Aircraft 28 VDC Bus Fail Channel A

TASK 73-21-00-810-912-A
Aircraft 28 VDC Bus Fail Channel A

1. Possible Causes

HARNESS-FAN, WF02 [4218KS]
[4000KS]

2. Job Set-up Information
A. Fixtures, Tools, Test and Support Equipment

REFERENCE	QTY	DESIGNATION
No specific	1	MULTIMETER - STANDARD
No specific	1	PLIERS - JAWED, SOFT

B. Referenced Information

REFERENCE	DESIGNATION
TSM TASK 71-00-00-810-834-A	General Troubleshooting Guidelines for Additional Troubleshooting Information
TSM TASK 71-00-00-810-835-A	Component and Harness Intermittent Troubleshooting Procedure for Guidance Troubleshooting Intermittent Faults
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 70-70-00-910-803	SPOP 6 - Cleaning of the Contact Points and Wiring Harnesses
AMM 71-13-00-010-802	Opening of the Fan Cowl Doors
AMM 71-13-00-410-802	Closing of the Fan Cowl Doors
AMM 73-21-01-000-801	Removal of the Electronic Engine Control (EEC)
AMM 73-21-01-400-801	Installation of the Electronic Engine Control (EEC)
AMM 73-21-62-000-802	Removal of the EEC Wiring Harness (WF02)
AMM 73-21-62-400-802	Installation of the EEC Wiring Harness (WF02)
AWM 732503
AWM 732504
AMM 73-25-34-000-802	Removal of the Engine Interface Unit (EIU)
AMM 73-25-34-400-802	Installation of the Engine Interface Unit (EIU)
AMM 73-25-34-710-802	Operational Test of the Engine Interface Unit (EIU) with the Centralized Fault Display System (CFDS)
EEC Wiring Harnesses
Pylon Junction Box Connections
Aircraft 28 VDC Channel A Wiring Schematic

3. Fault Confirmation
Subtask 73-21-00-710-165-A ** ON A/C NOT FOR ALL

A. Energize the FADEC of the engine that has a fault report.
(1) Energize the aircraft electrical circuits AMM 24-41-00-861-002.
(2) On the ENG section of maintenance panel 50VU:
(a) Release the FADEC GND PWR/1(2) pushbutton switch (the ON legend comes on).

NOTE: After the FADEC is energized, wait ten seconds before you start the test. If you do not wait ten seconds, the interactive mode stops the EEC self-test and this can cause spurious faults.

(3) On the Center Pedestal, on ENG panel 115VU:
(a) Move the ENG/MODE rotary selector switch to CRANK position.
(4) On Multipurpose Control and Display Unit (MCDU) screen, check if the fault is displayed on ground scanning report.
(5) Check on ground scanning report to confirm this fault.

NOTE: The Electronic Engine Control (EEC) has detected that the aircraft 28 VDC power is not available to channel A. The EEC is located on the fan case at the 2 o'clock position.

Refer to the general troubleshooting guidelines for additional troubleshooting information (Ref. TSM TASK 71-00-00-810-834).
Refer to the component and harness intermittent troubleshooting procedure for guidance troubleshooting intermittent faults (Ref. TSM TASK 71-00-00-810-835).

(6) If the maintenance message does not come back, examine the harness and electrical connectors as described in the component and harness intermittent fault isolation procedure (Ref. TSM TASK 71-00-00-810-835).
(a) If no problems are found, return the engine to service. Monitor the engine for reoccurring maintenance message.
(7) If the maintenance message comes back (hard fault), go to the Para. Fault Isolation.

4. Fault Isolation

WARNING: BE CAREFUL WHEN YOU DO WORK ON THE ENGINE AFTER SHUTDOWN. THE ENGINE CAN STAY HOT FOR A LONG TIME. IF YOU DO NOT OBEY THIS WARNING, INJURY CAN OCCUR.

WARNING: REFER TO THE MSDS FOR ALL MATERIAL USED AND THE MANUFACTURER'S SAFETY INSTRUCTIONS FOR ALL EQUIPMENT USED. IF YOU DO NOT OBEY THIS WARNING, INJURY CAN OCCUR.

CAUTION: MAKE SURE YOU REMOVE ELECTRICAL POWER FROM THE EEC BEFORE YOU REMOVE THE ELECTRICAL CONNECTORS. IF YOU DO NOT OBEY THIS INSTRUCTION, DAMAGE TO THE EEC CAN OCCUR.

Subtask 73-21-00-810-181-A ** ON A/C NOT FOR ALL

A. Do the operational test of the EIU with the CFDS AMM 73-25-34-710-802.
(1) Look for EEC A SUPPLY FAULT message and troubleshoot.
(a) Do the test given in Para. 3.
(2) Open the fan cowl doors AMM 71-13-00-010-802.
(3) If no problems are found or if fault continues:
(a) Examine the WF02 (4218KS) harness, connector P1 (4000KS-P1) - EEC (4000KS) and connector P402 (402VC-A) - Pylon junction box, for problems which are possible between the EEC (4000KS) and the pylon junction box.
1 Inspect for harness damage.
2 Inspect for loose or damaged connectors.

NOTE: Loose connectors must be disconnected and examined for further wear or damage.

3 If a problem is found with the harness or connectors:
a Replace HARNESS-FAN, WF02 [4218KS] AMM 73-21-62-000-802 and AMM 73-21-62-400-802.
b Do the test given in Para. 3.
(4) If no problems are found with the harness or the fault continues, use a PLIERS - JAWED, SOFT to disconnect the electrical connector P1 (4000KS-P1) from the EEC (4000KS) and connector P402 (402VC-A) from the pylon junction box.
(a) Examine the pins and sockets (contacts) for alignment and damage. Look for contacts pushed rearward into the connector face. Replace parts as necessary.
(b) Clean the contacts with contact cleaner and clean cloth per SPOP 6. Blow dry with air (at less than 30 psi) across connector face AMM 70-70-00-910-803.
(c) Reconnect the electrical connector P1 (4000KS-P1) to the EEC (4000KS) and connector P402 (402VC-A) to the pylon junction box.
(d) Do the test given in Para. 3.
(5) If the fault continues:
(a) Disconnect the electrical connector P1 (4000KS-P1) from the EEC (4000KS) and connector P402 (402VC-A) from the pylon junction box. Use a MULTIMETER - STANDARD to test the aircraft 28 VDC channel A circuit in the harness. Refer to Table 1. Look for:
1 Wire-to-wire shorts
2 Shorts to ground
3 Continuity (less than 5 ohms resistance).

NOTE: A short to ground is found when the resistance is less than 100 000 (100K) ohms.

Table 1: EEC to Pylon Junction Box Aircraft 28 VDC Channel A Circuit Connector Contacts

WF02 (4218KS) Harness P1 (4000KS-P1) EEC (4000KS) Connector Contacts	WF02 (4218KS) Harness P402 (402VC-A) Pylon Junction Box Connector Contacts
H	21
J	21
K	23
G	23

(b) If a problem is found with the harness or connectors:
1 Replace HARNESS-FAN, WF02 [4218KS] AMM 73-21-62-000-802 and AMM 73-21-62-400-802.
2 Do the test given in Para. 3.
(6) If no problems are found with the harness or the fault continues, do a check of the aircraft wiring containing the aircraft 28 VDC channel A circuit between the EIU and the pylon junction box:

For fault on ENG 1, refer to AWM 732503 and Table 2.
For fault on ENG 2, refer to AWM 732504 and Table 2.

(a) Disconnect the connector P402 (P402VC-A) from the pylon junction box and remove the EIU-1 (1KS1) for fault on ENG 1 or remove the EIU-2 (1KS2) for fault on engine 2 AMM 73-25-34-000-802.
(b) Look for:
1 Shorts to ground
2 Continuity (less than 5 ohms resistance).

NOTE: Short to ground is found when the resistance is less than 100 000 (100K) ohms.

Table 2: Pylon Junction Box to EIU Channel A Connector Contacts

P402 (402VC-A) Pylon Junction Box Connector Contacts	(1KS1(2)) EIU Connector AC Contacts
21	33

(c) Check for ground at pin 23 of connector P402 (402VC-A) of the pylon junction box.
1 Repair any problems found.
(d) Reconnect the connector P402 (402VC-A) to the pylon junction box and install the EIU-1 (1KS1) or EIU-2 (1KS2) AMM 73-25-34-400-802.
(e) Do the test given in Para. 3.
(7) If no problems are found with the aircraft wiring or the fault continues:
(a) Replace [4000KS] AMM 73-21-01-000-801 and AMM 73-21-01-400-801.
(b) Do the test given in Para. 3.

5. Close-up
Subtask 73-21-00-860-101-A ** ON A/C NOT FOR ALL

A. Put the aircraft back to its initial configuration.
(1) Make sure that the work area is clean and clear of tools and other items.
(2) Close the fan cowl doors AMM 71-13-00-410-802.
(3) De-energize the aircraft electrical circuits
AMM 24-41-00-862-002.

[Rev.8 from Aug 2018] 2026.04.04 04:36:43 UTC