W124 300CE - Center vent closes on acceleration
 

Greetings!

Though I have searched and perused many posts looking for my specific problem, I don't see any mention to it specifically. When I include accelerate and vents in the same search, very little comes up. I have a 1992 300CE W/M104.

The symptoms are; HVAC works great at idle, level cruise and deceleration. During acceleration, center vents close and shift to defrost. When I decelerate, they return. If I want the center vent to operate, I accelerate slightly, decelerate and then smoothly reapply pressure to the accelerator pedal. This works great until there's a slight upgrade, then I have to wait until there's a slight downhill stretch. When the vents switch from default (defrost) back to center, I can hear a "whistling noise" coming from behind the dash, albeit briefly.

All the posts speak of and lead to "pod" failure. But, I'm thinking the pod is working correctly, since the center vent opens and closes, but not when it should.

On this 300CE, there is no glove box. In place of it is an air bag. Hopefully, I won't have to remove this passenger air bag to access the distribution manifold. That is if I even need to look there.

Years ago, on "american cars" there was a device called a "vacuum canister", that when it went bad or leaked would cause problems similar to what I'm experiencing.

Thus, my questions. Is there a vacuum canister on the 300CE? I know there's a check valve somewhere. Maybe this has failed? If its not what I'm guessing, where might one begin to look for this problem?

Thanks!
Michael B.

You have a vacuum leak from one of the elements involved. The normal A/C condition will have the center duct, the defrost duct and atleast one stage of the recirculate duct apllied with vacuum. A leak in any of them and they can't stay held when vacuum goes away under engine load.

I had the same problem in my older 420SEL. Turned out one of the 2 recirc vacuum "pods" has a leak. Its a cheap fix, just a pita to get to....

Thanks for the replies.

I wonder how many pods are accessible without pulling the entire dash. Would you say I should test the pods with a Mighty-Vac from the distribution manifold located under the dash? Or bite the bullet, spend a weekend removing the dash and replace all the pods?

I think I already know the answer.

I would evaluate the leak and fix what is wrong. If it requires the dash to be removed I would do all of them. We replace them all when we do an evaporator