Message Name:

APU DOES NOT START

The APU does not start. This condition could be observed on the ground or in flight.

• APU Starter Motor (B4)
• Ignition Unit (A173)
• Igniter Plug
• Exhaust Gas Temperature (EGT) Sensor (TC2) - (also known as Thermocouple T5)
• APU Fuel SOV (HP4)
• APU Low Oil Pressure Switch (S110)
• APU Wire Harness
• Load Control Valve (V8)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Full Throttle Blog/Forum Articles/Infoservice/Newsletter: None

Flight Operation Notifications Manual (FONM): None

NOTES:

• The APU Load Control Valve may cause an APU No Start condition. Review the Current Faults in the MDC looking for MDC Faults associated with the Load Control Valve.
• The Load Control Valve is spring loaded closed and has a visual position indicator that can be referenced for valve position.

1. Review the NOTES found in the Troubleshooting Tips: element of this page prior to the start of troubleshooting.
2. Access the MDC to view fault messages. Are there any APU or APU component related fault messages?

   1. If YES, use the appropriate SmartFix Plus to troubleshoot these messages first.
   2. If NO, continue with next step.

3. Access the APU enclosure and disconnect the APU Fuel SOV solenoid connector.
4. Apply power to the aircraft and perform an APU start.
5. Monitor the APU speed. The speed should increase to 26-28%.

   1. If the speed is good, continue with next step.
   2. If the speed is not good, replace the starter.
      NOTE: If the APU starter is removed, make sure the starter clutch is inspected for damage.

6. When the start is attempted, can you hear the "ticking" sound of the igniters?

   1. If YES, go to step 8.
   2. If NO, continue with next step.

7. Remove the APU ECU. Disconnect the Ignition Unit connector A173P1. Perform a continuity check as follows:
   

   From	To	Result
   A39CP1-2	A173P1-B
   A39CP1-4	A173P1-A

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

8. Perform a continuity check as follows:
   

   From	To	Result
   A39CP1-2	Ground
   A39CP1-4	Ground

   1. If there is continuity, replace the Ignition Unit.
   2. If there is no continuity, continue with next step.

9. Perform a detailed inspection of the APU igniter plug as per AMM 49-41-09-220-801. Check for erosion and gaps.

   1. If defects are found, replace the igniter plug and do close out.
   2. If no defects are found, continue with next step.

10. Perform a wiring check between the APU ECU and the Low Oil Pressure (LOP) Switch as follows:
    

    From	To	Result
    A39AP1-12A	S110P1-A
    A39AP1-5D	S110P1-B

    1. If wiring defects are found, repair defective wiring as required and do close out.
    2. If no defects are found, continue with next step.

11. Do wiring check between the APU ECU (A39) and the EGT Sensor (TC2) for the following wires

    From	To	Result
    A39AP1-2A	EGT Sensor Lug + (CR)
    A39A91-2A	EGT Sensor Lug - (AL)

    1. If the check is satisfactory, continue with next step
    2. If the check is not satisfactory, repair defective wiring
       NOTE: Lugs must be disconnected from the EGT Sensor while doing the wiring check.

12. Replace the EGT Sensor (TC2). Is the fault message still present?

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

13. Using WM 49-42-05, perform a wiring check between the APU ECU and the ADCPC APU Fuel Valve Relay (K96).

    1. If no defects are found, continue with next step.
    2. If defects are found repair defective wiring as required and do close out.

14. Perform wiring checks between the ADCPC and the Fuel SOV.

    1. If no defects are found, continue with next step.
    2. If defects are found repair defective wiring as required and do close out.

15. Contact the Bombardier Customer Response Center (CRC) for further troubleshooting steps.
16. Do close out.