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Old 07-22-2016, 01:03 AM
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W212 AC self control, behavior and habits

I had W124s for many years. I know the 124 very well. Replaced two evaporators along the way. I recently bought a W212 E350, 2012. I am curious about the AC. I notice that when I shut the car down with the AC on, the car sits patiently for about an hour and then the blower starts up to dry the system. Seems to run about 30 minutes unless you open a door, then it stops. When driving, I can turn off the AC button but the AC continues to cool for several minutes. I notice the recirc door is activated even when the recirc switch is off. The system has a mind of its own. The W124 was very responsive to the driver's commands. This W212 seems to think it is smarter than the driver. Some of this is mentioned in the owner manual, like the recirc being on when the switch is not and the AC staying cold a while after you turn off the AC switch. Seems to be less info on these newer cars. How can I get details about how this system actually works? Thanks!
2012 E350
2006 Callaway SC560
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Old 07-25-2016, 10:16 AM
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I have a face-lifted W212 and I've never observed the blower motor running when the car is off? I'll have to check mine, next time I turn the car off the with the A/C on I'll return in 20 minutes and see if the blower is running.

I'm also a long-time W124 owner and I'm very, very familiar with the HVAC system on this series mainly due to having to fix it so damn much. When its working correctly its world-class in all respects but the number of failure points (vacuum pods, aspirator motors and tubes, vacuum lines, vacuum control blocks, blower regulators, control control valves) on the W124/W123 and W201 series cars is just mind-blowing. The transition to variable displacement A/C compressors in the later cars was a real game changer, much more efficient and easier on the mechanical bits as instead of the full-on/full-off clutches of the earlier designs. I tried to use my mityvac a couple of weeks ago and found it inoperative after sitting in the case for 7-8 years. In my W123/W124 days it seems like I was using it at least every other month to diagnose vacuum leaks.

In stark contrast I know absolutely nothing about the HVAC controls in any post W202 series car I've owned because I've never had any problems with the control circuitry. Pure guess on my part but I'd guess MB used way fewer vacuum controlled devices on later control systems.

I really like the way the W212 manages the blower speeds in the AUTO setting, the fan kicks in high-speed and then drops off as the cabin cools/heats. I've also noticed the HVAC system seems to communicate with the engine management systems to enhance cooling performance. When I start the car after its been in the sun a couple of hours and the interior is 140+F the transmission shifts at higher RPM's until the cabin cools off. It appears the me that the engine management system uses higher engine speeds to enhance compressor performance. Pretty clever.
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Old 07-25-2016, 10:21 AM
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I have been able to learn that the fan starts up on low one hour after shutdown. It runs for thirty minutes. It's in the MB literature and my car behaves exactly like that. On my car you can see the fan by removing the ductwork over the battery in the engine compartment. The recirc door is also visible there.
2012 E350
2006 Callaway SC560
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Old 07-25-2016, 10:22 AM
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The blower running to dry the evaporator feature has been linked to battery draw problems and is usually coded out to prevent it.
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Old 10-18-2017, 06:30 PM
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Originally Posted by duxthe1 View Post
The blower running to dry the evaporator feature has been linked to battery draw problems and is usually coded out to prevent it.
Do you know how this is coded out to prevent the running constantly without also completely disabling it? I want to be able to tell my Indy how to do it.

Thanks so much!
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Old 10-19-2017, 01:10 PM
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Join Date: Sep 2002
Posts: 2,541
Your indy will need an MB SDS to code out the "blow dry" feature. Keep in mind that the result may be a foul odor from the vents when first turning the AC on.

To the OP, the reason the system continues cooling after AC OFF is selected is that the compressor is of the variable "swash-plate" type. It's not switched off immediately, it ramps down according to current conditions.
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