Looking for some answers here (after searching). I've got a '83 300SD with two stacked ones, an '84 300SD with nothing there, and an '85 300D with an electric contact in place of the switchover valves. What the story? The reason I'm even digging into it is the '83 SD is not shifting into third and beyond, and some seem to think the valves affect the shifting, while others think it's just an EGR issue. If it is affecting anything, how is it being accomplished on the '84 SD where there is nothing there being actuated by the black piece on the linkage?

The 85 elec switch is probibly the full throttle AC cutout switch, I know 85 SD's had them.

My 83 had the stacked switches and after careful observation and testing, I yanked them. I see no reason for em other than egr. The only *maybe* is for pulling max vacuum at idle for smooth downshifts and such, but even that I doubt. I was content with my conclusion at the time.

As a test for your problem, the trans should shift, albeit hard, with no vacuum at all.

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

Matt,

We did have quite a discussion regarding those valves. In some vehicles, the valves only function to control the EGR. My SD is one of those vehicles. There are three vacuum lines running to the black box. One supplies input vacuum, one supplies output to the EGR valve (thru the coolant switch) and the final black line is a vent. None of these lines runs to the transmission vacuum control valve on the back of the IP.

Now, if you have two valves on the '83, there is a good possibility that one of those valves is for the transmission and one is for the EGR. Take a look at how many vacuum lines are going to the black box. If it is three, like my '84, then the valves are probably just running the EGR, however if it is four or five, then one of those lines probably goes down to the transmission vacuum control valve and affects the shifts. Check it out and report back.

Many of the forum people are unclear on the matter because the matter is not cast in stone. There are 240 vehicles where those valves affect the transmission and there are 300 vehicles where those valves have nothing to do with the transmission.

While on the subject, who has those valves for the least money?

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

O.K., the '83 has the two stacked together white valves and three hoses going to it, one from the vac line, one to the EGR, and a black vent one. Does anyone know what they are doing? Does it keep the EGR from actuating at full throttle, and why is it a two stage process, where it also changes at part throttle? On the '85300D, it makes sense that there is a full throttle cut out for the compressor, so is the EGR being controlled by the thing labeled "electric switchover valve"? And on the '84SD, it looks like it has the same electric switchover valve, but nothing on the valve cover, so is there a cutout for the compressor somewhere?

I just removed the two switchover valves on my 300sd. I determined that they are for full throttle cutout and idle cutout of the egr. Only difference is ever slightly more vacuum available now to the transmission due to the overall reduction in the volume of the system. One line goes out from them to the egr. One line goes out from them and joins the line from the vacuum regulator going to the cabin (vent). One line joins the vacuum line where the regulator gets its vacuum. I haven't seen a setup where the switchover valves are directly involved in the transmission. Good luck.