Originally Posted by Mech_Man
I did read a DIY a while ago, about someone who had this problem often enough that he found out that the AC coil drain pan could be reached by the hump by the drivers right knee. He drilled a small hole into the drain pan and put a colored cover on the hole. When he got the smell, he unplugged it and had some deodorizer / mold-killer with a long plastic tub he could put in the hole and spray the core.
I registered to E90post only to answer to this problem. I work for a company selling A/C's and accessories in Europe. The mold build up is a common problem with A/C units, household, industrial and in cars aswell. Household units (better brands) have usually a built in workaround, where the indoor unit fan is blowing down the moisture from the evaporator thus drying it after the unit has been switched off (uncontrollable by the customer). Cars unfortunately rely on our manual controls, so to prevent the mold build up the best way is to switch off the A/C 4-5 minutes before you reach your destination and you know you will shut down the engine.
If it's too late and the mold is already present (smelly odor from the vents) there are A/C coil cleaning products available in form of a spray usually. There are two basic cleaning methods, depending on the type of product.
First is aplicable directly to the coil, but not by drilling a hole close to the evaporator, but via the drain pipe present on the bottom of the car. You can easily find the pipe, it's the one that leaks water when the A/C is running (and you car is sitting on a parking spot). After locating the drain pipe (usually black rubber pipe around 10 cm long) you have to spray the content of the cleaning agent up the car (the A/C and the ventilation must be switched off !), wait at least 10 minutes, and then switch on the A/C with ventilation. The moisture mixed with cleaning agent should wash down the mold. The process should be repeated for better effect.
Second method is indirect application of the cleaning agent by switching on the internal ventilation, set fan to maximum, A/C on, spraying the cleaner inside the car, and close it. Let it run for 15-20 minutes, so the cleaning agent will 'wash' the coil (evaporator) by running through it.
To find the proper product you should find a household A/C shop selling accessories, they should have a A/C coil cleaner, which is usable for cars too. Better looking for a distributor/wholesaler supplying installation companies.
Hope this helps.