Hi, my first post here. I have read through everything I can trying to troubleshoot Mercedes problems at my shop over the time I have known this board and other. I have always found the answer, but this time I am stuck.
There is a 1985 300D in my shop with vacuum issues that I cannot pinpoint. There is no airflow out of the floor vents, and the center vents have very little. When defrost is selected, it blows hard and HOT! No matter what temp you are dialed in at, it is full hot. The temp selector appears to work, I can run the a/c, and get freezing cold, but the second I switch to defrost, with cold still selected, it is raging hot, too hot to even drive around the block. I have replaced the monovalve, it was cracked. I checked the vacuum element for the floor, and it will move and hold in place with my hand vacuum. I also checked the dual element for the defrost near it, and it holds vacuum on both sides. The element behind the glove box will function with my hand vac, but the one up higher in the corner does leak when vacuumed by hand. I could not reach the center element for testing, but not sure I need to. I would be delighted to have heat on the floor, on demand, and still keep air condition functions. The defrost situation is liveable, but I would like to fix it if I can. I appreciate any help, I have used the info from this forum for quite a while and have always found my answers. Thanks in advance from Kansas!

Someone will correct me if I'm wrong, but I'm pretty sure defrost is supposed to do exactly what you describe. It blows fully heated air at full blast.

As for the vacuum problems, you need to get under the climate control panel and the panel above it and test the lines going from the switchover valves to the pods. You're experiencing a lack of vacuum, and it's probably because one of the pods is leaking in air.
Just cross your fingers and hope it's not the center vent pod (on the books, the center pod is impossible to replace without pulling the dash, but in reality, it's possible but painful).

EDIT:
Looked a little bit closed (I somehow bleeped over your statement about that pod in the corner).
As I recall, the line to that pod is green. Be careful testing it, because the inside diameter of that line is VERY thin, and when the pod is still actuating, it will appear to be a leak. If it still leaks after you pump for about 30 seconds, it's a leak and not standard.

Mine leaked too, at one point. This is the recirculating air flap pod, and it's a pita to remove. I ended up drilling the rivets holding its bracket to the blower housing and reattaching the bracket with machine screws and locktite.
I'm not sure what position it is in when it is not actuated, but judging from your description of ice cold A/C, it must be in the recirculate position by default.



You can safely test the pods from the lines in the center console. The lines rarely leak, and it gives you a quick screen to see which pods are causing the trouble.

__________________
1984 300DT turbo 138k mi

Still hauling me to school and back.

Handy Site:
http://www.dieselgiant.com/

I do know that the one to the far upper right, the fresh air, recirculate element is leaking. At least when I hook my hand vacuum to it, it does not hold vac like the others do. I did not test the center one. It seems difficult to get to.

The center one can be tested from the switchover valves behind the two wood panels in the center console. The line goes from the middle (third) switchover valve to the center pod and the defrost halfway connection. If you plug the defrost halfway connection, you can connect your vacuum tester to that line and see if the line and pod hold vacuum.

__________________
1984 300DT turbo 138k mi

Still hauling me to school and back.

Handy Site:
http://www.dieselgiant.com/

So, the center pod shares vacuum with one of the lines on the dual vacuum pod above the throttle pedal, correct? I may not understand what you mean.

Just a quick runthrough here to make sure we're both on the same page:
You already know about the footwell pod - it's the one that opens the footwell vents (imagine that?)
The one above the gas pedal that has two stages is the defrost pod. The nipple on the end opens the defrost vents all the way (for defrost mode) and the one on the side just opens it halfway (to keep your windows from fogging during normal operation)
The one directly behind the glovebox that never fails (even though it's the one everyone wishes was blown) is the main air flap (ever quite found out what that one did)
The one on the right corner way in back is the recirculating air flap
In the center is the dreaded center vent pod, responsible for opening the center vents

Not trying to be offensive here, just making sure we're on the same page (when I first started working on my vacuum system, I certainly got confused).

Now, to the switchover valves.
They are numbered from left to right, and are closed b default. When an electrical current is applied, they open and allow air to be evacuated out of the line and pod(s) that are connected.
#1 is connected to the full defrost
#2 is the footwell pod
#3 is connected to a three way connection in back that connects to both the center vent pod and the defrost halfway connection
#4 is the main air flap
#5 is that recirculating air flap


This helps when tracking down hard to find leaks, although you seem to have yours pretty much located.
To check the center pod, just disconnect the line from #3 and see if it holds a vacuum. You will be testing the line, the three way connector, and both the center vent pod and the defrost halfway pod, but the lines and connectors rarely have problems, and you already confirmed that the defrost halfway connection is holding vacuum. Therefore, by simple logic, if there is a leak, it is probably the center vent pod, although it may be the line.


One other thing to check for - while you have the switchover valves uncovered, either right down the color coding system and where each line goes, or label them somehow. Disconnect all the lines from the switchover valves to the pods (never disconnect the vacuum lines on top - those are from the pump and the green main line that goes through the firewall). Then pop the hood and see if the green main line holds a vacuum. This confirms that no switchover valves are leaking in air when turned off.
Unfortunately, for me at least, the one that leaked only leaked when on. I just put the temperature wheel to minimum (turn it until it snaps into that setting) and made sure the mode was set to economy. Under these conditions, the ACC is trying to blow cold outside air, and thus will try to actuate the center vents by applying a current to switchover valve number three. You can take advantage of that by removing the switchover valves and plugging them into the number three wire. This puts them to the open condition, allowing you to cap the end that would normally connect to a pod and put your tester on the top connection (where the main line to the vacuum pump normally connects) and see if the valves themselves are leaking in the open position.


This is how to test the switchover valves, and while I recommend doing the leaking while closed test, leave the leaking when open test off unless you find there is still a leak when you fix the recirculation pod. It can be somewhat time consuming, and the screws holding the valves in are a pain to r&r without dropping them.

__________________
1984 300DT turbo 138k mi

Still hauling me to school and back.

Handy Site:
http://www.dieselgiant.com/

Great! So far we are on the same page. I will try out these tests in the morning to see what turns up. I suspect the center pod is my current issue. Will the recirculate pod leak give me issues right now?

One thing I'm not clear on is what really the problem is. I am reading that you think there is a vacuum leak because air is not coming out of the registers in specific settings.

Not to switch gears drastically but here's my take on this. From what I read so far, I have experienced the same symptoms. I do not expect a vacuum leak on the pods especially since this is an 85. Also, its not a typical area to leak. Usual leaks are on the door elements and sometimes the trunk and gas door elements

Rather than focusing on the vacuum components, my suggestion is to clean the CCU terminals. These lose their contacting integrity overtime and can cause the symptoms you described (make sure you disconnect the battery before working on the ccu or any electrical component). Sand each male and female pin lightly to re-establish the connections.

__________________
the sooner you start... the sooner you'll get done If it ain't broke, don't fix it.. Its always simpler to tell the truth...
2007 Honda Accord EX
2007 Honda Accord SE V6
96 C220
97 Explorer - Found Another Home
2000 Honda Accord V6 - Found Another Home
85 300D - Found Another Home
84 300D - Found Another Home
80 300TD - Found Another Home
Previous cars:
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87 Camry
84 Cressida
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80 Fiesta
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Contessa

Well, he has confirmed that the recirculation pod is leaking, so that's at least part of the problem.

The trick here is that a leak anywhere after the restrictor between this part of the vacuum system and the main line would cause this. You are simply getting no vacuum pressure.
Does the car turn off when you turn the key? Does it turn off whether the doors are locked or unlocked?
This is a simple test, but if the car stops either way, the climate control pods are probably your only problem.

For now, just test and fix what you can, and be sure to do a test of the door locks by connecting the line to your tester, locking the doors and seeing if a vacuum is held, and unlocking the doors to see if a vacuum is held.

There are two door lock lines, one to the reservoir under the trunk, and one that actually connects to the locks. These are behind a check valve, allowing the vacuum pump to evacuate the reservoir and locks, but holding the vacuum in the reservoir for exclusive use in the door locks.
You can tell them apart because one will be relatively quick to evacuate and one will make your hand sore before you can get 10 inches of vacuum.


Personally, I have had no problems with the CCU not operating the switchover valves as it should, rather, I had problems with it engaging the fan in two of the modes. My take on that is that while it could be a problem, test and handle any vacuum leaks first. Even if the CCU is operating the switchover valves properly, there will be no effect if air is leaking into the system.

__________________
1984 300DT turbo 138k mi

Still hauling me to school and back.

Handy Site:
http://www.dieselgiant.com/

I pulled the line from the #3 valve and put vacuum to it, it does hold, and I can see the center flap moving. I plugged the line going to the reciculate pod to isolate that leak. Still no change.
I pulled the conectors off of the climate control unit, and lightly sanded all of the pins, and reconnected it. Still no change. I have not yet found my problem, but will update if something happens! Thanks for the help so far, I will do more tests as soon as I can.

During my first tests to check the pods, I was connecting directly to them. Out of curiosity, I connected to the line from the valve to the foot pod, and it was blocked! I cleared the blockage, and reconnected the lines and still nothing. I pulled the line off of #2 valve and connected my gauge to it. Nothing at any setting. I am not getting vacuum at this spot, but I think I should be.

Which line do you mean?
You pulled the line off the front of it, put the tester on where the line to the pod would normally be, and there was no vacuum that way?
Did you set the temperature wheel all the way to heat, turn the blower to low (to keep yourself from burning up) and set it to economy? That should try to actuate the floor pod.

You have to remember that most of the pods actuate based on the temperature setting and not on which mode the ACC is in. You will never actuate the food pod in any mode if it's trying to cool (defrost excepted because it is full blast heat no matter what the temperature is).

__________________
1984 300DT turbo 138k mi

Still hauling me to school and back.

Handy Site:
http://www.dieselgiant.com/

Just a quick check you should do - try to pull a vacuum in the system from the green main line under the hood. This is the main line that goes to the huge rubber connector that goes across the tops of the switchover valves.
If you do it while the car is off, you should have no leaks, or something bad is happening.

__________________
1984 300DT turbo 138k mi

Still hauling me to school and back.

Handy Site:
http://www.dieselgiant.com/

Fixed! I don't know how common this is, but 3 of the switchover valves were non operational. Got some used ones, and done. This is of course in combination with the blocked line, that would have still been blocked if there was vacuum or not, and the bad diaphragm, that was bad, vacuum or not. All in all, 3 switchover valves, one diaphragm, one monovalve kit, a few feet of vacuum line, 13 headaches, and 2 days of wondering why I had no answers. All in the line of duty. Thank you to everyone who gave suggestions, and to everyone that makes this board stay alive. Our best regards...
Adams Street Automotive
Hutchinson Kansas

Wow, three of them were bad?
I had one bad, but it was just leaking in the open position (which is also the hardest problem to find - Murphy's law in action).
Well, I'm glad you got it fixed!

__________________
1984 300DT turbo 138k mi

Still hauling me to school and back.

Handy Site:
http://www.dieselgiant.com/