Fault Message:

APU GEN OPEN/SHORT LINE CT FAULT

Fault Code:

2420123LPS

Associated CAS:

APU GEN FAIL
(Advisory)

Auxiliary Power Unit (APU) Generator has tripped off line or will not come on line.

• Auxiliary Power Unit (APU) Generator Control Unit (GCU) (A109)
• Line Current Transformer Assembly (LCTA)
• 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. Get access to the ACPC bottom section.
3. On current transducer 5 (LCTA 5), disconnect connector PL5.
4. 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 continuity, go to step 6.
   2. If there is no continuity, continue with next step.

5. Replace LCTA 5.

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

6. Disconnect connector APU GCU (A109P2).
7. Perform continuity checks (open circuit = failure) at connector 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 continuity, go to step 10.
   2. If there is no continuity, continue with next step.

8. Disconnect connector ACPC (A54P4).
9. Perform wiring checks (open circuit = failure) between A109P2 and A54P4 as follow:
   

   From	To	Result
   A109P2-49	A54P4-E
   A109P2-57	A54P4-F
   A109P2-63	A54P4-G
   A109P2-62	A54P4-H

   1. If wiring checks are not good, repair defective wiring as required and do close out.
   2. If wiring checks are good, go to step 12.
      NOTE: Call Bombardier for decision to repair ACPC wiring or replace ACPC.

10. Disconnect connector A54P4 (ACPC) and leave PL5 disconnected.
11. Perform wiring checks (short circuit = failure) as follow:
    

    From	To	Result
    A54P4-E	A54P4-H
    A54P4-F	A54P4-H
    A54P4-G	A54P4-H

    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.

12. Replace ACPC.
13. Do close out.