What was found
THE APU FIRE WARNING LIGHT BEGAN TO FLASH INTERMITTENTLY APPROXIMATELY FIVE MINUTES INTO THE FLIGHT WITH THE APU SHUT DOWN. THE APU FIRE HANDLE WAS PULLED AND THE FIRE LIGHT FLASHING CEASED AFTER 45 SECONDS. THE AIRCRAFT WAS RETURNED TO EWR WHERE IT LANDED WITHOUT INCIDENT. MAINTENANCE FOUND THE APU TAILPIPE FIRE LOOP WIRES BURNT AND TOUCHING TAILPIPE. THE BARE WIRES WERE TEMPORARILY TAPED AND THE APU FIRE DETECTION SYSTEM WAS PLACARDED INOPERATIVE PENDING REPAIR OF WIRES. IN ADDITION, THE APU FIRE BOTTLE WAS REMOVED AND REPLACED.