AC BUS 4 FAIL
(Caution)

AC BUS 4 is not powered or is reported failed by the ACPC.

• AC Power Center (ACPC) GLC4 (K13) Contactor
• AC Power Center (ACPC) GTC(s) Contactor(s)
• Secondary Logic Card (SLC) 4
• Solid State Power Contactor (SSPC) Card 10
• AC Power Center (ACPC) (A54)

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

Troubleshoot other Electrical system CAS messages with CAIMS messages before troubleshooting AC BUS 4 FAIL (Amber).

If the AC BUS 4 FAIL is associated with a GCU 4 failure related to over current, the ACPC then will isolate the AC BUS 4 (BUS shorted situation). Remove all the loads related to AC BUS 4 and re-apply all loads one by one to find where the short comes from. Observe AC synoptic page during power-on steps.

NOTE: Verify C/B status on both pilot and copilot EMS CDUs.

NOTE: Please review the "ACPC Contactor Tack Welding" observed fault before carrying out this procedure.

1. Apply power to the aircraft in the configuration that the AC BUS 4 FAIL CAS message is posted.
2. In the flight deck, access AC Synoptic page.

   1. If AC electrical power is not supplied by VFG 4, go to step 4
   2. If AC electrical power is supplied by VFG 4, continue with next step.

3. Access ACPC Primary Subassembly. Do a 115 VAC voltage check as follows:
   

   From	To	Result
   K13-A1	Ground
   K13-B1	Ground
   K13-C1	Ground

   1. If there is voltage, go to step 6.
   2. If there is no voltage, troubleshoot involved contactors upstream of K13, leading to VFG 4 output. Replace defective contactors as required.

4. Access ACPC Primary Subassembly. Do a 115 VAC as follows:
   

   From	To	Result
   K13-A3	Ground
   K13-B3	Ground
   K13-C3	Ground

   1. If there is voltage, go to step 5.
   2. If there is no voltage, troubleshoot the involved contactors upstream of K13, leading to the power source.

5. Do a continuity check as follows:
   

   From	To	Result
   K13-A3	K13-A2
   K13-B3	K13-B2
   K13-C3	K13-C3

   1. If there is continuity, continue with next step.
   2. If there is no continuity, replace contactor K13.

6. Open the circuit breaker to all the loads that are fed by AC BUS 4. Verify if AC BUS 4 is now powered.

   1. If AC BUS 4 is now powered, push in one circuit breaker at a time to find what circuit or system is shorted.
   2. If AC BUS 4 is not powered. Continue with next step.

7. Remove power from the aircraft. Remove SSPC #10. Verify if the AC BUS 4 is shorted to ground by performing a continuity check as follow:
   

   From	To	Result
   K13-A2	Ground
   K13-B2	Ground
   K13-C2	Ground

   1. If there is continuity, troubleshoot the short inside the ACPC, you may have to replace the ACPC.
   2. If there is no continuity, replace contactor K13.

8. Is the AC BUS 4 FAIL message still present?

   1. If NO, do close out.
   2. If YES, continue with next step.

9. Swap SSPC card #10 with SSPC card #11.

   1. If fault follows, replace the defective SSPC.
   2. If fault remains, continue with next step.

10. Swap SLC #4 with SLC #1, #2 or #3.

    1. If fault follows, replace the defective SLC.
    2. If fault remains, continue with next step.

11. Remove SSPC card #10. Do a continuity check as follows:
    

    From	To	Result
    P420-2	K13-A2
    P420-5	K13-B2
    P420-8	K13-C2

    1. If there is continuity, continue with next step.
    2. If there is no continuity, replace ACPC.

12. Remove SLC #4 and do a continuity check as follows:
    

    From	To	Result
    P420A-20	P410B-13
    P420B-20	P410C-13
    P420C-20	P410C-14

    1. If there is continuity, continue with next step.
    2. If there is no continuity, replace ACPC.

13. Do a continuity check as follows:
    

    From	To	Result
    P420A-20	Ground
    P420B-20	Ground
    P420C-20	Ground

    1. If there is continuity, replace ACPC.
    2. If there is no continuity, inspect contacts for SLC #4 and SSPC #10 and make sure the cards are properly seated when re-installed and replace ACPC as required.

14. Do close out.