Fault Message:

APU GEN [PHASE NEUTRAL CROSSED]

Fault Code:

2420109LPS

Associated CAS:

APU GEN FAIL
(Advisory)

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

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter:

• Forum Newsletter Volume 13 issue 16 August 2016

NOTE: The Generator ground stud connection may need to be visually inspected for signs of arcing.

A GCU patch cable (GSE REF # 24X-24-02) can be used to swap GCU 1, GCU 2, GCU 3, GCU 4 and the APU GCU into a different position (for troubleshooting only). You can monitor the GCU faults in CAIMS via the active fault page.

1. At the APU Generator, do a check of the output connections at the terminal block. Are the connections as shown in the table below?

   Connection	Color
   T1	White
   T2	Blue
   T3	Orange
   G	Green

   1. If NO, reconnect or repair as required and do close out.
   2. If YES, continue with next step.

2. At the ACPC, do a check of the APU Generator output connection at the APU 5 LCTA. Are the connections as shown in the table below?

   Connection	Color
   T13	White
   T14	Blue
   T15	Orange

   1. If NO, reconnect or repair as required and do close out.
   2. If YES, continue with next step.

3. Perform a bonding test of ground stud GS678.

   From	To	Expected result	Result
   GS678	Ground	<0.010 ohms

   1. If the resistance value is higher than 0.010 ohms, repair ground stud and do close out.
   2. If the resistance value is good, continue with next step.

4. Swap APU GCU with GCU #2 (physically or using the GCU patch cable). Is the fault still present?

   1. If NO, replace APU GCU and do close out.
   2. If YES, continue with next step.

5. Replace APU Generator.
6. Do close out.