What was found
(CAN) ON APPROACH, THE ENGINE FIRE WARNING BELL SOUNDED CONTINUOUSLY, THERE WERE NO ENGINE WARNING LIGHTS ON. ON THE GROUND MAINTENANCE INSPECTED ALL ENGINES NO FAULTS WERE FOUND. MAINTENANCE WAS UNABLE TO DUPLICATE PROBLEM AGAIN ON THE GROUND. THE FIRE DETECTION DETECTOR UNIT P/N 65-24920-3 WAS REPLACED . REMOVED AND INSPECTED AND CLEANED PIN/SOCKETS AN ALL THREE ENGINE FIRE DETECTION POWER OPERATING UNITS P/N 1011B-1. THE SYSTEM WAS TESTED WITH NO FAULTS. THE AIRCRAFT WAS RETURNED TO SERVICE AND OPERATED WITHOUT ANY FURTHER PROBLEMS.