Fault Message:

APU SHUT DOWN - OVERSPEED

Fault Code:

4960125APU

Associated CAS:

APU OVERSPEED
(Warning)

An Auxiliary Power Unit (APU) shut-down occurred, because of over speed. APU speed was greater than 106%. Automatic shut-down will occur on ground or in flight.

NOTE: An asterisk mark (*) in any row above means that the automatic protection shut-down on that row is inhibited if the associated fault occurs in flight. The respective CAS message, or an advisory message reading APU FAULT will be displayed in the CAS window and the APU will not shut down automatically in flight. However the pilot may select to shut-down the APU manually by placing the APU control switch to "OFF".

• Auxiliary Power Unit (APU) Full-Authority Digital Engine-Controller (FADEC) (A108)
• Fuel Control Unit (FCU) (A3)
• Speed Sensor (MT4)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

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

   1. If damages are found, repair as required and do close out.
   2. If damages are not found, continue with next step.

2. Disconnect the MPE1P1 connector from the LCV and inspect for contamination and damage.

   1. If contamination or damage found, repair as required and do close out.
   2. If contamination or damage not found, re-connect MPE1P1 connector and continue with next step.

3. Disconnect the A3P1 connector from the FCU (A3) and inspect for contamination and damage.

   1. If contamination or damage found, repair as required and do close out.
   2. If contamination or damage not found, re-connect A3P1 connector and continue with next step.

4. Disconnect the MT4P1 connector from the APU Speed Sensor (MT4) and inspect for contamination and damage.

   1. If contamination or damage found, repair as required and do close out.
   2. If contamination or damage not found, re-connect MT4P1 connector and continue with next step.

5. Perform a continuity check between APU FADEC connector A108AP1 and ground as follows:

   From	To	Expected Result	Result
   A108AP1-F2	Ground	33 Ω to 41 Ω
   A108AP1-G2	Ground	33 Ω to 41 Ω

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

6. Perform a continuity check between APU FADEC connector and ground (this is the FCU shut-off solenoid) as follows:

   From	To	Expected Result	Result
   A108BP1-A1	Ground	18 Ω to 33 Ω
   A108BP1-B1	Ground	18 Ω to 33 Ω

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

7. Perform a continuity check between APU FADEC connector A108AP1and ground (this is the speed sensor (dual channel)) as follows:

   From	To	Expected Result	Result
   A108AP1-G11	Ground	≈24 Ω
   A108AP1-H11	Ground	≈24 Ω
   A108AP1-J11	Ground	≈33.9 Ω
   A108AP1-K11	Ground	≈33.9 Ω

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

8. Re-rack APU FADEC.

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

9. 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 channel 1, Page 2 is channel 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 (give or take 5 seconds), so have an assistant change the sensor physically (temp/ pressure) and verify the changes in CAIMS.
10. Swap speed sensor.

    1. If system checks are good, replace defective speed sensor and do close out.
    2. If fault remains, continue with next step.

11. Replace the APU FADEC.
12. Do close out.