Fault Message:

APU GEN CURRENT XFMR/WRG FAULT

Fault Code:

2420121LPS

Associated CAS:

APU GEN FAIL
(Advisory)

Auxiliary Power Unit (APU) Generator Control Unit (GCU) detects a discrepancy between the three line currents and the Generator currents.

• 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

A GCU patch cable (GSE REF # 24X-24-02) can be used to swap GCUs 1, 4 and 5 into position 2 or 3 (for troubleshooting only).

The GCU NVMs can only be downloaded at position #2 or #3.

1. Swap APU GCU with GCU #2 (physically or using the GCU patch cable).
   NOTE: Use PMAT to monitor the fault on GCU.

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

2. Disconnect connector A110P1 on the APU GEN and A109P2 on the APU GCU.
3. Perform wiring checks (open circuit = failure) between A110P1 and A109P2 as follow:
   

   From	To	Result
   A110P1-F	A109P2-41
   A110P1-E	A109P2-42
   A110P1-J	A109P2-50
   A110P1-D	A109P2-40

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

4. Perform wiring checks (closed circuit = failure) between A110P1 and ground as follow:
   

   From	To	Result
   A110P1-F	Ground
   A110P1-E	Ground
   A110P1-J	Ground
   A110P1-D	Ground

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

5. On current transducer 5 (LCTA5), disconnect connector PL5.
6. Perform continuity checks (open circuit = failure) as follow:
   

   From	To	Result
   PL5-1	PL5-4
   PL5-2	PL5-4
   PL5-3	PL5-4

   1. If there is no continuity, go to step 11.
   2. If there is continuity, continue with next step.

7. Perform continuity checks (open circuit = failure) in ACPC between A109P2 and PL5 as follow:
   

   From	To	Result
   A109P2-49	PL5-1
   A109P2-57	PL5-2
   A109P2-63	PL5-3
   A109P2-62	PL5-4

   1. If there is no continuity, go to step 13.
   2. If there is continuity, continue with next step.

8. Perform continuity checks (open circuit = failure) in ACPC between connectors A54P4 and PL5 as follow:
   

   From	To	Result
   A54P4-E	PL5-1
   A54P4-F	PL5-2
   A54P4-G	PL5-3
   A54P4-H	PL5-4

   1. If there is no continuity, go to step 13.
   2. If there is continuity, continue with next step.

9. Reconnect connector ACPC (A54P4).
10. Perform wiring checks (closed circuit = failure) between A109P2 and ground as follow:
    

    From	To	Result
    A109P2-49	Ground
    A109P2-57	Ground
    A109P2-63	Ground

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

11. Replace LCTA 5.

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

12. Replace APU GEN.

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

13. Replace ACPC.
14. Do close out.