No air from the center vent registers - PeachParts Mercedes-Benz Forum

SirMatt · #1 07-08-2003, 04:34 PM

Suddenly I've got no air from the center vents. Monovalve? Just a guess. I've seen it discussed here a while back but I actually have no idea what the monovalve does.

Thanks in advance for any help.

Rick Miley · #2 07-08-2003, 04:40 PM

No, that's a vacuum problem. Check the green line in the engine bay.

MVK · #3 07-08-2003, 10:15 PM

Check the old posts by me under MVK. I had similar problem. I have some nice photos there too.
If its not the greeen line then look for the above mentioned posts.

MVK

P.E.Haiges · #4 07-09-2003, 12:08 AM

Sir Matt,

I have the same problem ion my '87300 SDL. If you are able to solve your problem, please tell how you did it.

P E H

sixto · #5 07-09-2003, 04:13 AM

P E H, check/clean the in-line filter about 4" from the engine vacuum pump along the smaller line. A good blow from the pump side to the cabin side should do it. Have you checked how much vacuum you get downstream of the yellow check valve on the forward firewall ahead of the brake master cylinder? One line goes into the cabin, the other line goes to a vacuum reservoir somewhere below the left hood hinge. I can help you diagnose the distribution system under the dash but let's start with the simples things.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD

muscle_up · #6 07-31-2003, 06:45 PM

I just had the same problem repaired on my 1991 350SD. I originally though it was the vacuum diaphram, after taking the whole dash apart, found out there was no vacuum entering the cabin. It is best to start from the engine back. After taking it to my local MB repairman, found out the Vacuum Transducer EGR valve was bad (about a $90) part from MB. After replacement, all vents now work properly. Turns out that if any problem fails with the vacuum system involving the vents...it defaults to the defroster vents always open. Long and short, probably a vacuum leak somewhere...easier to start from the main source prior to agravating yourself under the dash.

kerry · #7 08-01-2003, 06:43 AM

Do a search on center vents. Lots of posts on this topic. Often a bad actuator diaphragm or bad switch over valve. You will find threads indicating how the actuator diaphragm can be rebuilt without removing the dash (which is what most people do). You can also just permanently lock the vents open with a bent piece of metal as a quick fix.

Dr. J · #8 08-01-2003, 08:57 AM

Kerry: How do you lock these open with a bent wire?

kerry · #9 08-01-2003, 10:42 AM

Take the glove box out, reach in and remove the rubber boot on top of the vent box, open the vents by pulling back on the lever attached to the vacuum actuator and put something in the vent tubes to stop them from closing. I used a piece of thin flat stock aluminum bent to hook over the edge of the vent box to stay in place. Others have used a piece of rubber hose. Replace the rubber boot and glove box. Voila!

84300DT · #10 08-02-2003, 02:15 PM

i was so tired of the center vents not operating with the a/c on and frustrated trying to trace a vacuum problem etc (couldn't find any..) failed to jam the door open.. so..

i did something really radical today --- i took the durn flap door out!

no more problem

Dr. J · #11 08-04-2003, 06:32 PM

What happens if I lock the flap open, or remove it so the center vents always are open? Will that reduse the effectiveness of the the defroster later this winter, or can I just use the thumbwheel to keep air from coming out the vents when I need to use the defrost?

84300DT · #12 08-04-2003, 07:00 PM

before i took the offensive flap out for good i tested the defroster setting and it worked as before - full heat and a/c compressor on with full air to the windshied which is the default.

(i also thought of the winter.. even if it is 80 deg out ..)

if you do not want heated air coming from the center you can always shut it off manually at the individual vents.

lorenztl · #13 08-09-2003, 08:37 AM

I used dark gray packing foam from computer shipments stuffed between the two center vents to keep the center flap door open. Removing the door is definately a permanent fix. The center vents can be manually shut off with the thumbwheel at the vent opening as well and it's 100% reliable.

brightem · #14 08-11-2003, 12:12 AM

I had that problem on my '82 SD. I got behind the CC unit and found that 2 (I believe) vacuum-controlled valves controlled the flaps. They are plastic cylindrical-shaped with a vacuum hose attached on one end and the control arm on the other end. Suction is applied to one end and the arm retracts to open the flaps. Test the valve by putting a spare vacuum hose on the end and apply suction. the control arm should retract and the valve should hold the suction. If not, take the valve out and take it apart. There's a rubber diaphragm inside the valve that creates a seal to hold the vacuum. More than likely, the rubber will have a split in it somewhere from the countless times of flexing over its life. I was able to do a quick repair on mine by using a tube tire patch to seal the crack. It has held for over a year now. Hope that helps somewhat.

84300DT · #15 08-11-2003, 09:44 AM

the only problem on a w123 is that the vac element that controls the center vents is way under the dash far forward almost next to the windshield, so there is no way that i could see to remove it without removing the entire dash. that is something i for one did not want to do