Transmission downshift clunk
 

My car is experiencing a clunk while coming to a stop. The clunk is happening at approximately 15 mph very low rpm

I plan on buying a mityvac hand pump.

With the mityvac I will disconnect the line going to the transmission modulator and the apply 12" of vacuum. If the vacuum holds for 5 minutes or slowly leaks down over 5 minutes, my guess is the modulator is good?

I am also experiencing slow shutoffs(2-3 seconds after key is turn off).

I will check the shutoff valve also.

EGR is still connected. I plan to disconnect all vacuum lines going to the EGR and 2/3 valve and patch where they were tied to other vacuum lines.

Vacuum to doors is plugged at firewall(the yellow valve). Center vent actuator doesn't work and I assume it could be a culprit to vacuum leak?

Does my plan sound correct?

My SD does too, not sure what it is.

It is a vacuum leak some place, from what I remember.

The clunking sound during down shift could be your trans vacuum modulator. its located on the drivers side of the trans in a very tight spot. its easy as hell.
The two adjustments on your transmission are the throttle pressure (shift timing) and vacuum modulator (quality of shift).

If the shift is too soft or too hard, you can change the quality of shift by adjusting the modulator. Turning the adjustment “T” clockwise (right) will increase the pressure and harden the shift. Counter clockwise will drop the pressure and soften the shift. You can find all the info here under transmission technical information http://www.mercedesdismantlers.com
hope this helps

Answer:
 

Diagnostic test:
Disconnect and plug/cap the main vacuum line feeding the climate control at the bulkhead, go for a test drive.
If the issue is gone, and the engine shuts-down quick = leaking vacuum elements in the climate control are your main issue.



Have a great day..

My 85 300SD has this problem as well and i do not have a vacuum leak in the climate control .... have replaced almost all rubber vacuum hose under the hood and cant seem to find a leak anywhere.... my next step is to go through all of the orifaces and see if there is any blockage and "tune" the vacuum system. There is a good very thorough DIY on this here.

The problem with adjusting the modulator on the transmission is that; from what I understand, you can actually make the shifts from 3-4 too soft? Or the vacuum range could be incorrect throwing other shift points out of wack.(ie: instead of vacuum range of 0-14" if you adjust it too far you may have 3-18")

Yep. Check EVERY connection. It is tedious but you will find the leak if you take your time and do it right.. For me it was the piece of hose that plugged into the top of the vacuum modulater on the IP.

Good Luck!

Even quicker and easier. Push the far right button on the climate control and see if the clunk goes away. If it does, the leak is in the climate control system.

This doesn't always work if the leak is in the source vac line to the ACC. That's why it's best to plug the hose just before the firewall.

For me, this clunk went away once I removed the EGR 3/2 junk. Now I just need to get the fluid pressure monitored and determine how much I need to correct the VCV or modulator.

This I have done. It did not change anything with the clunk or the slow shut-down.

FWIW my brakes are excellent.

Is it correct to assume that if the Vacuum pump was going bad or was weak it would affect the brakes?

@toddyvol - I am having this issue on my 87' 300TD and it hasn't been warm enough for me to really dive into it. I had a bunch of the guys here help me determine that my VCV was bad and I replaced it and confirmed it to be shifting better however; my car shifts consistently at the same speed whether my foot is planted or lightly pushing, and some up shifts clunk like your describing. =/

How much vacuum is your trans. getting? My guess is it is not getting any at all and may be causing the clunk. At least that was the issue for one of my cars.

Yes. You likely have a leak in the locks (if it's not a 126) or somewhere else. Plug the doors and ACC and see what happens or just start pulling vacuum with a mity vac.

UPDATE!!!
 

I was able to dive into some of the vacuum problems today. What a mess!!

The previous owner(s) or mechanic(s) had unhooked the ALDA lines from the crossover valve. The line to the ADLA and the line to the intake were connected to the vacuum system!!

Of course I had no idea. So I took the line that comes from the ALDA and hooked it to the left side of the switch over valve. The line from the intake side I hooked in the middle of the switch over valve. My car halls arse now!!

I plugged the remaining openings left vacant but I still have a slight clunk and slightly slow shut-off.

I have not been able to go buy a vacuum tester yet. I'll have to drive a pretty good distance to get one(no one where I live seems to have one) and just haven't been able to find the time. There is a Harbour Freight that I can get one at.

Also, the vacuum line at the lower portion of the VCV is plugged tight and not going to anything. One went to the EGR and the other was the ALDA.

The EGR stuff is long gone. What do I do with the stuff left over on top of the valve cover?

The line coming out of the lower side of the VCV should be hooked up to a black vent line that ends in the cabin, for cleaner air.
You can leave the black box alone, or remove it.