Message Name:

SERVO TEST FAULT

Message Code:

B3-681732

Associated CAS:

AT FAIL
(Advisory)

AT SERVO TEST FAULT is posted if the Servo Test fails, either due to an interruption to the commanded lever movement (mechanical issue), an internal fault being detected during the test, or due to a failure of the TLA position feedback path.

• The TLA position feedback path is via the TQA RVDTs, the FADECs, and Avionics/ARINC 429 buses.
• The fault is reported by the Autothrottle (AT) computer. 

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

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

NOTE: Review the  Autothrottle "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. 

NOTE: The AUTOTHROTTLE (AT) will stop the lever movement when servo test failure is detected.
NOTE: The AT servo test failure state will stay latched until a subsequent servo test is initiated and passes successfully.
NOTE: The Servo Test may be manually initiated, by pushing the Engage button, however the Required Conditions must be achieved first. See the Required Conditions below:

1. Access MDC on MFD #2 by pressing "A/ICE", "ECS", "FUEL" buttons simultaneously.
2. Highlight "LRU INDEX/OPERATIONS" then press "ENTER" button. Highlight "AT COMPUTER" then press the "ENTER" button. Press the "SKIP" button. Highlight Label "271" then press the "ENTER" button.
   

   Label 271 BITS	Status	Description
   17	0	AT LA SERVO CURRENT FAULT
   18	0	AT LB SERVO CURRENT FAULT
   19	0	AT RA SERVO CURRENT FAULT
   20	0	AT RB SERVO CURRENT FAULT

3. Are all bits listed in the table set at 0?

   1. If YES, continue with step 5.
   2. If NO, continue with next step.

4. Access the MDC to view fault messages. Select MDC Faults displayed from menu below. If no messages are present, continue with next step.
   - Select Message -B3-681677 - LEFT A SERVO CURRENT FAULT ATB3-681680 - LEFT B SERVO CURRENT FAULT ATB3-681683 - RIGHT A SERVO CURRENT FAULT ATB3-681688 - RIGHT B SERVO CURRENT FAULT AT
5. A mechanical issue causing a delay or interruption to the commanded lever movement can cause the test to fail, access the Servo Motor Clutch Pack (SMCP) and the Throttle levers and do a visual inspection for any restricted mechanical movement of the Throttle Levers, RVDTs, SMCP components.
6. Are there any mechanical issues observed?

   1. If YES, repair as required.
   2. If NO, continue with next step.

7. Remove the Autothrottle (AT) computer (A252) and disconnect the Servo Motor Clutch Pack (SMCP) (B205P1). Do wire checks as follows:
   NOTE: Refer to the "Wiring & Connections" document found in the Troubleshooting Tips ↑ element of this page.

From	To	Expected Result	Result
A252P1-20	B205P1-H	Continuity
A252P1-21	B205P1-J	Continuity
A252P1-20	Ground	Open
A252P1-21	Ground	Open
A252P1-15	B205P1-F	Continuity
A252P1-16	B205P1-G	Continuity
A252P1-15	Ground	Open
A252P1-16	Ground	Open
A252P1-33	B205P1-C	Continuity
A252P1-34	B205P1-D	Continuity
A252P1-33	Ground	Open
A252P1-34	Ground	Open
A252P1-1	B205P1-A	Continuity
A252P1-2	B205P1-B	Continuity
A252P1-1	Ground	Open
A252P1-2	Ground	Open

1. If no defects are found, continue with next step.
2. If wiring defects are found, repair defective wiring as required and do close out.

8. Replace the Servo Motor Clutch Pack (SMCP). Is the fault still present?

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

9. Replace the AT computer.
10. Do close out.