dbackbmx3

Weird issue. Just picked up this clean C43. It didn't have working AC so I got it for a good deal. I installed a new compressor air is nice and cold but I noticed as I accelerate the AC air flow changes to the windshield then when cruising or idling the AC returns to the vents. Drives me crazy! Any idea on where to start looking?

benzslo

there is a leak in the vacuum system for the flaps. flap system is powered by engine vacuum and when you accelerate, the engine has very little vacuum. This is normally not a problem unless something is constantly bleeding vacuum, due to a leak. the large vacuum switchover block is behind the glovebox. from there you can test each line/branch off it to see whats holding vacuum and what isnt.

Super B

Thanks benzslo,

dbackbmx3

Thanks for the reply. Sounds like a pain to diag. Just curious is there any access to the flap that controls the flow without pulling the entire dash?

DRBC43AMG

The glovebox is easy to remove with a couple of screws holding it in and then you pull it all out. Take care to disconnect the light bulb. You can get behind to check the block Benzlo is talking about

Sulaco

Isn't there another valve on the passenger side of the hood? I know there was on my pre-update C280, and that box was prone to cracking. It sends vacuum from that box to the one behind the glove box.

But it could also be a leak on the vac line at the back of the engine. I accidentally unplugged it doing the CPS job and lost all control over my air vents. If its rubber connectors are cracked and leaking I imagine it could also cause it.

The good news is it will be a simple cheap fix once you find it.

Super B

Fighting same problem. Hope this helps https://mbworld.org/forums/attachmen...9&d=1316996641

DRBC43AMG

Nice find. Thanks for posting

Super B

You bet, the more I search the more it seems like this is a common problem on our cars. Some really good older posts.

dbackbmx3

So I got behind the glove box and started to play with the vacuum hoses. I watched to see which arms on the bellows move. My thought was OK let me disconnect the arms from the flap. Therefore the flap would not move. It sort of stopped the air flow from changing with engine vacuum but the problem is still present. Way better but its still there. With those arms disconnected I'm at a loss as to why the flap would still move ever so slightly.

dbackbmx3

I finally had a chance to vacuum test from the engine side to the vac box under the fender. It held vacuum just fine. I also tested the line going into the cabin. Also held fine. What am I left with? There is a bellow on the drivers side under the kick panel that controls the center vent correct?