What was found
FLT NR: ATN 842/KTOL TO KPHL. DURING CLIMB FROM KTOL THE NR 4 FIRE WARNING CAME ON. THE CREW ACCOMPLISHED THE APPROPRIATE CHECKLIST AND DISCHARGED BOTH FIRE BOTTLES. THE ACFT LANDED AT KPHL WITHOUT INCIDENT. MAINTENANCE INSPECTED NR 4 ENGINE FIRE DETECTION SYSTEM AND FOUND HIGH RESISTANCE AT PIN F, UPPER PYLON SOCKET PLUGS DISTORTED. MAINTENANCE REPLACED PYLON FIRE DETECTOR ELEMENT, SOCKET PLUG, FIRE DETECTOR CONTROL UNIT AND BOTH FIRE BOTTLES. OPERATIONAL CHECK GOOD IAW MM 26-10-0, 26-10-1 AND 26-10-2. AIRCRAFT WAS RETURNED TO SERVICE. (A)