Fault Message:

APU GEN OVERCURRENT FAULT

Fault Code:

2420115LPS

Associated CAS:

APU GEN FAIL
(Advisory)

Auxiliary Power Unit (APU) Generator Control Unit (GCU) has reported an over current on one phase of the APU Generator.

• Auxiliary Power Unit (APU) Generator Control Unit (GCU) (A109)
• Auxiliary Power Unit (APU) Generator (A110)
• AC Power Center (ACPC) (A54)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

Over-current detection specification:

• The output current on one phase of the APU Generator is more than 185 Amps

NOTE: Due to the nature of this CAIMS message, wiring checks should be performed prior to swapping any LRUs in order to prevent damage to LRUs or aircraft.

1. Perform a visual inspection for arcing evidences of ACPC primary section. Focus on K1, APU Generator and APU Gen feeder cables.

   1. If arcing evidence is found, repair/replace damaged components and do close out.
   2. If fault remains, continue with next step.

2. Perform operation test of APU generator.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.

3. Swap APU GCU with GCU#2.

   1. If system checks are good, replace APU GCU and do close out.
   2. If fault remains, continue with next step.

4. Check for short circuit to GND at T1, T2 and T3 on APU GEN.

   1. If short circuit is found, go to step 13.
   2. If fault remains, continue with next step.
      NOTE: Disconnect the power feeder cables on studs T1, T2, T3 and the ground cable (G) for short circuit test.

5. Perform wiring checks between ACPC, APU GCU and APU GEN.

   1. If wiring checks are not good, repair defective wiring and do close out.
   2. If wiring checks are good, continue with next step.

6. Check for short circuit to GND between APU GCU and ACPC wiring.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.
      NOTE: Disconnect A109P1 and A54P4 for short circuit test.

7. Check for short circuit to GND between APU GEN and ACPC wiring.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.
      NOTE: Disconnect power feeder cables on studs T1, T2, T3, T13, T14 and T15.

8. Check for short circuit to GND between APU GEN and APU GCU wiring.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.
      NOTE: Disconnect A109P2 and A110P1 for short circuit test.

9. Check for consistency of resistance for APU GEN coils by measuring all phases A110P1-F, A110P1-E, A110P1-J to A110P1-D (COM).

   1. If resistance is out of range, go to 13.
   2. If resistance is within range, continue with next step.

10. Check for consistency of resistance for ACPC coils by measuring all phases A54P-F, A54P1-E, A54P1-G to A54P1-H (COM).

    1. If resistance is out of range, go to 12.
    2. If resistance is within range, continue with next step.

11. Replace K1.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

12. Replace ACPC.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

13. Replace APU GEN.
14. Do close out.