As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE AT THE GATE AND WHILE PASSENGERS WERE BOARDING, GROUND PERSONNEL ADVISED THAT THE CARGO BIN APPEARED TO BE FILLING WITH SMOKE. CREW STARTED TO SMELL BURNING SMELL IN CABIN AT THAT TIME. DEPLANED THE AIRCRAFT AND TURNED OFF ALL ELECTRICAL POWER. THE AFT SUPP AIR COND UNIT WAS RUNNING AT THE TIME OF THE SMOKE. REPLACED NR 2 A/C MOTOR PN 1134104-1, SN ON 1404, SN OFF 2103