Message Name:

LEFT B SERVO CURRENT FAULT

Message Code:

B3-681680

Associated CAS:

AT FAIL
(Advisory)

Left Servo Coil B current fault is an indication that the current monitors in the Autothrottle Computer detect an absence of current flowing through the servo coil B of the Servo Motor Clutch Pack (SMCP).

• Servo Motor Clutch Pack (SMCP) (B205)
• Autothrottle (AT) Computer (A252)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

NOTE: The Left B Servo Current Fault could be a result of a broken wire or open connector within the aircraft harness, or a failure of the SMCP or Auto Throttle Computer.
NOTE: Review the Auto Throttle "AT Diagnostic Information", to find details about the MDC Data Reader Labels & bits.
NOTE: Bombardier recommends referencing the elements one (1) through eleven(11) of the "Wiring & Connections" Troubleshooting Tips document. The intent of the document is to ensure the Maintenance Technician has the information needed to perform a comprehensive check of an aircraft system wiring circuit, while also doing checks found in the From - To tables called out in the Troubleshooting Recommendations element.

1. Ensure "Auto Throttle" circuit breaker (CB2-A8) is CLOSED, N1 <15% & aircraft is WonW..

   1. Set both throttle levers to TO (TLA 47°).
   2. On "System Test Panel" (Aft Center Pedestal) turn knob to "Auto throttle" and push and hold for at least 1 second.

2. The following CAS message is shown:

   1. "AT IN TEST" (Status).

3. While the self-test is running, Press and release either the left or right TQA, "AT Engage" switch.
4. The following CAS messages are shown:

   1. "AT IN TEST" (Status) is still shown
   2. "AT SERVO TEST" (Status) is shown.

5. Observe both Throttle Levers:

   1. Both Throttle Levers move together from TO detent to IDLE position within 4 seconds.
   2. Both Throttle Levers move together from IDLE position to TO detent within 4 seconds.
   3. Left Throttle Lever moves from TO detent to IDLE position within 4 seconds.
   4. Right Throttle Lever moves from TO detent to IDLE position within 4 seconds.

6. At completion of the Servo Test the following CAS messages are not shown:

   1. "AT IN TEST" (Status).
   2. "AT SERVO TEST" (Status).
   3. "AT FAIL" (Advisory).
      NOTE: A short Circuit of the servo coils in not detectable by the AT current monitors; the impedance of the motor coils is too low. However, a short between coils or from a coil to ground will be readily detectable or observable as a failure of the throttle lever to move when commanded, and likely a brief vibration of the affected throttle lever followed by an automatic disconnect. It may also be detected by the Servo Test.

7. If the fault is no longer present, monitor and do close out.
8. Remove the AT computer (A252) from the rack and disconnect the Servo Motor Clutch Pack (SMCP) (B205).
9. Do wiring check as follows:
   NOTE: Refer to the "Wiring & Connections" document found in the Troubleshooting Tips element of this page.
   

   From	To	Expected Result	Result
   A252P1-15	B205P1-F	Continuity
   A252P1-16	B205P1-G	Continuity
   A252P1-15	Ground	Open
   A252P1-16	Ground	Open
   B205P1-V	Ground	Continuity

   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.

10. Replace the Servo Motor Clutch Pack (SMCP) (B205).
11. Does the fault remain active?

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

12. Replace the AT computer (A252).
13. Do close out.