C320 Inside Defrost Problem
 

I love my 2001 C320. It has been a perfect car but in the last two weeks, I have been having a major problem. As the weather has gotten very cold lately, I cannot get the inside windows to clear off. After ten minutes in the car, all of the windows are entirely fogged up. If I am in the car for more than 30 minutes, ice will form on the inside of the car! The rear defroster works, but as soon as it clears the back window and shuts off automatically, the ice starts building on the inside again. If I turn the front defroster on high, I can clear the windshield halfway up, just enough to see out if I needed to drive the car. I took it to my local mechanic and he cannot find a problem as he said the defroster is working fine. When you let the car sit and run and turn up the heat, all of the windows clear off fine, but whenever I am inside driving, the frost builds up. Maybe bad glass? Maybe I am incompetent? Any help is appreciated.

This sounds like you are recirculating inside air rather than bringing in fresh air. When it's cold out, you breathe out enough water vapor to really fog up a car's windows if the climate control system doesn't bring in fresh air.

I know nothing about your car's climate control system but hopefully someone else on the site can give more specific help.

mtburton

Not to belittle you but, give the owners manual a very good study, especially the heat/defrost section.
You may be surprised what you can learn. ;)

Defrost Problem
 

The car only allows for fresh air to come in so I know it is not a circulation problem. The owners manual for the entire defrost system is one page and it simply says to press the defrost button and it will adjust automatically.

Any other ideas?

Sounds silly, but you wouldn't be operating in EC would you. You need the airconditioning system to dry the incoming air. Make sure you are not operating in EC as that cuts out the AC.
The recirculation is controlled by a button with an arrow that goes in a little circle attaching the head to the tail. You should have one of those buttons since the recirculation is recommended for various operating conditions. If the AC is operating it should be able to dry the recirculated air but if you do have that button make sure you're not operating in that mode.

To add just another little tidbit.
When I park the car in my garage overnight, I always leave 1 window down.
The purpose is to allow some of the moisture accumulated on the floormats ( slush, snow, etc. ) to evaporate as much as possible.

I would bet you dollars to donuts that this is indeed a circulation problem (unless, as Manny suggests, you've got too much interior humidity from wet carpets, etc., or your air intake at the end of the hood is clogged with snow and ice). I would also bet you that your car does indeed have the capacity to recirculate inside air.

On my car, at least, the AC is cut off at ambient temps below about 40F, and in fact in most winter conditions you don't need the AC to keep the humidity level down in the cabin, especially when it's really cold out -- the outside air's relative humidity will be very low at low ambient temps, and so when it's warmed up to cabin temp, the relative humidity will be extremely low.

I only know W124s, but on those, the recirc switch automatically goes off after about 10 minutes when outside temps are low, to prevent interior fogging. Don't know about your car's climate control, however.

On older MBs, the vacuum motor which controls the flap which determines the mixture of inside/outside air can and do fail, which can produce the symptoms you're describing.