A/C vent vacuum problem

[cjscopp]

1991 300 SE. During WOT air stops flowing from center vents, goes to defrost or to feet depending on mode selection. Everything is fine with closed throttle and lots of vacuum. Vacuum reservoir is good (holds vacuum) seems like a check valve issue. Anyone have experience with this kind of problem? Thanks..

[sixto]

There's a yellow(?) check valve on the false firewall ahead of the brake booster. The front port goes to the intake manifold, the rear ports go to the cabin and the reservoir in the left fender.

The nice thing about Diesels is that you have vacuum for accessories regardless of throttle position

Sixto
95 S420
87 300SDL

[cjscopp]

I can't find a test for that one. I have to make some assumptions about its' function. When manifold vacuum drops I assume the check valve allows vacuum from the reservoir to operate the needed parts, but I am not sure. Without knowing it's good it's hard to test it and trust the results. Anyone else dealt with this?

[cjscopp]

The fix is in. For the sake of anyone who may experiance this in the future. 4 bad vacuum actuators for the mode doors 2 dual position acutators adn 2 single way actuators. The worst of them was the defrost actuator. The actuators can be checked at the vacuum distribution block behind the center console. Apply vacuum and see what holds. The tricky one to replace is the defrost since the actuating rod goes through the ducting. The easiest are the recirculation ones right behind the glove box/passenger airbag. Air blows real well from the center vents now and distributes quickly when modes are changed.

[126nut]

Did you have to pull the dash?

I remember doing all the elements on my 380SEL some years ago and I had to pull it, what a major amount of work.

I have a feeling my current 420 is gonna need them replaced soon. At times under hard acceleration the air will go full defrost.

[cjscopp]

Nope didn't have to take the dash out. Through the center of the dash you cut a 3 sided box around where the actuator lever goes through and fold back the flap to access the actuator rod. Then just fold the flap back and secure it with either tape or whatever adhesive you like. The other actuators are easy to get at.