Weird Air Vents 300E
 

On my 91 300E, I selected the Defrost Selector and my vents will not return to front only, front bottom only, etc. The Defrost vent is stuck in the open position and air always blows from the front, front sides, and defrost vents. Prior to slecting the defrost, everything worked perfectly. Any ideas on pinpointing the issue? AC still blows ice cold through front and side vents but aslso through defrost vents. if I switch to defrost, hot air blows out of the defrost vents, side fron vents, but cold air blows through front middle vents. Also, I cannot stop outside air from coming in by selecting the recirculating switch ether.

Sounds like vacuum pods went kaput.

It is WAY too muich trouble to change a pod just because you SUSPECT that it is bad. You should properly diagnose before replacing the pods since it is about a 15 hour job.

Remove the glove box liner and you will see a vertical manifold at the lower left of the glove box opening. Remove the lines top to bottom one at a time and use an auxilliary vacuum source to see if they leak. If you find one or more that leaks and it is NOT the defrost pod which can be changed without removing the dash, then buy ALL the pods and replace all of them while it's apart.

Sounds like I should just live with the problem. Thanks guys. I'll check out the vacuum lines and see what happens but with my luck nd age of the car, it will probably be the pods. Woo hoo!

Well, I decided NOT to live with the problem and got a vacuum pressure tester. Took apart the glovebox and figured out how to test the lines. Here is what I found:

On the manifold from top to bottom...first 2 lines did not hold vacuum pressure. Last line did not hold vacuum pressure (which goes to my defrost vent pod so it can't hold the vent in closed position, and is stuck in open position).

I tested all of the vacuum pressure coming from the manifold and every line had pressure so no problems there.

I was able to take out the defrost vent pod and exam it. I hooked up the vacuum tester to the rear nipple and was able to detract the pod's arm which would close the vents but the front nipple would not hold vacuum pressure. Somehow, I think this is prohibiting the pod from detracting when installed.

Any thoughts out there? Which Pod does the top 2 lines go to?

Thanks,

Steve

The defroster pod has three positions - fully closed, mostly closed, and open. Fully closed requires applying vacuum to both vacuum ports. It's typical for the upper vacuum chamber to fail first. When this occurs you can only get to the mostly closed position. This is why you get condensation on the windshield when running the air conditioning in humid weather.

The fresh/recirculated air pods (there are two) operate the same way, and have the same failure mode. Likely this is at least one of the lines that won't hold vacuum.

I'm at the office and don't have the information on which vacuum line operates which pod. It's been posted in the past, perhaps you can find it using search. The vacuum lines are color coded (e.g. red w/white stripe), the function is identified by color code.

Looks like I'll replace that deforster pod and then when I feel like tearing apart the dash, replace all others in there as well. I ordered the Defrost pod for now. That is easy. Thanks for the help!

Since this thread is here...Probably a good time to ask. I do not get any heat out of my center vents at all on my 260E. The side vents do, but nothing from the middle ones. I just get cold air all the time (incl A/C). Any ideas? Is this a pod issue?

I would have to say yes based on what I saw and tested when I got into the manifold. Mine does the same thing as it is letting in the outside air which is coming through the center vent because one of the 2 pods (or both) that controls the outside to inside airflow are ruptured and can't close. Supposedly the defrost vent has three positions which also may cause that issue. I bought a vacuum tester at Kragen Auto for 29.99 and it worked perfect for testing the vacuum leaks.

The system does not deliver heat out of the center vent by design. The logic in the climate control unit closes the center vents and directs air to the footwell vents when it is heating the cabin. The center vents are used only for cooling the cabin, whether via a/c or fresh air. This operation is described in the MB climate control manual for the 124 chassis.

The side vents deliver heated air, as noted. This is to keep the side windows defogged/defrosted in cold weather. I find this annoying, so close the side vents on cold days.

Good 'ol Mercedes Benz! What will they think of next! Thanks for the clarification. Got my new Pods today and the ones in my car are definitely bad as these react differently when vacuum pressure is applied than the ones still installed. I'm sure this will fix my problem.