Message Name:

SPEED SENSOR [THRESHOLD SIG CKT 1]

Message Code:

4960267APU

Associated CAS:

None

Full Authority Digital Engine Controller (FADEC) detected a low signal amplitude for Speed Sensor CKT1.

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

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

Notice To Operators (NTO): None

Flight Operation Notifications Manual (FONM): None

1. Get access to the CAIMS to view the above fault message and other APU related faults.

   1. If fault message is present, troubleshoot them first and continue with next step.
   2. If fault message is not present, continue with next step.

2. Re-rack APU FADEC and verify condition of connector AP1, as well as APU mounting tray connectors for damage.

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

3. Using an ohmmeter measure continuity on Speed Sensor receptacle MT4, perform continuity check between the pins of Speed Sensor receptacle (MT4) and resistance should measure as follows:

   From	To	Expected Result	Result
   MT4P1-A	MT4P1-B	54.08-81.12 ohms

   1. If resistance is out of range, go to step 7.
   2. If resistance is within range and fault indication remains, continue with next step.

4. Perform wiring checks between FADEC (A108AP1) connector and Speed Sensor 1 (MT4P1) connector as follow:

   From	To	Result
   A108AP1-G11	MT4P1-A
   A108AP1-H11	MT4P1-B

   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.

5. Perform wiring checks between Speed Sensor 1 (MT4P1) connector and J802 connector on APU firewall as follow:

   From	To	Result
   MT4P1-A	J802-13
   MT4P1-B	J802-14

   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.

6. Perform wiring checks between P802 connector on APU firewall and FADEC (A108AP1) as follow:

   From	To	Result
   P802-13	A108AP1-G11
   P802-14	A108AP1-H11

   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.

7. Replace Speed Sensor.

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

8. Replace FADEC.
9. Do close out.