What was found
DEPARTED SMF WITH 13TH STAGE MOD VALVE DMI TO OPEN POSITION, NR 1 BLEED OFF, LEFT PACK OFF. WHEN LEFT PACK TURNED ON IN FLIGHT, NOTICED A STRONG ODOR IN COCKPIT. THE ODOR INCREASED AS WE TURNED BACK FOR LANDING. RECEIVED APU FIRE INDICATION, PULLED HANDLE AND DISCHARGED FIRE BOTTLE. PUT ON EXTERNAL AIR ON AIRCRAFT, APPLIED BOTH PACKS AND 100 AMP LOAD. GROUND RAN ALL ENGINES AND PACKS WITH 100 AMP LOAD, OPS CHECK NORMAL. INSPECTED WHEEL WELL AREA FOR EVIDENCE OF FIRE, SMOKE AND ELECTICAL DAMAGE, NONE NOTED .