HELP!! I have had harsh shifting problems since owning this 1980 300 SD 116 even after having the motor and transmission from an 81 300D installed and having the transmission rebuilt ($$$) by a good shop. After reading various threads about this problem, I have determined that:1. The vacuum valves that reside on the valve cover(EGR and throttle position)are not working correctly. The EGR is not a factor since its vacuum supply has developed a plugged line. And, 2.There was NO vacuum going to the transmission.
Using my trusty Mitivac,I found that the throttle position valve would reduce the vacuum to zero at the slightest touch of the throttle mechanism.
Defective valve?? There is a connection on the vac control unit on the IP that is on the fender side and below where the "snubber" goes into the top. When I disconnect and plug this connection the vac goes to about 12 in. Applied to the transmission line the 12 in of vac results in silky smooth shifts,also increased power(or is that just my imagination). At this point the 12 in is being fed to the transmission and to the "snubber only. What I am calling the snubber is the green cylinder shown in the pic. Question: will a constant 12 in to the transmission cause any problems? Anyone?? The Poor Soul would really appreciate any input you might provide to avoid any potential problems and to further his MB education.Thanks in advance, Willie(Poor Soul)

Attached Thumbnails

 

Most likely.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

http://www.peachparts.com/Wikka/TransVacTune

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

response. What is the function of the line that is connected just below the "snubber " line? If this is connected into the system I can get no vacuum at all. As far as I can tell the vac was connected as per the 81D 123 diagram. I have not a clue as to what the problem is. Thanx,Willie(Poor Soul)

You have this in a 116 car? You should go for the simple system used there.

The reason I posted the article is you don't stand a chance of intelligently fixing/adjusting this system without a knowledge of how it works and a vacuum gauge.

The 116 car had the simplest of the systems. Basically a line runns from a huge constant vacuum source through an ORIFACE to the trans modulator. If this was all there was the modulator would see 20in of vacuum all the time.

The purpose of the oriface is to allow the vacuum beyond the oriface to be modified by a proportioned leak. The leak gets bigger in proportion to the throttle movement. So be sure the brake booster line outlet still has the oriface built in (not reamed out) and run a line to the trans. Now tee off a line to the proportioned leak (see the artcicle for the position). Now hook a gauge to the line such that it reads what the trans sees and you should have 10-15in at idle and 2-0 when full throttle.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

I did away with the top-of -valvecover valves from the 81D arrangement, reconnected the setup and regained proper vacuum control. vacuum varied as it should: more throttle less vacuum.Still nice smooth shifts,but shift points were wrong. Adjusted the pressure cable and now the transmission is just fine. I want to thank you for your contribution to resolving the longstanding problem with this car. I am not convinced that an ordinary transmission shop would have the expertise to have done this job.Thank you,
Willie(Poor Soul)