What was found
THREE TIMES DURING FLIGHT WE SMELLED AN ACRID ELEC BURNING SMELL. NO SMOKE NOTED & UNABLE TO ISOLATE. FIRST OFFICER ALWAYS SMELLED IT FIRST. ALWAYS WENT AWAY IN 1 MIN. NO SMELL IN COURIER AREA. IAW FIM 21-00 PROC 4, SINCE FAULT CAN'T BE DUPLICATED & ALL ELEC PANELS HAVE BEEN INSPECTED, IN PREVIOUS UPDATED, SUSPECT LT RECIRC FAN MAY HAVE BEEN OVERHEATING. R & R LT RECIRC FAN PRECAUTIONARY IAW AMM 21-25-01-4. OPERATIONS CHECKED GOOD, NO FUMES OR DEFECTS NOTED. FAN OFF PN 645705-3, SN 45-211, FAN ON PN 645705-3 & SN 102-86.