Sounds like a good reason for water to get in. Those drains can clog real easy if you park under trees at all.

Check out this training document for more than you probably want to know:
http://www.mercedestechstore.com/pdfs/507%20Systems%20I/507%20HO%20HVAC%20220%20(ACB,%20IC)%2010-30-02.pdf

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician