AC center vents not working - PeachParts Mercedes-Benz Forum

Gary F. · #1 03-31-2002, 10:12 AM

I have an 85 300d. Yesterday I ran the ac for the first time in a couple of months. Everything was fine, except I was getting almost no air flow from the center 2 vents.
Any help would be appreciated.

Give a man a fish, you feed him for a day. Teach a man to fish and you feed him for a lifetime.

tcane · #2 03-31-2002, 11:30 AM

My 300D had a similar problem years ago. I found most of the cold air was coming out of the defrost instead of the center vents. The problem was the vacuum actuator (a large valve and pink in color) that opens and closes the butterfly that directs the air flow to the center vents. A small piece of the valve actuator's plastic broke on the piston that moves the rod connected to the butterfly. Your valve actuator my also be broken because the plastic fits tightly around the rivot that connects the piston rod to the activating arm (later I had to replace another vacuum actuator and the rivot connecting the piston to the rod was so tight it barely moved, I removed the rivot and connected another rivot that allowed free movement of the rod). Or, there may be a vacuum leak to this valve actuator in a vacuum line or the vacuum line fell off. Or, the diaphragm in the valve actuator is cracked, split, etc. Or, there may be some other problem causing the butterfly to not work properly - like the switch over valve that receives the elec. signal to activate the vacuum actuator and changes the vacuum to the element so it moves the butterfly. Or, the plastic check valve has cracked that is part of the control system to the defroster and center vents actuators.

Also complicating things is the fact there are two subsystems that can be causing the problem because there are separate valve actuators/switch over valves - one for the defrost butterfly and one for the center vents butterfly.

More likely it is the center vents subsystem where the problem is. The vacuum actuator for mine was located above the accelerator pedal, behind the lower dash panels, and screwed to the heater-evaporater assembly. Your actuator looks to be in about the same place but higher up and more in the middle of the heater-evaporater assembly - meaning it is harder to reach than mine. I was able to fix mine with a bit of epoxy and this fix has lasted over 12 years - knock on wood. Perhaps another member with experience on your model year can tell you exactly where the center vent vacuum actuator is located.

I will say that these are pretty complicated climate control systems and fixing them can be a trying experience. In my opinion, M-B made the climate control system far more complicated than they needed to. In your case, the climate control system is not as complicated as mine that has the servo controlled climate control - whatever solace that provides. A shop manual is a needed tool to work on these systems - a good one can be found at your library in the Mitchell service manuals (at least my library has the Mitchell manuals). You might get lucky and the problem is simple, if not, then a shop manual is a must have.

Good Luck!
Tom

gsxr · #3 03-31-2002, 07:09 PM

Tom nailed most of what you need to check. Any vacuum leak - anywhere - can make the ACC system vents act up. Assuming that's not it, you can access the 5 solenoids by pulling the pushbutton unit out. Then you can remove 1 hose at a time, connect a MityVac, and see which ones don't hold vacuum. Every hose should - if not, the diaphragm is bad for that actuator.

One correction - the actuator for the center vents is up top, behind the vents themselves. It's visible by removing the glove box but you can't replace it without removing the whole dash.

The dual-chamber actuator above the accelerator pedal is for the defroster vents, IIRC. My center vent pod is bad too, I've just been dreading the dash removal job though. My defrost pod is 1/2 bad, I plugged the one end so it works without creating a vacuum leak for the rest of the system. :p

My advice is that if you do pull the dash to do this, replace EVERY actuator (or the internal diaphragms, if you choose to repair them). Otherwise you'll end up doing the job all over again when the next one fails.

engatwork · #4 03-31-2002, 07:28 PM

Shine a really bright light down into the center vents and see of you can see the door opening and/or if the actuator is attached. The pin fell out of mine and I was able to put a ty rap in there just by removing the rubber piece between the ducting and the vent section. I was able to do this without having to removed the dash.

#5 03-31-2002, 07:50 PM

Same problem here Gary~

The vacuum actuator that controls the center vents is the hardest to get at. Its replacement can only be acheived by removing the dashboard. I didnt want to do this when mine failed, so i have learned to live with it. Instead, the flow will be directed to the front windshield vents, hey~ something Mercedes Benz never invented but should have on these cars. Defog the front windshield and have A/C in the cabin at the same time!

tcane · #6 03-31-2002, 08:28 PM

On my car, the actuators are positioned differently and that's why I suggested another member that has your year model could tell you exactly where to look for the actuator for your model year. M-B changed the design (from the early servo) and in so doing moved lots of things around - my diagram shows the center vents actuator where I described it (inaccurate manuals are not unusual). Also, on my model the only way to access the switch over valves/solenoids is to remove the center console after removing the driver's seat (major PIA). Being able to access these switch over valves/solenoids by only removing the panel will make diagnosing a problem there a breeze.

If you have to remove the dash, then look for a thread with pictures (as I recall) by a member who replaced one. Good advice about replacing or repairing all the actuators if you have to R&R the dash - this is a job you do Not want to repeat because an actuator later leaks.

Good Luck!
Tom

Gary F. · #7 03-31-2002, 09:18 PM

Thanks to all of you for the information. I am dreading pulling the dash. I might just have to live with it.

MarkM · #8 03-31-2002, 09:23 PM

.....but when I was first looking for an MBZ diesel, I always had a special liking for the older 240s with manual shift, rollup windows, manual sunroof, and especially lever-controlled heat/AC vents....how simple.

I had non-operable center vents too, about a year ago....I was lucky to find the problem very quickly ...one of the little vacuum swithover valves (in the row of five) behind the pushbutton unit was bad. I got a replacement from a local junkyard for a few bucks.

Good Luck

mark

tcane · #9 03-31-2002, 10:24 PM

Mark:

I too wish the AC system was a lot simpler. Give me a three way switch for the fan, an on off switch for the AC compressor, and a heater valve control lever. The 300D engine I found to rebuild was in a Euro W123 (badly rusted body, esp. the wheel wells) with the simple AC system just described, but someone got to the car before me and took some of the parts - enough to prevent me from doing a conversion and throwing my AC servo system into the trash. I took some of the stuff and will keep an eye out for another W123 with the rest.

Tom

leathermang · #10 04-01-2002, 03:45 AM

Tom, that sure sounds like a good subject for a FAQ page... hope you take notes and pics as you do that ( convert ) ....
I just saw your statement about inaccurate manuals.... geeeeez.. is nothing sacred or trustworthy ?

Couldn't you use standard industry stuff on that instead of having to find an old mb with that system ? Greg

Gary F. · #11 04-02-2002, 11:37 AM

Yesterday I was off, and decided to take a look at the problem. I noticed there was a butterfly valve that wasn't opening. I took a screwdriver and was able to get the vent to open. I had the fan on high speed a some debris blew out. I noticed that the valve stayed open but it was oscillating, but at least it was allowing air to flow out the vents. However when I turned off the ignition and restarted, the valve wasn't opening. It appear something is making it stick closed. So I got creative. I didn't want to take the dash apart.(ugh!!!!). So I fashioned a piece of coat hanger wire and placed it so the valve would not fully close. Everything works great. Since it is getting hot here, it is ok if the center vent stays partially open all the time. I can pull out the wire later when the weather turns cold again. PS You can't see the wire.

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