Hi. I have a 94 280 with an intermittent air distribution problem. I've read posts with similar problems but could not find anything close to what I'm experiencing. to summarize the issue is as follows:

- air flow cuts out the center vents and is redirected to the defrost and side vents
- the issue can start when the car is first started or after the car has been running for a short time
- the air will cut on and off the center vents thought-out the day without any set pattern
- at times accelerating will cause the air to be redirected
- decelerating does not redirect air back to the center vents
- the air is cold and strong and the windshield gets a nice frost when the air get redirected
- when the air get cut from the center vents and directed to defrost, the air direction keys don't work
- once the system directs air back to the center vents, the air direction keys work
- there is no pattern, that I can see, as to how long air is directed through the center vents, it varies from 5 minutes to 20 minutes and then cuts off. Will stay off for longer periods before getting directed back to the center vents
- tried playing with the cabin temperature but the system still redirects air away from the center vents
- not sure if it's related, a pump in the rear / trunk of the car runs for a while once the car is stated then stops. Then after a while, the pump starts up again runs for a while and then stops, the cycle goes on as long as the car is running
- not sure if it's related, had a water leak in the cabin, front passenger side. tree stuff clogged some drain hole and water leaked in though some filter, which was replace and correct the leak. However, under the rug is a device that has a knob on it, not sure if it's connected with the air flow system or if it was compromised when the rug got wet.

I think that's it. Any suggestions or hints to getting this issue resolve is greatly appreciated. BTW, this is a great site, very informative.

Ronin

Hi all,

"I posted an update at the end of post"

Here is some more info on my air flow issue.

I've had the vacuum lines tested using a smoke machine that uses up to 15lb's of pressure and I think he said a coconut oil compound / mixture to help identify leaks. The mechanic said no leaks were found. I also noticed that when air flow starts to cut out of the center vents and redirected to the defrost vents it's done ether gradually or immediately, meaning:

Gradual air flow redirection observation:
- when air working and flowing through the center vents:
- air is starts at the center vents
- air then gradually flows to both the center and defrost vents with more air to the defrost vents
- air then cuts out on the center vents
- air flow direction keys do not work on the control panel once air is directed to the defrost vents
- when air cuts back to the center vents, its immediate and all the air flow direction keys work on the control panel

Immediate air flow redirection observation:
- when air is working and flowing through the center vents
- air is starts at the center vents
- air then immediately stops flowing from the center and is redirected to the defrost vents
- air flow direction keys do not work on the control panel once air is directed to the defrost vents
- when air cuts back to the center vents, its immediate and all the air flow direction keys work on the control panel

Any and all suggestions are welcome, I'm anxious to get this issue resolved. More so since the car gets really hot when the air get redirected to the defrost vents. Thanks for your help.

Ronin

Hi all,

Here is an update on this issue.

After several tests, my mechanic said that the vacuum distribution module, located inside the engine compartment on the passenger side of the car near the firewall was the issue. I had the module replaced and the problem persists, sooo were back to square one again. A few more observations on the issue, I'm not sure what is normal operation and what isn't so please forgive my ignorance.

- with the control unit off, outside air blows into the car from the side and defrost vents, it's very light but grows faster if the roof is open
- with the control unit off, on occasion, gusts of air hit the windshield and fog it
- once the windshield gets fogged, I can turn on the defrost and it works very well, I can then turn off the defrost and it stops without issue


Thanks all for reviewing this issue and any suggestions which can help fix this issue are welcome.

Ronin

Hey there,

I have a 96 C280 that did the same thing. I did a few things and finally located the problem. (Assuming you have the LCD temp control unit above the radio)

It turned out to be the control unit connector. Get two slim peices of metal to slide into the from of the unit, remove your glove box so you can use your hand to push the control unit out of the dash from the rear of the unit. Use the metal "keys" to release the clips that keep your control unit in place.

Slide the unit out, there are two connectors I believe. There are a little tricky, there is a release arm that when pulled away from the unit, pushes the connector out of the socket. On my unit the pins on both the socket and connector where corroded. I used a small metal brush and some contact cleaner to remove the corroson.

Push the connector in and out of the socket several times, then give the system a try!

This fixed my unit. It would act the same as you discribe and make a strange deep wind instrument sound too... foggy windows and all.

Before I figured this all out I also disconected all the Vaccum lines from the solenoid control module located behind the glove box. It's easy to find because all the vaccum bladders that change the position of the routing fins have a thin hose that goes to this unit.
So if you have worked with a volt meter you can pop the lid on the soleniod control and measure the coil load on the 8 solenoids, or disconnect the unit and trigger each solenoid with 12v. (Don't worry about damaging it, no IC circuits just a plain old Solenoids.)

I hope this helps! My motivation was 97 degree weather in the summer with all the AC blowing on the windshield and heat routed into the car!!! So on top 97 outside I had the heater stuck on!

Hi TobyWonKanobi,

Thank you for replying to this issue and for your suggested fix. I know then the issue was being looked into, the Control Unit was not removed and the connecters were not checked. As soon as I have some time, I'll look into this fix and let you know if it corrects the problem. Thanks again.

Ronin

The most common problem that causes vent operation problems is vacuum supply or loss. The fresh air vacuum diaphram has a high failure rate & can cause this any time the A/C compressor is on & you ask for A/C. Try pushing the EC button & then see if the vent problems go away.
ALSO under the hood there is a vacuum supply manifold(plastic) with 10 vacuum ports that CRACK & leak vacuum as well. There is also a vacuum storage tank under the rt fender that connects to that manifold as well. They also will crack as they are plastic as well.

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Hi M.B.DOC,

Thanks for your input to this issue. I had both the 8 - 10 connector plastic vacuum manifold replaced and the vacuum storage tank pressure tested. The vacuum storage tank I'm told passed the pressure test (whatever that was) and the vacuum manifold had a slight crack in it. After replacing it, the issue stayed the same.

In testing the fresh air vacuum diaphragm, how does one go about testing it and replacing it, since it has a high failure rate. I'll try playing with the EC button & see if the vent problems go away. I'll post the results.

Ronin

Hi M.B.DOC,

With the weather warming up, I looked into the air issue again. First, I played with the air flow direction buttons (top, center, top /down & down) and the air continued to blow to the windshield. I then pressed the EC button as you suggested and the air flow did start coming out of the center and lower vents. At first it was nice and cold and then steadily got warmer. I then pressed the EC button again to turn it off and for a very short while, the air flow stayed to the center and lower vent and then went back to the windshield. Pressing the EC button again, repeated the operation.

From this description, do you feel the issue is still the Fresh Air Vacuum Diaphragm. If it is, where is it located and are there more then one that need to be replaced. Thanks for your help with this issue.

Ronin