What was found
FLT 422 - MEX-MIA - DUIRNG TRANSIT CHECK, WHEN THE GROUND POWER UNIT WAS CONNECTED, SMOKE WAS DETECTED IN THE PASSENGER CABIN. BOTH A/C SYSTEM WERE TESTED BUT SMOKE CONTINUED. THE WATER SEPARATORS WERE REPLACED AND SYSTEM WAS TESTED IAW MM 21-20-02, ALL CORRECT. DURING OVERNIGHT SERVICE, IN MEX STATION THE APU WAS VERIFIED DETECTING INTERNAL DAMAGES AND AN INTERNAL OIL LEAKAGE. THE APU WAS REPLACED AND SYSTEM TESTED IAW MM 49-00. (M)