bypassing vacuum box on top of valve cover
 

84 300sd. I can’t seem to get the vacuum sorted out. Shifts aren’t quite right and I have intermittent trouble shutting off. Ive been through every connection and cleaned or replaced anything that was even half suspect including checking the connections to the ignition and that whatzamagig climate control thing under the driver side dash.
It also seems unusually sensitive to kickdown cable adjustments. A couple rotations either way and it behaves very differently.
To help diagnose where my problem I bypassed the “vacuum switchover valve” on top of the valve cover I did this by simply eliminating each T that sent a vacuum line to it and back from it from the area near the injection pump . The EGR had already been blocked off by a metal gasket but all vacuum lines to it remain. (testing purposes only of course)
Does that black box on the top of the valve cover only serve to control the EGR, or does it play a role in shifting and other vacuum functions? If it is there only to control the EGR did I bypass it correctly by just straight lining any lines that were going to it? Or, should I T together the vacuum lines that used to go to that box?

First you need a Mity vac to test the systems. Just because the connection to the vacuum lines seem OK does not mean the actual component is not leaking. The only way to verify this is with a mityvac.

One of the air switches on the top of the valve cover does effect transmission shifting. The other is for the EGR valve.

The vacuum system is quite extensive.
Your AC system has Air switches behind the Ac controls. Vacuum lines and connections to each element. 5 vacuum elements in the dash.
Your door lock unit has 1 main control valve and 5 elements as well as a pump.
Your engine area has numerous vacuum units including the modulator valve at the transmission.
So check all the lines, connections and units with the mity vac. Isolate the AC vacuum lines by using golf tee's in those lines while trying to work the engine lines. some thing with the engine shutoff brown lines. If it still acts up then you'll know its an engine problem and not the Ac or shutoff.
Be meticulous with the checks. You checking vacuum and a small leak can effect the transmission shift. If all these check out OK then there are other transmission shifting adjustments. The Vacuum control valve on the injection pump, the modulator valve in the transmission, the air switches in the engine compartment, the air valve on the valve cover and the metering valve where the brake booster line is bled off for the transmission.

Dave

of course very familiar with mighyvac.
Uh oh then but that is kind of good news and means that I just have to replace those lines and I should be good to go.
Now, what lines go where? I took a pic with my camera phone before I took it apart but the pic is pretty inadequate.
Ive attached a pic from one of my vacuum repair bookmarks. The schematic is for a 123 but it suggests my lines were not plugged into the right holes in the switch on the VC anyway. My lines were plugged into the bottom right two holes as you view from the front instead of top and bottom right.
Is this right?

Dave, on all SD's from '81-'85, the two vacuum switches on the top of the valve cover serve to operate the EGR exclusively.

They have no direct operation or effect on the transmission shifting.

I've recommended to everyone who has these engines to remove all the Tecalan lines that lead to and from the black box.

One line is the supply vacuum.........one line is the vent.........and the third line leads to the thermo vacuum valve in the thermostat housing and then onto the EGR valve.

These switches frequently leak and have an undesirable effect on transmission shifting however, they have no direct function reagarding control of transmission vacuum.

Please realize that older engines, prior to 1981, will likely have a different configuration.

You do not have to replace those lines. All the lines to and from the black box can be eliminated. Follow them back to the T's and remove the T. Connect the two remaining lines that were at the T with a short section of hose.

Thanks for the info.

Dave

is that also true of the 83 300TD?

Yes, the '83 W123 with the 617 had the same configuration.

Yep, those valves underneath the black box cover will leak and the transmission shifts will get harsh. Just remove all the lines to and from the black box and most of your problems will be solved.

that is what I thought. I was sure I didn't get the idea out of thin air. I remember talking to someone on the board here last year about doing it but I lost my saved files in a computer crash.
Brian, it sounds like I bypassed it correctly by just eliminating the Ts that send a vacuum line to that box.
The lines going to regulating valve and the shut off valve do not need to be connected to each other right?

Also, mityvac gauge on the control valve doesnt hold vacuum. Can the valve be rebuilt or should it be replaced outright?

When you eliminate a T, there will be two Tecalan lines sitting there wide open. These two lines must be connected with a short section of hose.

Repeat for second T.

Connect Mityvac directly to line going to transmission (bypassing VCV). See if the Mityvac holds vacuum.

What is the usual pressure for the vacuum? I did a driving test with the mityvac because my transmission is shifting a little hard. My pressure hovers around 4.5-5 PSI and then sometimes drops to 3 PSI when I go from a full stop or go from a rolling stop.

what timing!
 

My shifting got a little hard after I did a valve adjusment over the weekend. When I checked my vacuum lines I found that one of the vac lines going into the black box was unplugged. When I plugged it back in it broke, so I bought some more line. I unplugged the other line when I was putting the new one in, and I couldn't remember which line went in which hole. Blah blah.

Point is, I was getting hard shifts with one of those lines unplugged at the black box. But that was probably because it was leaking out. I can see it being unecessary, but if you don't seal off the T, your transmission will probably shift weird, right?

Also, besides the three vac lines that go into the black box, there are a few more holes with nothing going into them. What are they for? Two of them, in my car, have screws screwed into them. If there are no vac lines going into that box, should the black box holes be sealed with screws or something?

One of the holes is a vent hole. If the transmission is getting little to no vacuum, the shifting will be hard and abrupt.

Follow Brian Carlton's advice on disabling the 3-2 valves and EGR valve. It really helped me out. Now I need to fix the door locks and the HVAC vacuum systems.