82 300SD ACC problem making me crazy
 

In ECON mode get full flow thru the dash vents, switch to AC and center vents get almost zero flow. checked all switchover valves and they are fine, Tried 2 pushbutton units and same problem. However if I plug switchover valve 5 (17 in manual) that controls fresh air/recirc flap then flow is good in center vents. Any ideas on what to check next.

Bob H.

Bob, my reading suggests the problem often lies with a vacuum switch that is located behind the center of the dash in a spot that, apparently, you can't really get to without removing the dash. My car's center vents don't work either and its bothersome as the A/C frosts up the windshield when operating.

I'm going to just wait until I can buy a nicer dash (without cracks) and replace every vacuum switch in sight when the dash is off.

Good luck...maybe someone else can give alternate suggestions.

Nic
'85 300CD @ 151k miles

Seems like you have a leak in your fresh air flap dash pot, I think there are 2, you can get diaphrams for about $5.

Hi bob:
Do a search under my name MVK. I think about 2 months back I had a nice and detailed post written for this one. Also I had posted some pictures of component involved.

I also hope you did the switch-over valve tests for each of those 5 valves correctly. I still have the feeling that you have a bad switch-over valve.( assuming all vac lines and rubber connectors on the 5 valves and related components are ok)

set the dial on full hot and econ. wait till you get hot air in foot-wall. Now turn dial to cold and watch for moving flaps in the center vents using a flash lights. If you see flaps in center vents moving, your center vent vac-control valve is good( which is PIA to replace nd this is good news). Also test the function of defrost flap vacuum element. I think its the pink one. See the pictures I have posted. If that one is working fine then its the switch over valve for sure. My previous posts on ACC are a must read for your problem. Good luck
MVK

MVK, the SD's have 6 valves and I checked them according to the MB manual. I got out my mityvac today and started checking the vacuum elements, there is a slow leak in the 2 that control fresh air/recirc. What really bites is that I had the dash out about a year ago to replace the gears for the foot flaps and the center flap element diaphram, a weekend in hell. Guess I get to go back there this weekend. :)

One day i want to meet the sadist that came up with the Mercedes ACC system, and kill him..............


Bob H.

6 valves?? Are you talking about the big vacuum elements that contols the flaps directly ot are you talking about the electrically activated vacuum change over valves that are in the same line behind the wood panel above the radio?

When I had the same problem, it was the change over valve one of those five valves that are in a raw.

Vac element behind the dash in center is PIA.

MVK

MVK, Mercedes in their ultimate wisdom and to cause more grief with the ACC system decided that the fresh air flap would need 2 switchover valves and 2 vacuum element, so yes the 126 has 6 valves not 5.

Bob H.

Sorry I give up. I have never owned a 126. But I will watch this thread. Good luck.

MVK

jerryb has your probable answer. The actuators (two) are behind the glove box. Remove glovebox and you'll see them (not a terrible location!). Diaphrams can be had from George Murphy or here http://www.autoluxmbz.com/ . Fastlane didn't have the diaphrams only when I checked, but if you feel more confident replacing the whole actuator, rather than just the diaphram, buy from FastLane.