Fault Message:

APU GCU [OVERCURRENT SIG INVALID]

Fault Code:

4960329APU

Associated CAS:

None

A signal from the Auxiliary Power Unit (APU) Generator over current input is sent to the Full-Authority Digital Engine-Controller (FADEC) when the APU ready to load is not true.

• Auxiliary Power Unit (APU) Generator Control Unit (GCU) (A109)
• Auxiliary Power Unit (APU) Full-Authority Digital Engine-Controller (FADEC) (A108)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

1. Check the AC wiring-harness for bare wiring, kinks, worn areas and other damage.

   1. If there is damage, repair defective wiring as required and do close out.
   2. If there is no damage, continue with next step.

2. Disconnect the connector P619 at the AFT pressure bulkhead (861 LHS) and remove the FADEC (A108). Perform a continuity check as follows:

   From	To	Result
   P619-79	A108BP1-G10
   P619-80	A108BP1-H10

   1. If there is no continuity, repair defective wiring as required and do close out.
   2. If there is continuity, continue with next step.

3. Perform a continuity check as below. The resistance should measure as follows:

   From	To	Expected Result	Result
   P619-79	Ground	>1 megaohm
   P619-80	Ground	>1 megaohm
   A108BP1-G10	Ground	>1 megaohm
   A108BP1-H10	Ground	>1 megaohm

   1. If resistance value is not more than one megaohm for each wire, repair defective wiring as required and do close out.
   2. If resistance value is more than one megaohm for each wire, continue with next step.

4. Disconnect the P601 connector, perform a continuity check as below. The resistance should measure as follows:

   From	To	Expected Result	Result
   P601-A	A108BP1-G5	<1 megaohm

   1. If resistance value is not less than one megaohm for each wire, repair defective wiring as required and do close out.
   2. If resistance value is less than one megaohm for each wire, continue with next step.

5. Perform a continuity check as below. The resistance should measure as follows:

   From	To	Expected Result	Result
   P601-A	Ground	>1 megaohm
   A108BP1-G5	Ground	>1 megaohm

   1. If resistance value is not more than one megaohm, repair defective wiring as required and do close out.
   2. If resistance value is more than one megaohm, continue with next step.

6. Disconnect the connector J619 at the AFT pressure bulkhead (861 LHS) and connector P320 (STN 485). Perform a continuity check as below. The resistance should measure as follows:

   From	To	Expected Result	Result
   J619-79	P320-118	<1 megaohm
   J619-80	P320-117	<1 megaohm

   1. If resistance value is not less than one megaohm for each wire, repair defective wiring as required and do close out.
   2. If resistance value is less than one megaohm for each wire, continue with next step.

7. Perform a continuity check as below. The resistance should measure as follows:

   From	To	Expected Result	Result
   J619-79	Ground	>1 megaohm
   J619-80	Ground	>1 megaohm
   P320-118	Ground	>1 megaohm
   P320-117	Ground	>1 megaohm

   1. If resistance value is not more than one megaohm for each wire, repair defective wiring as required and do close out.
   2. If resistance value is more than one megaohm for each wire, continue with next step.

8. Disconnect J601 connector at the AFT pressure bulkhead (861 LHS) and P325 connector (STN 485). Perform a continuity check as below. The resistance value should measure as follows:

   From	To	Expected Result	Result
   J601-A	P325-11	<1 ohm

   1. If resistance value is not less than one ohm for each wire, repair defective wiring as required and do close out.
   2. If resistance value is less than one ohm for each wire, continue with next step.

9. Disconnect the connector J320 (STN 485) and connector A109P2 from the APU generator (A109). Perform a continuity check as below. The resistance should measure as follows:

   From	To	Expected Result	Result
   J320-118	A109P2-64	<1 ohm
   J320-117	A109P2-66	<1 ohm

   1. If resistance value is not less than one ohm for each wire, repair defective wiring as required and do close out.
   2. If resistance value is less than one ohm for each wire, continue with next step.

10. Perform a continuity check as below. The resistance should measure as follows:

    From	To	Expected Result	Result
    J320-118	Ground	>1 megaohm
    J320-117	Ground	>1 megaohm
    A109P2-64	Ground	>1 megaohm
    A109P2-66	Ground	>1 megaohm

    1. If resistance value is not more than one megaohm for each wire, repair defective wiring as required and do close out.
    2. If resistance value is more than one megaohm for each wire, continue with next step.

11. Disconnect the J325 connector (STN 485). Perform a continuity check as below. The resistance should measure as follows:

    From	To	Expected Result	Result
    J325-11	A109P2-53	<1 ohm

    1. If resistance value is not less than one ohm for each wire, repair defective wiring as required and do close out.
    2. If resistance value is less than one ohm for each wire, continue with next step.

12. Perform a continuity check as below. The resistance should measure as follows:

    From	To	Expected Result	Result
    J325-11	Ground	>1 megaohm
    A109P2-53	Ground	>1 megaohm

    1. If resistance value is not more than one megaohm for each wire, repair defective wiring as required and do close out.
    2. If resistance value is more than one megaohm for each wire, continue with next step.

13. Check if EICAS messages that are related to the APU generator are displayed on the EICAS primary display.

    1. If YES, perform the applicable Troubleshooting for APU generator and do close out.
    2. If NO, continue with next step.

14. Replace FADEC (A108).
15. Do close out.