Failure of the APU Generator Exciter-Field or its Wiring

TASK 24-23-00-810-809-A
Failure of the APU Generator Exciter-Field or its Wiring

1. Possible Causes

GEN-APU [8XS]
wiring
GAPCU [24XG]

2. Job Set-up Information
A. Referenced Information

REFERENCE	DESIGNATION
ASM 242301
AMM 24-23-51-000-001	Removal of the APU Generator
AMM 24-23-51-400-001	Installation of the APU Generator
AMM 24-41-00-861-002	Energize the Aircraft Electrical Circuits from the External Power
AMM 24-41-00-862-002	De-energize the Aircraft Electrical Circuits Supplied from the External Power
AMM 24-41-34-000-001	Removal of the Ground Power Control Unit (GPCU)
AMM 24-41-34-400-001	Installation of the Ground Power Control Unit (GPCU)
AMM 49-91-41-200-002	Remove and Discard Oil Filter Elements (131-9(A))
AMM 49-91-41-920-001	Discard Pressure Oil Filter Element (APS 3200)
AMM 49-91-41-920-002	Replace AC-Generator Scavenge Filter-Element (APS 3200)
AMM 49-91-45-200-002	Discard Lube and Alternator Scavenge Filter Elements (GTCP 36-300)
AMM 49-91-45-200-003	Remove and discard Lube Filter and Generator Scavenge Filter Elements (GTCP 36-300)

3. Fault Confirmation
Subtask 24-23-00-869-050-A ** ON A/C NOT FOR ALL

A. Test
Not applicable.

NOTE: As the APU GEN was set to OFF because of a true failure, the fault confirmation is not necessary.

4. Fault Isolation

CAUTION: BEFORE YOU START THE CHECK OF IDG WIRING THAT INCLUDES CONNECTORS, EXAMINE EACH CONNECTOR TO MAKE SURE THAT THERE IS NO CONTAMINATION IN IT. IF YOU FIND CONTAMINATION, CLEAN THE CONNECTOR AND CONNECT IT AGAIN. IF THE FAILURE CONTINUES, DO THE TROUBLE SHOOTING PROCEDURE AS GIVEN BELOW.

Subtask 24-23-00-810-057-B ** ON A/C NOT FOR ALL

A. If the fault symptom is identified by the CFDS message GAPCU (24XG) EXC FLD/ GEN APU (8XS) and the upper ECAM-DU warning APU GEN FAULT:

Do a check of the resistance of the APU generator exciter-field between pins A/9 and A/10 of the APU generator (7.67 ohms plus or minus 0.73 ohms) ASM 242301.
Do a check of the wiring for a short-circuit between:
. Pin A/9 of the APU generator and APU generator case
. Pin A/10 of the APU generator and APU generator case.

NOTE: Do the check:

GTCP 36-300: for metal contamination of the lube filter and the generator scavenge filter element AMM 49-91-45-200-002 or AMM 49-91-45-200-003.
or
APS 3200: for metal contamination of the pressure oil filter element AMM 49-91-41-920-001 and the AC-generator scavenge filter element AMM 49-91-41-920-002.
or
131-9(A): for metal contamination of the oil filters AMM 49-91-41-200-002.

(1) If the resistance values are out of the specified limits:

Replace the GEN-APU [8XS] AMM 24-23-51-000-001 and AMM 24-23-51-400-001.

(2) If the resistance values are in the specified limits:

Do a check of the wiring ASM 242301 for a short to ground, a short circuit or an open circuit between:
. Pin A/9 of the APU generator and pin C/12 of the GAPCU
. Pin A/10 of the APU generator and pin C/13 of the GAPCU
. Pin C/7 and pins C/12 and C/13 of the GAPCU.

(a) If the wiring is not correct:

Repair or replace as necessary.

(b) If the wiring is correct:

Replace the GAPCU [24XG] AMM 24-41-34-000-001 and AMM 24-41-34-400-001.

Subtask 24-23-00-860-052-A ** ON A/C NOT FOR ALL

B. Make sure that the aircraft electrical circuits operate correctly in APU configuration.
(1) Energize the aircraft electrical circuits from the APU AMM 24-41-00-861-002.
(2) Let the APU generator operate for five minutes to make sure that it operates correctly.
(3) De-energize the aircraft electrical circuits from the APU AMM 24-41-00-862-002.

[Rev.8 from Aug 2018] 2026.04.04 04:26:39 UTC