Last week my center dash cooling vents stopped opening. I pulled the Vacuum line off the electrical vacuum control for the center vents and applied vacuum with my hand tester. The vents open and stay open with vacuum applied so I'm sure the Vacuum servo is OK. I measured the voltage on the connector to the electrical vacuum switcher and found it had + 12 VCD across the 2 pins except when in defrost mode. I checked to see if any vacuum was present on the output connection from the switcher where the hose goes to the Vacuum servo. No vacuum So I suspect I have a bad electrical control valve for the center vents. However I wonder if the Pushbutton unit might actually be the culprit. Part of this reasoning comes from the fact that during cold weather when heat is called for the foot vents do not open. however if I put the system into bi-level mode the foot vents open in both heating and cooling modes with out failure. Does Anyone have a schematic diagram of the Pushbutton unit so I can figure out if the PBU is flakey or I simply have a bad Electrical vacuum switch.

Thanks Russ

It sounds like a bad change over vacuum valve. Its one of those five valves behind your ACC. Thank God your vacuum element for the center vents is holding the vacuum. Otherwise its PIA to change that element. it requires you to remove the whole dash board.

Now in cold weather foot vent will not open at all until you have acheived sufficient temp during engine warmup. This way it wont blow cold air on yr foot. Only after the coolant temp has reached above a certain value, the legroom vent will open.
Now that its warmer weather, start the car, let it come to about 50C or more, press economy push button, and set temp dial to hot do not engage in hot, and see what happens, if it opens the leg room vents( in about 5-10 seconds) I think you are fine. If not
you have 2 possible senario( assuming that all vacuum elements and vacuum lines are fine)
1. A bad ACC unit.
2. One or more bad change over valve.

While you have access to all 5 valves at this point, I would test the function of each of them. Its a very simple test described by MB repair CD( I can attach PDF if you send me your e-mail). Once you have done this and found the bad valve, replace it because you will need to replace that valve( or valves) irrespective of your ACC console was good or bad. Once you have replaced them test the system again and see if all flaps open and close as per the owners manual. If so you are all set. Its only the valves and not ACC.

Also no or little or irrational behaviour of heat can be due to a bad monovalve. Its very simple to inspect and repair( about 25/- bucks for repair kit) if necessary. A second reason for delayed heat esp during idle is a non functional auxillary water pump. its funcionality can be checked very easily too. Locate the Auxillary water pump, remove a little black cap on the back side of the pump with a fine screwdriver( see my previous posts for a photo)
Now start your car press defrost button, and touch the back side of the auxillary water pump. you should feel a rotating shaft there, this tells you your pump is working fine. Now press the economy button and set the dial on heat(red), you should again feel the rotating shaft. This is good. Now press turn the dial to full cold and feel the shaft. After a few seconds(and as much as about 30-60 seconds) feel the shaft on the back end of that pump again. It should have stopped now. If it did stop Congrats your Aux water pump is fine.
If during the "shaft feel test" in defrost mode, the shaft feels not turning you have a non functinal aux water pump. Next step is to disconnect the pump, from the socket and test it with a direct supply from the battery or any other 12V DC, if you feel the pump is working(shaft can be felt turning) then the power supply from ACC to this pump is not working. To confirm this press the defrost button while engine is on, and measure the volts across the the 2 points on the female plug it should be about 12 V. If there is no voltage its an electrical problem and is mostly due to a Fried ACC or broken connection.

I know I gave you too much details but I went thru a hell to learn all this last week and I wanted to be as easy for you and other as I can be. This description applies to my 1985 300D turbo 123.

Let me know how it goes. My hunch is you have atleast one bad change over vlave, a busted or nearly busted monovalve. and a nonfunctional auxillary water pump(or may be not).

Good luck
MVK

__________________
One penny saved is three penny earned.

1985 300D Turbo 179,000miles

MVK:

I have an alert ear to the advice you are giving Russo. I am experiencing the same issues in my 79 300 CD (123 - with "servo' styled climate control (MB Manual - ACC II).

Your troubleshooting steps give me excellent incentive to takle this issue myself. I have a mityvac, and will begin my testing.

My question is do you think I should start with the vaccum elements just behind the ACC, or should I look at the ones that are under the dash first? I think, upon looking at my MB repair CD manual, the ones under the dash are called"flap" elements. One for the legroom flap-lever, one for Defroster Nozzle Flap, and ( this one is a b*tch ~ Center vent flap. Luckily, I get air out of my center vents, but just a little cause its all going out the defroster vents.

First I will describe the symptom:

My symptom is that most air flows through the defroster vents during all stages of the climate control. This happens with the compressor switch on or off, temp dial set to lower temperatures and higher temperatures. Some air flows through side vents, very little air flows through center vents, no air flows from leg vents. The temperature of air flowing through these vents changes in response to temperature dial settings and / or compressor switch changes, but the vents do not open or close in response to changes in ACC button selections. The fan / blower speed responds correctly to button selection changes, as does the force of the air, but, as stated earlier, most air flows through defroster vents, with only very little (leaked / incidental) air flowing through side and center vents.

Additionally, at someone’s suggestion I tried the following:
1. Without starting the car, but activating the ignition, choosing the DEF button resulted in airflow (through defroster vents), choosing Auto-high, no airflow.
2. Start Car. Press Auto-Hi, blower starts once auto reaches temp (about 45-60 seconds)
3. Turn off ACC, Switch off ignition. Wait 10 minutes, turn key to just before starting, select Auto-Hi. Airflow immediately

I believe this test was to confirm that there is not a vacuum leak coming into the ACC system, and that perhaps my troubles are with a leak within the ACC system. Also service records from the PO show that the servo, amp and center dash vent actuator were replaced in June of 1999 along with a Freon recharge.

So, after all that, here are my questions:
1. Do you think that I should start with changing the Vacuum Changeover Valves? (FastLane # 14045-12000, or 04045-70190, I am curious about the differences between these two.)
2. Should I change other vacuum elements:
a. Defroster Nozzle Flap 04010-25703
b. Legroom Flap Lever 04010-25709
c. Defroster 04010-25709
d. Legroom Flap VE 04010-73599
e. Center Vent Circulating Flap 04010-56034

Have you seen this symptom in the past? What do you recommend? I hope that I am not asking too much, but hope that you can offer a bit of guidance.

Thanks,
James

__________________
James

85 300SD 285k
Charcoal Gray/Grey MB-Tex

79 300CD 142000mi "Rabenshwarz"
Black / Black MB-Tex, Burlwood Int. TOTALLED - 10/24/02 --

Hi James:
Well lets first say this My advice in previous post is only for my model or comparable. My car is MB 300D Turbo 1985- 123.
This car uses the ACC Type 3 or third version.

Your car uses ACC type 2 or second version. So things like location of vacuum elements and location of change over valve will be different.
I am not a mechanic and I have never owned anyother Benz so I have very limited experience with your model.

So it would be not a good idea to give you a wrong advice. But as a rule of thumb you start trouble shooting with simplest things first.
1. Vacuum line attachments and connections in engine compartment.
2. Visual check of flap movement in center vent. This is what you do. Engine running., car at normal temp( about 60-80C), set to the highest heat and look with a flash light into the center vent,while looking in there change the temp dial to cold. If you see the flap opening( does not matter is air flow is very little) then mostly(99.9%) the vacuum element that controls the center vent flap is good and I will not mess with it.
3. If your legroom flaps open and close as they are supposed to during the heating operations, then the vacuum elenemts for those legroom flaps ar fine as well (99.9%).

If # 2 and # 3 above are passing the test then

4.Start trouble shooting defrost vent. Defrost vent in my car stays open if you do not apply vacuum to it. So if that vacuum element was bad due to internal leak, then it will never close and most air will go to the defrost vent on top. Second assuming that defrost vacuum element was good but still stays open, then your problem is with the vacuum not reaching to the defrost vac element. and no or insufficient vacuum to the defrost element means it cant close and stays open. so most air will go to thar defrost vent.
Why the vacuum not reaching to the defrost vac element.
a. Bad connection - leaking or detached line between the change over valve anf the defrost vac element.
b. non functional change over defrost valve. staying closed so that no vac goes to defrost vac element.

If a. in above a abd b is good, then b. is your problem.

There are simple tests for testing vac elments and change over valves. Check the benz cd. Its available from ebay for about 20-30 bucks. Its worth buying. It will make your life very very easy. It has a whole section on ACC. covers many different model years. look for 123 benz cd in ebay.

Well thats all I can say hope this helps. I am going out for 10 days.

MVK

__________________
One penny saved is three penny earned.

1985 300D Turbo 179,000miles

Finally! Found some info on the vacuum change over valve for the climate control not the engine one. My interest is that I have '83 300d turbo that has the third version CCU good strong vacuum pump but no pod activity(even tho I tested them all as functional). I can at least confirm that the CCU puts 12 volts to the change over valve for the green line to the right most pod near the passenger A-pillar behind the glovebox. I first applied 12v from the radio red and black but I found that caused harsh switching. You can set the CCU to full AC and unplug and replug the switch harness from the CCU and here it click to let you know its doing something. I may have more issues with these change over valves but I found that my problem is somewhere near the firewall where the vacuum is divided between brown ignition hose and the central locks yellow lines. The line was purposefully plugged to the locks but I'm still not getting enough vaccuum for the pods. I may have an orifice looped in where the vacuum manifold input goes the switches too. More investigating is needed to get this working. I'd like to get my dash back in sometime before next year.

I was just working on this problem yesterday. It could be that you have a leaking defrost pod. With total ventilation, defrost pod dont use vacuum. On all other heating modes, it uses vacuum. If you have heat coming out of the foot vents during during total ventilation its because no vacuum is needed for the defrost pod. Check your main air pod too.

While your in there, check the switchover valve for continuity. They should have continuity.Replace as needed.

Attached Thumbnails

 

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