ATT:stevebfl VCV adustment 1987 300D - PeachParts Mercedes-Benz Forum

#1 05-09-2002, 07:01 PM

Hi Steve,

Thank you for your article at the Contenintal Imports site on Transmission/Vacuum adjustments. With the original VCV, I could not get any vacuum to show on the guage at all so, I have replaced it. I can now get about 12in at idle but it builds to 12 VERY slowly. When the accelerator is quickly depressed, it drops to 0 and then quickly recovers to 12 once the accelerator is released.

Even though I have put about 2 counterclockwise turns on a new vacuum modulator, I still get pretty hard shifts at light throttle, especially from 2-3. It is better than it was, but not right. I am trying to start by getting the VCV adjustment correct. How much movement does it take to make a significant change.

I think that I have found most of the leaks and all of the other vacuum related functions of the car seem to operate properly.

I would welcome comments or any other member...

Once again, thanks for all of the help that you've left in the archives already.

BR,

Steve (8C>

Gilly · #2 05-09-2002, 09:34 PM

One thing I have found to be a problem is the vacuum port for the transmission/engine controls at the brake booster line. You will often find vacuum at the port, but the vacuum has to also be considered a "volume" not just a vacuum measurement to be measured. A restricted port (by caked on deposits inside the booster line) can allow a good vacuum measurement, but a lousy "volume" of vacuum. You need to locate the proper vacuum nipple (port) on the brake booster line, there is usually 2. It's fairly easy to figure out. One is for the door locks, engine shut-off system, and climate control, the other is for engine control devices, such as the trans modulator in question, also i believe the EGR valve. Find the port supplying vacuum to the engine control systems and after CAREFULLY removing the rubber hose from the port, try to clean out the port, either with a small drill bit (1/16" or less) or a tip cleaner for a torch, or maybe even a straightened paperclip. If this makes a difference (and it may very well require an immediate readjustment of the vacuum modulator) then a proper repair would be to replace the booster line and then do a final readjustment of the modulator.
Gilly

#3 05-10-2002, 06:05 PM

Hi Gilly,
Thanks so much for your reply. You absolutely pounded the nail! Although I was getting some vacuum through, the white plastic orifice coming out of the vacuum pump was nearly completely blocked.

Should I be able to put a straightened paper clip all the way through this orifice? I got it cleaned out enough to cure my hard shift blues but I'm wondering if I still have a partial blockage.

Thanks again for taking the time to help us DYIers with our old cars. They are a lot of fun to work on, especially when the results are so dramatic.

BR,

Steve (8C>

Gilly · #4 05-10-2002, 10:26 PM

Steve:
This is where you have to bite the bullet, so to speak. If you look at those ports, somewhere on the top is listed the bore size. That's pretty much how accurate it has to be, so you can find maybe an appropriate size piece of wire or a tiny drill bit, etc. to clean it out.
But in my opinion, the problem will be back since there is crud inside the booster line. I'd recommend just getting a new brake booster line, shouldn't be too darn expensive.
Gilly

#5 05-10-2002, 11:07 PM

Gilly,

Thanks for the follow up. I understand regarding the port size. Since I have a 124 (1987) 300D, it actually has two separate lines coming from the Vacuum pump. The brake booster line seems to be fine and I have nearly 25hg of vacuum on this line. The clogged port is the other one. I have no problem with replacing every vacuum hose on the car. I think, however that I will start by replacing the white plastic junction (for lack of a better term).

One more thing, if I may... I want to disable the EGR valve as I am fairly certain that this is the primary source for the crud in my vacuum lines (The overboost protection line was full as well). When I check the EGR (hot), it does not hold vacuum. Will simply eliminating the vacuum source defeat the EGR valve or will I need to fabricate a block off plate?

Again, many thanks!

BR,

Steve (8C>

Gilly · #6 05-11-2002, 12:05 AM

I'm not all that familiar with this topic of disconnecting things Steve, maybe SteveB will have an idea. My take on it is that some crud in the booster hose, after alot of miles,is normal. I believe it gets in there from slight oil seepage from the vacuum pump. I don't know how it gets in the booster line with the pump producing vacuum, but I've heard it does somehow. I'm not sure what the white junction is you mean. What I am recommending is the whole thick line from the steel line that screws down on the vacuum pump, back to the power brake booster. I believe this will solve your problem for many many miles.
Gilly