I have a 1988 300E with 223,000 miles. The car is perfect except for one problem that crept up in the last week: When the AC is engaged, cold air blows only out of the windshield defogger vent and the left-most vent to the dash. Nothing whatsoever comes out of the center dash vents or the righ-most dash vent.

Any ideas as to the source of my problem? Is there a malfunction in the pushbutton control unit that is causing these vents to be closed off from inside?

And, yes, I have manually opened the center and right vents with the wheels next to the vents.

Thanks in advance for your help!

Ed

I'm not sure about the side vent but your loss of the center vent with accompanying air from defroster is a common problem. The system defaults by spring pressure into the defrost position, for safety reasons.

The vacuum servos that close the defrost vents and open the center are all on the same vacuum source. Thus any loss of vacuum including ruptured servos (2 defrost, one center vent)or open lines or mulfunctioning switch over valves or bad source vacuum will cause this problem. Usually it is one of the servos.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Steve,

Where do I find these two servos?

Ed

Deep in the dash. Do you have the 124 CD-Rom? It would give enough info to diagnose the problem and maybe enough to fix it.

I have never replaced these servos myself. I only do diagnostics. We put all the servos in when we do an evaporator as some of them are next to impossible to change without removing the dash. Maybe Donnie can help here; I know he has done this a few times (bg).

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Thanks Steve!

We do have the 124 CD Rom. My technician and I will venture forward tomorrow with this project. I will let you know what transpires.

Ed

Hi! I did change those actuators and, unfortunately, the entire dash has to come off. The good news is, with the manuals, a DIY like myself can do it with no problems. It is a long process but "doable". I would recommend to change all the actuators while you are at it, and check the evaporator for freon leakage.
I hope my reply is on time, otherwise, somebody else can use it...
J. Hidalgo
86 300e

Thanks Steve!

Your reply suggested that the source of my problem was either: one or more of the vacuum servos, a bad switchover valve, or a vacuum leak.

Well, the problem turned out to be a vacuum leak at the one of the vacuum lines that goes from the engine compartment (near the instrument cluster) to the climate control area. Once this vacuum leak was fixed, the system operated normally again!

Thank you for saving my technician and I alot of time!

Ed

Ed,

Can you identify which hose causing the problem? Was it inside the car or under the hood?

Thank you

David

David,

The line was under the hood, in the area behind the master cylinder.

Ed

Ed,

Thank you for the info.

David

I have the same problem on our 126. 85 300SD Turbo.
Using the normal vent the air goes to the defrost B-U-T using the defrost vent the air goes to the center vents. And in both modes a little goes to the defrost and fogs up my windows in our South Texas humid nights :)
THnaks for the insights. Leon