I am looking into why the heater only blows through the defrost.
There is a valve right behind the ACC that moves when I go to and from defrost. But nothing changes. None of the other items move. So after looking at the vacuum diagrams I think that I have no, or little vacuum to the distributor for the vacuum. Where is this located, and then where is the reservoir at. Is this fed from the same source as the engine, or the electrical pump for the locks?

thanks
Doug

Has anyone actually tested the ACC output with an ohm meter? can you give me a coupe to check so I can see if it is working or not?

I am lost here....

thanks
Doug

If you have a vacuum guage (MityVac or similar) it is pretty easy to test out the SD ACC system from the passenger side footwell.

Remove the carpeting, the kick panel under the dash and the side carpet (held by one screw up near the front and bottom). Then the side carpet slides toward the firewall and out.

Once you get it removed you will find the vacuum distributor with all the vacuum valves in a neat little holder down the side of the center console behind the radio. You can get yourself some vacuum tubing and few golf tees and start testing.
There are two compnents to look at here.
1. The vacuum pods themselves, they can be tested by putting a vacuum on them and seeing if they bleed off. These are fed by the main vacuum lines that plugs in series into all the vacuum valves and then the outlet side is tucked neatly back where your fingers just barely go. Remove the two secrews holding the vacuum valve mount and you may gain an inch or so to work. If any of the pods bleed off then the diaphragm inside the pod is bad and needs replacing. Depending upon which one is leaking you can sometimes get away with plugging off the source and it will help the footwell heat issue.
2. The vacuum valves themselves. These are not so easy to test but in theory they are meant to hold vacuum on the source side when not energized and then open when energized to allow vauum to pull the pods open. You can check them by running through the settings that should in theory open each one. Full heat should open the footwell pod valve etc...

Note that the dash center vents and defrost flap partial draw are connected together via a coupler back behind the valve. It is possible to have a good working center vent yet have the partial defrost diaphragm bad however this is usually not the case.

If you have good vacuum coming in the source line then there is a very real chance that you have a bad vacuum valve that has also caused a pod diaphragm to go bad because it has been pulling on it for a long time. This will have the same effect as no vacuum at all from the source since it will just bleed off everything in the system.

I have been chasing this systema nd trying to get a better understanding for the last 2 years and am still surprised every now and then by soemthing else that I find. Like the vacuum valve that opens just fine on the workbench but will not operate at the voltage level within the car. I do not believe tht this system is perfect but sometimes you can fudge connections around a little and it makes things work (albeit a little screwy). As an example, I had a controller that would not open the center vents so I took the plug off the center vent vacuum switch and put it on the (green) flap. This has caused my center vents to allow ambient air to blow through in small quantities and also I now get heat through these vents which is not a normal setting but hey, my center vents now work...

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'78 Ski Nautique

It must be a very common problem in the 300SD's to not have center vent operaton. Every one I looked at buying had this problem, including the one I now have. I put a screw in behind the one actuator that you can see when you remove the glove box, and this forced the center vent to stay open so I could get some air blowing. I had to take it out last week though so I could have a funtioning defroster.

Vacuum pods are pretty easy repair on the 300SD. Except, and a big exception, for the center vent actuator. Replacement diaphragms are available here: www.autoluxmbz.com Do an advanced search for "vacuum pods" "center" and my username and you should come up with a thread where I went through this with someone else.

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Sam

84 300SD 350K+ miles ( Blue Belle )

So I am trying to find out why nothing else works.

thanks
Doug

Defrost is the default mode.

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Have you taken apart and checked the solder joints where the circuit boards meet? Touch them up if they look in the slightest way questionable. Find a friend who does this sort of thing if you don't have experience.

Vacuum diaphragms are the other thing.

Once those are taken care of you will have probably a good operating climate control system. It is not an expensive repair, it just takes time.

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Sam

84 300SD 350K+ miles ( Blue Belle )