What was found
DURING PREFLIGHT OF FLIGHT 3031, WHILE PARKED AT CMI ON THE GROUND AFTER A/C QUIT, WITH EXTERNAL POWER ON, SMOKE DISCOVERED COMING FROM AVIONICS BAY. SMOKE IN THE CABIN ON PREFLIGHT, DEPLANED PASSENGERS. MAINTENANCE INSPECTED AVIONICS BAY/ AVIONICS EQUIPMENT FOR CONDITION, SECURITY, CHECKS ARE GOOD. SUSPECT SMOKE WAS COMING FROM SUPPLEMENTAL A/C UNIT, DEFERRED. RAN AVIONICS FAN ON EX POWER, NO DISCREPANCIES NOTED. RAN AVIONICS FAN FROM GEN POWER WITH A/C PACKS ON, NO DISCREPANCIES NOTED. MAINTENANCE REPLACED COMPRESSOR MOTOR, RECEIVER/DRYER COMPRESSOR BELT GAUD REF MM 409 144407 AND CONTINUATION PG 071315 &16 REPLACED BELT COVER ASSY.