Message Name:

OP CKT-START MOT DC PWR WRG/SEN WRG

Message Code:

4960112APU

Associated CAS:

None

The start relay feedback voltage is less than 1 volt. This shows a possible open in the signal lines to the Full-Authority Digital Engine-Controller (FADEC).

There is a high resistance between the Auxiliary Power Unit (APU) Full-Authority Digital Engine-Controller (FADEC) and the APU Starter. Experience has shown that (assuming the sense circuit wiring is intact) the problem is Starter Brush wear/sticking.

• Auxiliary Power Unit (APU) Full-Authority Digital Engine-Controller (FADEC) (A108)
• Auxiliary Power Unit (APU) Starter Motor (B55) (MPE3)
• Auxiliary Power Unit (APU) Starter Contactor Assembly (ASCA) (A48)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Full Throttle Blog/Forum Articles/Infoservice/Newsletter:

• Honeywell SIL APU-137 - Service Information Letter
• Honeywell SIL APU-99 - Electronic Control Unit (ECU) “No Fault Found” Removals

Flight Operation Notifications Manual (FONM): None

1. Remove the APU FADEC and disconnect the APU Starter motor. Inspect the pins/sockets/lugs for damage, corrosion and/or push back. Perform a continuity check between APU Starter Motor (-) negative and (+) positive terminals (MPE3) as follows:

   From	To	Result
   A108BP1-H9	MPE3-NEG
   A108BP1-G9	MPE3-POS

   1. If there is continuity, go to step 3.
   2. If there is no continuity, continue with next step.

2. Perform a continuity check between the APU connector P1 and the engine branched wiring harness as follows:

   From	To	Result
   APU P1-5	Negative cable
   APU P1-4	Positive cable

1. If there is no continuity, go to step 5.
2. If there is continuity, repair defective wiring as required and do close out.

3. Perform a continuity check between the APU FADEC and relay K2 inside the ASCA as follows:

   From	To	Result
   A108BP1-C5	A48P2-C

   1. If there is no continuity, go to step 5.
   2. If there is continuity, continue with next step.

4. Perform continuity check between relay K2 inside the ASCA and the APU Start Motor B55 (MPE3) as follows:

   From	To	Result
   MPE3-POS	A48P2-T5
   MPE3-POS	Ground

   1. If there is continuity, go to step 6.
   2. If there is no continuity, continue with next step.

5. Replace engine branched wiring-harness. Is the fault still present?

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

6. Perform a relay check inside the ASCA (A48) on the APU Start Contactor relay K2.

   1. If relay does not check out, go to step 9.
   2. If relay checks out, continue with next step.

7. Replace APU Starter Motor.

   1. If APU Start Motor engages, do close out.
   2. If APU Start Motor still does not engage, continue with next step.

8. Replace APU FADEC.

   1. If starter motor engages, do close out.
   2. If starter motor will not engage, continue with next step.

9. Replace ASCA.
10. Do close out.