked on it sometime in the past.

__________________
1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver

Don, I wish you would have asked me prior to buying those valves. I recently tore out all the 3/2 valves, all the Tecalan lines to the 3/2 valves and all the lines from the 3/2 valve to the EGR temp switch and the valve itself. Cleaned up the underhood environment completely.

The SD never knew the difference. Completely the same shift quality and shift points.

Connect the Mityvac to the line down to the transmission using a suitable "T" and route the gauge to the inside of the vehicle so you can read it while driving.

Then take it for a drive. As mentioned above, you should have very close to 20 inches at idle and it should steadly drop as you open the rack and add more fuel. At maximum power, it should be very close to zero. The valve is adjustable, slightly, and the vacuum curve can be shifted left or right by rotation of the valve on the IP. I have not done this yet, but would like to attempt it to firm the shifts slightly on the SD.

Brian, sorry but you'll have to slow down a bit for me. One aspect of this car I don't know much about is one I have to face now: the vacuum systems. I read your post about cleaning out the hood of your 300SD several weeks ago but I didn't actually realize then it was related to what I had planned to do later (today). So tell me, what are these 3/2 valves everyone is talking about? Are they the valves inside the black plastic box on top of the valve cover? Those are the valves I just replaced. I'm not informed on how all that stuff works either. But I will learn slowly.

__________________
1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver

I decided to call my indy to ask him about my transmission issues and he said he was very familiar with all of them and could solve them for just $50. I took him up on the offer, took the car to his shop and he softened the shifts a little but not too much to avoid damage to the clutches. Turned out my transmission modulator is fine, but there was such a big hole on my vacuum line that it was getting no vaccum. He solved it and the downshift clunk is no more.

Damn, I feel like kicking myself for living with that annoying clunk for so long when it could be fixed for just $50. The car drives like never before. Even my girlfriend noticed the absence of those clunks tonight when we drove to the movies.

__________________
1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver

1984 300d Turbo 135k
No Down Shifting, And Engine Speed Up 2500 To 3500 Rpm Shift 3-4. Trans Was Slipping. Adjustment Of The Vacuum Control Valve From 15 In To 17 In Vacuum With A Small Hand Vacuum Pump Now It Works Good Downshifting. Up Shifting Is Good Also. Do A New Filter And Atf Before Adjustment. And You See The Adjustment TOP SIDE REAR IP ON THE VCV Is Under The 1/2 In. Round Clear Cover Take Off, . USE VISE GRIP TOOL TO REMOVE COVER AND ADJUSTMENT And Remove The Green Dash Pot And PLUG IN A Hand Vacuum OR RUN THE ENGINE AT IDLE,T INTO THE GREEN DASH POT ADJUST For 17 In. Vac Pump The Vac Pump Each Time For For Adjustment With Engine Off. If Vacuum Is Low To Trans, You Have To Move VCV By Let Up The Power To Shift And When Sliping Of Trans , NOW THE ENGINE FEELS LIKE A LOCK UP CONVERTER AND THE FUEL MILES WILL GO UP, IF TRANS SLIPS ENGINE SPEEDS UP, MODULATOR WILL HAVE LOW VACUUM RELEASE POWER IT WORKS LIKE A CLUTCH IN MANUAL TRANS Your Filter May Be Plug Up Use Pep Boys New Gallon Atf And They Will Take Back The Old Atf What You Remove Is The Same You Put Back In Only Put 3.5 Qts In Start Engine And Move Gear Shift To All Gears Than Add More Atf 3 Qts Their Is A See Gage On Side Of The New Gallon Atf 6.5 Qts Dip Stick Will Read Low When Cold

Glad to hear its fixed.

__________________
1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair

My 81 300 sd does the same thing some times. My tranny is fresh 6 months ago and If I come up to a stop sign no clunk. my clunk is when the tranny needs to jump down from three to two because of throttle position or foot feed position I think it is a vac issue

Really I Am, But Isn't That More Trouble Than It's Worth? I Just Don't Understand - Your Description, That Is.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.