2004 S-class - Strange issues with inetrnal heating.

[V8 Supercharger]

Hello all

Iv had my car for just over 2 years now and the air vents for the rear passengers have always blown hot air, regardless of where you set the temperature, or regardless of wether the A/C is on or off. Always hot air going into the back. I was trying to get to the bottom of this while the front was still working.

The front used to work perfectly, however today its started doing something very strange. If the car is idling and the engine is at full temperature, i only get cold air from the front vents, even when the temp. is set to "Hi".

I thought it must be a lack of coolant, but it was topped up perfectly with no losses.

However if i start driving around it starts blowing hot air out like its supposed to. Then if i drive slow or come to a stop, its back to cold air again. And all this while the temperature is set to its highest.

I thought it might be the Thermostat but then it starts blowing hot air when driving!!!

So now iv got ONLY hot air in the back, and hot and cold in the front but not when i want, but when ever the car decides.

Any ideas?

 

[Major774]

Duo valve, change it

 

[Vilus]

The w220 has a complex AAC system which has two duo valves.
On my E class w211 when the duo valve was faulty, red hot air was pumping out of the rear vents, but there was also heat coming from the front, which the A/c system was working overtime to counteract. I only noticed because of the large amount of condensation dripping from under the car in the summer. The Duo valve only distributes the air to left or right.

On some S class there is a separate a/c system for the rear I believe.

The a/c system has a feature to increase the flow of air through the cabin when stationary throught the blower. This may explain the cold air when at a standstill.

Welcome to the S-Class

The climate control/ventilation section in this link may be of help to you.

 

[Vilus]

Link to the US forum w220 Wiki, post 8 refers to climate control issues..

W220 S-CLASS ENCYCLOPEDIA - START HERE (major update Apr 2013; minor ones frequent) - Mercedes-Benz Forum

 

[AdamG]

My w220 recently stopped blowing hot air unless the engine was revved and the "rest" heat function stopped working.

It was caused by the electric auxiliary pump mounted next to the duo valves. The brushes had worn. New brushes were about £10 but the motor is available from Mercedes for £100.

The aux pump is used to boost the coolant flow at idle.

Adam