Need help with early ACC (servo system) part: main vacuum switch
 

I have my 1980 300td at an electrical specialist right now, having given up trying to troubleshoot my ACC II (servo system).

He he has found issues with the vacuum hoses and is trying to find the main vacuum switch (term may not be exact).

He says that according to the wiring diagram and other docs I provided, the blower won't run without that switch working.

He can't find it inside the dash. I don't recall ever hearing of it.

I have tried googling it but what comes up is the switchover valve on the firewall on turbo models.

Yeah I have no idea what he is talking about.....The blower motor will always work, the default settings for the servo system is defrost, if the system won't even work on defrost....there has to be a bad fuse...

This is usually how it goes down....

The aux water pump will seize, thus intern frying the amplifier behind the dash...

The small motor in the servo will also seize thus the servo cannot move to the right positions....

The heat valve under the servo will disintegrate from the improper coolant thus you will end up with heat 24/7...

What you need to do is to do this...

Replace/rebuild aux water pump and fuse the wire
Replace servo with the Aluminum version
Replace amplifier behind the dash
Replace sensor tube running to sensor in passengers dash speaker....

Then you can diagnose....

Personally people find it cheaper to convert to a 240D CC or to by the updated kit but that doesn't control the fresh air flap....

Either way count on 2 grand to get it all working but may fail again do to bad vacuum pod in a few months...

There is actually a main vacuum switch in the system that is required for the blower to work if I remember correctly. I was helping another member troubleshoot his ACCII and we were pouring over the diagrams when we found it. I had always assumed as well that the defrost function bypassed EVERYTHING and would work no matter what. Bur from what we found in the manuals that wasn't true.... I think it was located in the center console behind the ash tray somewhere.... I'll see if I can dig up the thread...

Ahh here it is
need help!! no heat!! brrrrrrrrr.... - Mercedes-Benz Forum

lots of good info in there as well

give the entire thing a read through

I agree, but then again until this afternoon I would have said there was no vacuum related to the blower motor function, either.

There is indeed a vacuum/electrical switch that connects to the servo -- via the green vacuum line. There are actually two identical switches, the other is for the AC and I think is the yellow line.

The wrench is using Mitchell and he says it begins by requiring a full vacuum functional check before moving on to the electrical side. Not surprised if the switch isn't working as I am pretty sure I delivered the car to him with the servo unplugged.

I am heading there after work to see what is up.

Excellent thread, thanks!

Actually Google picked it up when I was searching earlier... but I tend to pass on the threads with odd names. I wish list moderators would "fix" thread names to make them more relevant for later searches. Roy Hunter used to do it sometimes here.

Thanks to all for your input.

No problem, let us know how it goes, good luck!