1980 300SD 116 has 82 617 engine and transmission. The switchover valve has been bypassed since I have had the car. Reading the various threads about overboost results made me want to reconnect this system. I began by removing the valve to check its operation. This is what I found: when 12 volts is applied to the electrical terminals there is a straight thru path for air flow from the intake manifold to the ALDA. When 12 volts is removed from the terminals the pressure immediately goes to a low level and then slowly bleeds off to zero. -- The Intake Manifold Switch. -- I found that this is is a normally open switch. That is to say, with no pressure applied the switch is open. Applying a small amount of pressure will cause the switch to
close.-----If I understand the way the system is supposed to work,isn't the direct connection from intake to ALDA supposed to be the normal condition?
And isn't an overboost condition supposed to reduce the pressure that the ALDA sees to a lower value? --- If I assume that these assumptions are correct then it would seem that the actions of the two components would be opposing the desired results. That is to say, given the way the relay works, should not the manifold switch be normally closed and open on pressure?? One electrical terminal of the valve is connected to 12 volts on run or start.The other terminal of the valve is connected to the active terminal of the switch. The other terminal of the switch is of course, grounded. I checked the intake manifold switch from the parts car and find that it is also the normally open variety.-- My questions : Do I have the incorrect manifold switch/ Do I have the incorrect valve?/Do I have an incorrect understanding of the way the system works?---- Have not seen the answer to this in any of the threads. can someone give me a clue? Thanks in advance. Regards, Willie

Sounds like you have the wrong switchover valve. It should be open when unpowered and closed when powered.

The rest seems okay - manifold switch completes ground when there is overboost in the manifold.

Someone looking to install remote start in an MB Diesel might want the switchover valve you have.

Sixto
93 300SD 3.0

Thanks Sixto The fact that both my manifold pressure switches were of the normally open variety prompted me to go back over everything I had done before. I pulled the switchover off the car again and retested it with my trusty mitivac. The valve is shaped like a silo with a vent covered with a plastic cap on the top, with two air connections coming from the bottom. As you look at this unit from the front of the car the air connections are side by side. I plugged the one on the right side(R)and applied vacuum to the one on the left(L). Without 12 volts applied I pumped about 16 in hg and found that the vacuum would leak down very slowly. Pumped vac back to about 16 in hg and applied 12volts. The vacuum dropped immediately to about 6in hg and held there for an extended period while 12 volts were applied. It seemed to me that this was likely what the unit is supposed to do.I put the unit back on the car and connected the ALDA line to connection (L) and the manifold line to (R). Test drive shows no loss of power so perhaps all is well. Further review of the wiring diagram shows that the active terminal of the manifold switch is also connected to terminal 1 of the OL protection Module. I expect this would enable the means to protect the transmission from over pressure . (Trans pressure switch on terminal 5 of OL protection module.)-- Connection of the manifold switch to pin 1 of the OLM would eliminate the possibility of using a normally closed switch.
Went back to the notes I had made on the original test on the switchover and made no sense of them. Sorry about the confusion. Moral of the story-do your work when its warm and take your time.
Does anyone know if the 82 transmission has a pressure switch for transmission protection? I noted an unconnected wire hanging down.