L MAIN BUS FAIL
(Caution)

The amber 'L MAIN BUS FAIL' caution CAS message indicates the Left main power bus has no power, though it is available.

• This could be indication of faulty Fuse(s), Generator(s) or Contactor(s).

• Left Essential Contactor (LESSC) K77
• Left Bus Tie Contactor (LBTC) K79
• Right Bus Tie Contactor (RBTC) K80
• External DC Contactor (EDCC) K81
• Left Aux Contactor (LAUXC) K75
• F1 Main Fuse (LDCPC)
• PCB3 Logic Card (LDCPC)
• Left Direct Current Power Center (LDCPC) A35
• Associated Wiring

Advisory Wire/Service Bulletin:

• AW300-24-0162 - Service Bulletin 100-24-20 - DC Power Center (DCPC) System - Inspection and Replacement of Internal Fuses in the L/H and R/H DC Power Center (DCPC)

Forum Articles/Infoservice/Newsletter: None

1. Check for other ATA 24 CAS Messages on EICAS and proceed with respective troubleshooting prior to continue with next step.
2. Interrogate the MDC as follows:

   1. On the cursor control panel, ensure the toggle switch is to the right and select Anti-Ice, ECS and Fuel buttons simultaneously. The MDC main menu should now be displayed on Display Unit #3.
   2. Using the joystick, select Current Faults (if the fault is active) or Aircraft History to research stored faults. From Aircraft History you can then select Fault Message History and locate the fault. Going into advanced diagnostics will provide an equation id for the fault identified.
   3. Select the MDC fault or equation from the drop down below to provide additional information to assist in troubleshooting.

3. Select an MDC Fault Code/Message:B3-006668 - FAULT [EXT DC CNTOR]B3-007014 - FAULT [L AUX BUS CNTOR]B3-007019 - FAULT [L BUS TIE CNTOR]B3-007071 - FAULT [L ESS CNTOR]B3-007105 - L LOGIC/LBTC/RBTC FAULT [L LOGIC PCB]B3-007106 - FUSE BLOWN [L MAIN FUSE]B3-007688 - FAULT [R BUS TIE CNTOR]B3-269321 - SHORT CIRCUIT [BUS TIE FEEDER]