Fault Message:

APUGCU SENSE/GEN CONT SW FAULT

Fault Code:

2420254LPS

Associated CAS:

APU GEN FAIL
(Advisory)

The Auxiliary Power Unit Generator Control Unit (APU GCU), has detected a fault in the Generator Control Switch (GCS)/wiring or in its internal GCS sensing circuit. The GCU GCS sensing circuit has detected the OFF signal as being the same as the ON signal.

• Auxiliary Power Unit Generator Control Unit (APU GCU) (A109)
• Generator Control Unit (GCU) 2 (A57)
• Electrical Control Panel (AP3)
• 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.

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

2. Perform disconnect connector, on both APUGCU (A109P2) and overhead control panel (AP3P2).
3. Do wiring checks (open circuits check) between interface wiring GCU/Electrical Control Panel as follows:

   From	To	Result
   A109P2-21	AP3P2-F
   A109P2-28	AP3P2-G

   1. If an open circuit is detected, repair or replace wiring as required and do close out.
   2. If an open circuit is not detected, continue with next step.

4. Do wiring checks (short circuits check) between interface wiring GCU/Electrical Control Panel) as follows:

   From	To	Result
   A109P2-21	Ground
   A109P2-28	Ground

   1. If a short circuit is detected, repair or replace wiring as required and do close out.
   2. If a short circuit is not detected, continue with next step.

5. Do a check for continuity checks (circuits check) between AP3P2 and ground stud as follows:

   From	To	Result
   AP3P2-Y	GC160

   1. If a short circuit is detected, go to step 7.
   2. If an open circuit is detected, repair or replace the defective wiring between AP3P2-Y and ground stud GC160 and continue with next step.

6. Check the integrity of ground stud GC160.

   1. If the fault remains, repair or replace the ground stud and do close out.
   2. If the system checks are good, continue with next step.

7. Inspect the electrical control panel internal wiring and the generator control switch (GCS-SDS7).

   1. If a defect is found with the wiring, repair or replace wiring as required and do close out.
   2. If a defect is found on the GCS, continue with next step.

8. Replace the GCS.
9. Do close out.