My Mercedes 350DL air conditioning works great while highway driving. When slowing down or at stops it blows heated air…any ideas?
Does it have an electric radiator fan? If so, is this fan working? It should probably run when ever the AC is on. Open the hood and observe fan. Start car and observe fan. Turn on AC and observe fan. Report back.
I agree with beadsandbeads. Check the fan. A dodge I bought back in 2000 had the exact same symptoms. It was brand new so I took it to the dealer. After 2 days of checking everything they could think of and being at the point of ordering a new wiring harness for the vehicle, a tech saw that the electric fan was not plugged in properly…the connection was just loose enough to not make the electrical connection. They plugged it in properly and it worked as it should. If yours has worked fine in the past but isn’t now, then I’d guess that the fan motor has probably died or the connection is dirty.
If the fan is operating, and keeping in mind I don’t consider myself a Benz expert, you might consider the possibility of a blend door acting up.
If this model uses engine vacuum as a means of controlling the mode operation it’s possible that a vacuum leak, faulty mode switch, bad vacuum pod, etc. could be causing the blend door to become inoperative. This will then allow air to pass through the heater core instead of bypassing it.
This can be a common problem on many vehicles with vacuum operated doors. Maybe someone with a better knowledge of your vehicle can either verify my comment or dismiss it as wrong.
Is there anyway you can check when the system is blowing warm is the compressor still engaged? Iam think low pressure cut off switch.