HI I AM NEW I HAVE A 1999 S320 .WHEN I CUT MY AC ON IT BLOWS FINE THEN AIR STOPS BLOWING THRU THE CENTER VENTS AND STARTS TO BLOW THRU THE DEFROST.ALSO AT TIMES IT WON'T COME ON IN AUTO AT ALL THEN ALL OF SUDDEN IT BLAST OUT.PLEASE HELP I LIVE IN THE SOUTH I NEED AC, THANK YOU.
Welcome!
I am researching the AC problems on my w140. First of all flaps controlling the air flow are operated by vacuum that is generated by the engine and also by a pump module that sits under the rear bench in a foam box. The vacuum pump has a tendency to fail. Secondly the AC Pushbutton Controller sets all flaps via a vacuum solenoid block that is located behind the Glove Compartment. The AC controller has many test features to test manually all flap functions. You could subscribe to Alldata.com ($24 for the year) for you specific vehicle and year to have online diagnostics and troubleshooting help. It certainly helped me to troubleshoot and understand the functionality of the AC. Alldata provides a table to test the function and also how to read trouble codes from the display of your AC controller. If the manual test does not operate the flaps that divert the air, than you definitely have no vacuum to operate the flaps as in my case. There is also an aspirator blower (a sensor with a little blower inside sits close to the sun-roof switch behind the little louvers) that controls the function (speed of air and flaps via the AC Controller). This aspirator blower has a tendency to fail. Alldata tell you how to test it.
You have to dig a little in, its kind of complex!
Hey, more brains is better than one, and please post your steps as we can all use it. Especially me, I have the exact same problem where the middle vents are direct up to the defrost vents. And my ac system has a small leak somewhere too. I found a tube dangling in the engine compartment, it's light gray in color and has been disconnected from it's source. It's on the driver side and I also see a location on the back side of the throttle controller (intake)that looks like a tube should be plugged into it. I will buy some vacuum tube and connect to see if will fix the middle vents. I know we all like Benz cause they look cool and make us feel upper class but dang they sure such in quality as we all have the same problems over and over.
look at my posting about the AC middle vents
I just fixed the one on the 600. I believe all other models are similar.
The Ac Air Flaps are operated by engine Vacuum only! So if the gray line has a disconnect at the engine, no vacuum will get to the AC. The default position of the AC Flaps without vacuum is air from the defrost vents to prevent fogging up the windshield. The Pump sitting under the rear bench does not supply vacuum to the AC. As the engine does not provide vacuum during hard acceleration the vacuum reservoir act as a buffer to provide vacuum. The gray line from the engine connects to a vacuum distribution and valve block in the passenger inside wheel well area. A red/white line connects to the vacuum reservoir. and a green line connects to the AC vacuum solenoid block controlling the Air Flaps. Now the interesting finding is that also a pressure line coming from the central lock pump (rear bench) connects to that vacuum distribution block. In my opinion the pressure operates a valve inside the distribution block, because if the pressure coming from the Central lock pump is missing due to a leak or a defective pump, the AC will not receive vacuum from that distribution block. In my case the distribution block had a leak.
Hope that helps
I just fixed my middle vent issue on my 95 s320. On mine, there was a gray tube on the firewall in the engine compartment on driver side. I got some vacuum tube extension and plugged it onto a piece around the back side of the intake manifold. This cannot be seen from front of car, you'd have to get a flash light and look around the back-lower side of where the intakes comes together. Good luck.
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