what is the NORMAL operation of center vent?
 

Okay, I am officially befuddled by the w124 climate control.

When I test drove the car a couple weeks ago the AC worked great and I swear cold air was coming out the center vent. I know i checked it because I know it can be a problem. My wife confirmed this memory. I also remember that, because the car needs a new belt tensioner, when I was testing it out, the AC kicked off and we got warm air through the center vents for a bit. It was, I should mention, super hot then, over 90 degrees and the car had been sitting out all morning.

Now i get nothing out the center vent on AC. side only with a small amount from the defrost all the time in the regular a/c setting. But its been a lot cooler, with the temperature setting of the control almost the same as that of outside temperature. It does not seem like any vents are blowing as hard either. but that may be my imagination. The whole car gets icy cold if i crank it right down, and then the fan clicks down to almost nothing. So its cooling right at least.

So here is my question: does this sound the dreaded vacuum valve(s) is gone or going, or is it normal for the center vent to cycle on and off depending on how cold the car thinks it should be. I have read that it can and that it is normal for the defrost to send a but of air up the windshield, and then I have read that this is a sign of vacuum doom.

Also, the AC just went through a big refurb not long ago with new comp, evap, new control panel, new center vent, etc. I would assume when they pulled the dash that all the vacs were replaced as well, for good measure, but maybe not. I guess I need to sit down with the PO's repair records or call the mechanic up and ask.

But before I do that, just thought I would ask here because either: 1) I just entered the joys of MB ownership and have to cough up 1k to open the dash or become very handy very quickly; 2) the car is smarter than me.

BTW, besides that, the car the DREAM! I love it.

Vacum actuators
 

Same chasis, same problem.

The vents are operated by vacum actuators that open/close the flaps inside the vents and when they fail, there is a default position: center vents flap stays closed, defroster stays open (security reasons) and recirculating stays off (no reciculating).

Fixing that is complicated because you need to remove the entire dash to replace the actuator for the central vents flap.

What I did is insert a pice of wood to keep the central vent flap open all the time. I get cold air but can't control the flaps anymore (I don't care because I have hot weather all year long).

There is a chance you have some isue with a 6 position vacum valve located under de glove box on the passenger side or a bad vacum hose. That's the easy part if you can find the leak. I didn't know how to find that valve in my car because mine has an airbag instead of a glovebox and I don't know how to manipulate airbags.

I hope this help.

that's novel... how did you access the center vent and put wood in it...

that sounds like something i would do for the summer... i won't have time to do vents until the fall or winter....

sounds interesting....
 

yes tell us, tell us how you managed this feat! did you shove it through the vent or something?

its also usually hot here and a/c is more necessary than heat, so this might work for a while.

anyone know if it would mess up the system at all running it this way?

I too have an airbag; I have read of someone jamming something in place by removing the glove box and getting to the arm, and I know its possible in theory to remove the airbag as well..but it would be nicer to avoid that if possible.

The "official" procedure for testing the pods does not require removing the glove box or air bag.

It is in the ACC manual.

What I did
 

Ok. This is what I did.

If you use a long screwdriver you can actually insert it thru the vents and push down the flap and check if you get air blowing.

Inside the central vent, there is a screw. I removed it and pull the left side of the vent just enough to insert a stick of wood using some pliers to lock the flap down. It's a little complicated but after some trial I got it locked.

I wanted to remove the hole vent but It looks like you have to remove the airbag cover to acces a screw on the right side of the vent.

BREWTO: we would really appreciate if you tell us how to test the vacum system without removing the airbag.:confused:

It's more than I can just type up here.

Basically, though, you attach a mity-vac under the hood at the firewall where the vacuum line comes in to feed the switchover block. Then, when selecting different buttons on the control unit in the dash, you check for leaks. Since you know which pods are activated at each setting of the control unit, if a circuit leaks, you know which pod(s) are faulty.