I am finally revisiting my '82 300D-T's vacuum system as it pertains to the trans control. A few months ago i replaced the switchover valves hoping to fix my trans' hard-shift-all-the-time problem. These are the white vacuum valves located on the valve cover under a black plastic cover. The old ones not only had the plastic finger worn off but the valves themselves were no longer porting correctly. They influence (with vacuum) the trans modulator, the EGR and the injection pump. I replaced them with a brand called MTC (not genuine MB). They look the same, have the same part # and worked OK upon receipt (port 5 and 2 open at idle and port 5 and 3 open at partial to full throttle. I guess they work by providing vacuum to the trans modulator at low throttle (soft shift) and no vacuum to the modulator at partial throttle (hard shift).

In any case after just a few months the switchover valve no longer ports vacuum correctly, 5 is open to both 2 and 3 at vacuum. The genuine MB switchover valves are double the price at about $40 compared to MTCs $20. Anyone else had experience with faulty switchover valves by MTC? I hate paying for these again especially at $40!

I already avoid using non genuine trans pan gaskets and frame/subframe bushings (I also confess to only using MB coolant on my MBs). Are MTC switchover valves another one to avoid?

__________________
1972 450SL
1982 300D Turbo

Those valves exist for only one reason: to operate the EGR valve. Their effect on the transmission is only coincidental; if they leak they rob the vacuum system of vacuum that the transmission needs. Vacuum to the transmission (assuming that it is available) is regulated by the valve on the rear of the injection pump. If you disconnect the vacuum supply line (usually a white line)to the switchover valves and plug the connector, you will eliminate the affect of the leaking switchover valves. If your vacuum system is otherwise intact, your transmission operation should return to normal.

Thanks tangofox007,

You're right, the transmission modulator is controlled by the vacuum control valve by the IP.

__________________
1972 450SL
1982 300D Turbo

Hold on there, if you refer to the big grey books, you'll see those valves shunt high vacuum to the modulator at closed throttle to lessen the feel of downshifts, which can often thunk when things are tuned to give a good upshift.

And yes, the non-OEM valves are known to have bad batches, member WillRev here had some, ended up going to the dealer.

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

Thanks, I found the post about defective aftermarket switchover valves.

vac valve on top of valve cover defective?

I guess i'm not the only one that ran into a bad one.

I did some other searching and was wondering this:
One of the switchover valves feeds vacuum to the vacuum control valve next to the IP. If the vacuum control valve adjusts vacuum as needed to the trans modulator why even bother going through the switchover valve. i think tangofox007 would agree if I understood him correctly that the switchover valve is not needed for trans control...but... the switchover valve actually stops providing vacuum at higher throttle positions. So maybe the switchover valve is needed afterall by providing all or no vacuum to the vacuum control valve. If the switchover valve was bypassed the vacuum control valve would always get vacuum and maybe it was not designed to do the entire 0-22inch HG control.

As much as i like the idea of bypassing the switchover valves (they are ridiculously expensive) I'm not so sure i want to load the vacuum control valve with max vac all the time.

__________________
1972 450SL
1982 300D Turbo

so wait a minute, is there a definitive answer here? I removed my switchover valves because they appeared to me to do nothing other than operate the egr at different throttle settings. One of the lines to them was vacuum, taken ahead of the conrol valve, and the other seemed to join the bleed. Seems they control only the egr, (but I do have hard downshifts )

You really need to refer to the vacuum diagram for your car. They are not all the same, you may be comparing apples to oranges. Someone else's good advice may be bad advice for you if you are talking about different cars with different vacuum routing.

Make sure that you understand the difference between vacuum lines and vent lines.

To be clear, my comments were in reference to the '82 300D, the subject of the thread.

Tangofox, remember that there is not "by the book" setting for the trans vacuum for one of these cars with a quarter million miles on them (give or take). You may reach a point, such as my 300D was, that the range of vacuum is not suffecient to give a good shift and still have high enough vacuum at closed throttle to prevent downshift clunks(a common complaint because someone that knows just enough to be dangerours had a soft shifting car and cranked the vacuum up when the valves were not up to par). Those valves shunting very high vac at closed throttle allow one much more leway in mid throttle vac adjustment. This has been discussed to death if you search around a bit. On a tight trans you could probibly get away with it, but it's not *right*, and will require a lot more screwing to get what passes for a good shift.
This comes up about every three months..

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

At closed throttle, the switchover valves are closed so that they do not "use" vacuum. Notwithstanding leaks in the system, the vacuum available to the transmission circuit is the same when the switchover valves are closed as it would be if the switchover valves were bypassed. The switchover valves have no mechanism by which they can increase the vacuum to a level higher than that in the supply line, which is shared with the transmission circuit. In fact, there is an orifice in the vacuum supply line between the EGR and the vacuum control valve. The purpose of the orifice is to further reduce the vacuum level at the control valve. The orifice is supplied in five different sizes to accomodate variations in individual vehicles.

If you can think of a good thread on this, would you post a link please.

Could do this by venting (controlled leak) when not at idle.

I'm sorry, I just don't have time. Maybe my 83 was different than the 82, but it's all in the factory manuals, just not very plainly spelled out more often than not. They are not JUST for EGR, even the dealer told WillRev this as I recall. Have fun. No more auto's for me.

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

Oh here, I'm a sucker.
http://www.continentalimports.com/ser_ic20242.html

If you put a vac gauge on a properly working setup in the line to the modulator, and slowly open the throttle, you'll see what he's talking about.

As I said, you don't have to have it, but it's there for a reason, and can become crucial to get a satisfactory shift on some well worn examples.

Most people don't see the relationship, and I've had this conversation more than once. I did finally contact Mr. Brotherton on this matter and he confirmed I was not nuts.

Enjoy..

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

If you are referring to the black line that connects to side of the IP, it might be helpful to understand that it is a vent (leak) line. It does not "feed vacuum." At near full throttle, the second switchover valve is actuated. At that time the vacuum to the EGR valve is vented so that the EGR valve closes. At that point, the vacuum in the EGR system is already quite low, as the vacuum control valve is doing a significant amount of venting itself, reducing vacuum available to the EGR circuit. It is also worthwhile to understand that the vacuum control valve is always leaking vacuum, even at idle. So the vacuum level in the transmission/EGR vacuum circuits should always be significantly below the level that exists in the primary vacuum system. As a result, the vacuum control valve should never be subject to a vacuum level anywhere close to 22 in Hg.

At closed throttle, with functioning switchover valves, it should be darn close, and jump down a good bit with the slightest movement of the throttle.

Everything in this system goes hand in hand, and was designed by some very bright fellows. If you much with one thing, you will much with another in some way. Fascinating, and infuriating.

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