I have been having an intermittent hissing from behind the ACC unit in my 85 TD. Today I got into it and found that there are several vac switches behind the acc panel. Each one appears to have a hole in the bottom of it and the one labeled number 3 just dumps vaccum whenever the thermostat wheel is in a position calling for heat in the cabin. It does this whether the ACC is in the off position or in the economy mode (I didn't check other options).
When you rotate the thermostat to a point where it senses the need for cooling the hissing stops.
Otherwise the heat and AC work pretty well. The center vents have been blowing with the heat on for the last year or so and I have verified that the pod is functional but I am not sure if that is related to the vac leakage issue.
My general question is whether I should be looking at the electrical ACC unit as the cause of my problem, or the vac switch itself. The vac switch seems to be turning on and off in response to an electrical signal from the ACC, but I do not know whether the hole in the bottom of the vac switch is supposed to be a dump valve or is malfunctioning.
It is annoying to have hard shifts every time the heat is on.
Any direction would be helpful.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

......intermittent hissing ....that is a SNAKE !!!!

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/showthread.php?threadid=10414

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/showthread.php?t=156207&highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Turns out Google is much better at searching for answers then the search function on this site. changeover valve for the center vent has failed and the hole I was plugging with my finger is a vent for it.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

I dont recall seeing or hearing any vent on those switchover valves.

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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles