APU Detection Loop A and/or Loop B Inop Signal Output

TASK 26-13-00-810-803-A
APU Detection Loop A and/or Loop B Inop Signal Output

1. Possible Causes

FDU-APU [13WG]
wiring

2. Job Set-up Information
A. Referenced Information

REFERENCE	DESIGNATION
AMM 26-13-00-710-002	BITE Test of the APU Fire and Overheat Detection System
ASM 261301
AMM 26-13-34-000-001	Removal of the APU Fire-Detection Unit (FDU)
AMM 26-13-34-400-001	Installation of the APU Fire-Detection Unit (FDU)
AWM 315218

3. Fault Confirmation
Subtask 26-13-00-710-086-A

A. If the Electric Ground Supply energizes the aircraft:

Operate the switches only in this order:

(1) MAINT BUS to "ON"
(2) EXT PWR to "ON"
(3) BAT 1+2 to "ON"

Do the BITE test of the APU fire and overheat detection system AMM 26-13-00-710-002.

Subtask 26-13-00-710-088-A

B. If the APU energizes the aircraft:

Do the BITE Test of the APU Fire and Overheat Detection with the CFDS' after the APU starts AMM 26-13-00-710-002.

NOTE: If FWC and FDU are connected and all other systems operate satisfactorily, the message "Test OK" is displayed. The "CHECK FDU APU LOOP A/ or B WARN CKT" will be cleared next "Ground/Flight" transition.

4. Fault Isolation
Subtask 26-13-00-810-065-A

A. If the test gives a different maintenance message:

Do the trouble shooting procedure related to the maintenance message.

Subtask 26-13-00-810-081-A

B. If the test gives the maintenance message CHECK FDU APU LOOP A WARN CKT:

replace the FDU-APU [13WG] AMM 26-13-34-000-001 and AMM 26-13-34-400-001.

(1) If the fault continues:

do a check and repair of the wiring between
FDU 13WG-A/M and the first terminal block
ASM 261301 and AWM 315218.

Subtask 26-13-00-710-087-A

C. Do the test as given in the Para. 3.A.

Subtask 26-13-00-810-082-A

D. If the test gives the maintenance message CHECK FDU APU LOOP B WARN CKT:

replace the FDU-APU [13WG] AMM 26-13-34-000-001 and AMM 26-13-34-400-001.

(1) If the fault continues:

do a check and repair of the wiring between
FDU 13WG-B/L and the first terminal block
ASM 261301 and AWM 315218.

Subtask 26-13-00-710-055-A

E. Do the test as given in the Para. 3.A.

5. Close-up
Subtask 26-13-00-860-052-A

A. Put the aircraft back to its initial configuration.

[Rev.8 from Aug 2018] 2026.04.04 04:27:17 UTC