Fault Message:

APU OVERTEMPERATURE

Fault Code:

4960134APU

Associated CAS:

APU OVERTEMP
(Warning)

During the start sequence or while the Auxiliary Power Unit (APU) is in operation, the temperature of the Exhaust Gas Temperature (EGT) Sensor has increased higher than the over temperature limit for two seconds or more. The Full-Authority Digital Engine-Controller (FADEC) will cause an APU automatic shut down if the aircraft is Weight on Wheels (WOW). In Flight the APU will continue to run, but the same CAS message will remain.

• Auxiliary Power Unit (APU) Inlet Door
• Auxiliary Power Unit (APU) Inlet Duct
• Load Control Valve (LCV) (MPE1)
• Auxiliary Power Unit (APU)
• Surge Control Valve (SCV) (MPE2)
• Exhaust Gas Temperature (EGT) Sensor 1 (TC1-1)
• Exhaust Gas Temperature (EGT) Sensor 2 (TC1-2)
• Auxiliary Power Unit (APU) Full-Authority Digital Engine-Controller (FADEC) Controlling Sensors
• Auxiliary Power Unit (APU) Full-Authority Digital Engine-Controller (FADEC) (A108)
• Fuel Control Unit (FCU) (A3)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

APU EGT exceeds a Schedule limit which is a function of APU speed, P2, T2 and operating conditions (ECS and MES). In this condition the APU will auto shutdown on the ground only.

1. Examine the visual position of the LCV. It should be closed when the APU is not running. If it is open when the APU is not running, investigate why you have no fault on APU BITE check before start.
2. Examine the position of the Surge Control Valve (SCV) and ensure that it is closed while aircraft is WOW. This valve only opens at altitude, with APU running and has a visual indicator. It remains closed on the ground (WOW), when APU is off or running.
3. Examine the bleed ducting for any air leaks or damage from the APU bleed port to the APU fire wall.
4. Examine the inlet door for blockage or damage.
5. Examine the inlet duct and top of oil cooler forward section for blockage using a borescope through the inlet door (APU switch to Run only).
6. Examine the parts of the APU that move to make sure that there are no unusual noises. With the APU off and cool, remove the APU exhaust, fwd. lower bellmouth assy panel. Reach in to the rear of the APU and move the Turbine wheel and ensure it turns freely, with no unusual noises.
7. Remove the cover off of the rear of the starter and turn the starter with a ¼” square drive. Ensure free rotation with no unusual noises.
8. Disconnect the MPE1P1 connector from the LCV (MPE1) and Perform a continuity check between A and B on the Load Control Valve receptacle (MPE1). This is the torque motor of the LCV.

   1. If continuity is not within the range, repair defective wiring as required and do close out.
   2. If continuity is between 65 to 85 ohms, continue with next step.

9. Disconnect the A3P1 connector from the FCU (A3) (WM 49-12). Perform a check for continuity between A3-C and A3-D on the FCU (A3) receptacle (WM 49-12).

   1. If continuity is not within the range, repair defective wiring as required and do close out.
   2. If continuity is between 30 to 38 ohms, continue with next step.

10. Remove APU FADEC and Perform a visual inspection of the FADEC rack, pins and sockets for damage or FOD.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

11. Check for 68 Ω to 88 Ω on APU FADEC connector A108AP1-J1 and A108AP1-K1. Check each pin (J1/K1) individually to ground and ensure circuit is open. This is the LCV torque motor circuit.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

12. Check for 33 Ω to 41 Ω on APU FADEC connector A108AP1-F2 and A108AP1-G2. Check each pin (F2/G2) individually to ground and ensure circuit is open. This is the FCU torque motor circuit.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

13. Check for ground and open conditions at the APU FADEC connector A108AP1-K5 and A108AP1-G6. This will verify the cockpit APU Bleed switch (S7) is operational. You will need an assistant to move the switch. When the switch is closed, open on A108AP1-K5, ground on A108AP1-G6. Opposite for when the switch is open. Verify no continual grounding condition on any of the 2 pins.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

14. Rerack the APU FADEC.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

15. Access CAIMS/System Diagnostics/ATA 49/FADEC/LRU TEST/APU DATA STATUS page 1 and read EGT DATA. Back out and go to APU DATA STATUS page 2 and read EGT data. Compare the two and ensure they are the same or close to the same value. (Page 1 is EGT 1, Page 2 is EGT 2). Compare the rest of the data on these two pages to current conditions and ensure it is accurate. If possible read this data with and without the APU running to have as much data as possible. If you suspect a sensor, remember what you are reading is real time so have an assistant change the sensor physically (temperature/pressure) and verify the changes in CAIMS.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

16. Access Active Faults, System Diagnostics and Flight/Ground Faults. Look for faults that are active, stored or shown as faulty.
17. Perform a wiring check between APU FADEC and APU controlling Sensors not already checked.

    1. If wiring checks are not good, repair defective wiring as required and do close out.
    2. If wiring checks are good, continue with next step.

18. Replace APU FADEC.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

19. Replace the APU FCU.

    1. If system checks are good, do close out.
    2. If fault remains, continue with next step.

20. Replace APU.
21. Do close out.