A
737-300/400/500MAINTENANCE MANUAL
 YAW DAMPER SYSTEM - TROUBLE SHOOTING
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 1.  General_______
 A.  The following trouble shooting procedures are intended to help trouble shoot the Yaw Damper system down to a component level. Trouble shooting charts are based on possible failure reports which may occur and the corresponding isolation procedure to detect and replace the faulty component. The charts are listed under the system heading and are separated into engage problems or operational problems.
 B.  Yaw Damper System
 (1)  
Trouble shooting the Yaw Damper system operational faults can be accomplished fairly quickly through use of the BITE system test circuits. If the system shows faulty by the BITE tests, the Yaw Damper coupler is usually at fault.

 (2)  
Yaw Damper coupler has BITE controls on the front panel. Operation of BITE controls checks the majority of the signal paths in the system (Ref Yaw Damper System - Adjustment/Test). Proper analysis of the BITE indications, while performing a test of the coupler, helps indicate whether the coupler is at fault or a fault is at the rudder hydraulic power pack (Ref Yaw Damper System Trouble Shooting Chart and BITE Test Failed Chart). It should be noted that the autopilot warning light illuminates if the BITE switch on the Yaw Damper coupler is positioned to ARM.

 (3)  
Trouble shooting Yaw Damper system engage faults involves checking the operation of the AFC accessory unit, (or the Integrated Flight System Accessory Unit) and Yaw Damper coupler. If the Yaw Damper system does not engage or has nuisance trips, the fault will probably be in the accessory unit (Ref Yaw Damper System Engage Trouble Shooting Charts).

 (4)  
The solenoid valves, transfer valves and position transducers on the hydraulic power pack have proven to be very reliable. Operational problems will more than likely be traced to relays in the AFC accessory unit (or the Integrated Flight System Accessory Unit). Hydraulic power pack problems are usually indicated by operational faults when the yaw damper system is not engaged, such as sloppy centering of the rudder due to hydraulic wear or leaking seals (Ref Chapter 27, Flight Controls).

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 CAUTION: DO NOT REMOVE YAW DAMPER COUPLER FOR AT LEAST 3 MINUTES AFTER
_______ REMOVING POWER FROM THE SYSTEM. RATE GYRO MAY BE DAMAGED IF COUPLER IS MOVED BEFORE GYRO IS ALLOWED TO SPIN DOWN.
 (5)  
Before trouble shooting the Yaw Damper system, ensure hydraulic and electrical power is applied to the airplane and the rudder operates normally without the Yaw Damper system engaged. The flight control system B switch must be ON in order to engage the Yaw Damper system. Ensure the Yaw Damper system, master caution, master dimming bus indicator lights, dim and test, Autopilot engage interlock and warning light circuit breakers are closed before attempting to engage the Yaw Damper system. To prevent misleading indications, test operation of the Autopilot disengage lights by pressing the light caps to ensure bulbs illuminate. It should be noted that the Yaw Damper engage light is normally illuminated whenever the Yaw Damper system is not engaged. The Digital Air Data Computer does not have to be energized for self-test of the Yaw Damper system since no interlock voltages are provided to the Yaw Damper systems. However, the air data computer must be installed since the Yaw Damper "q-potentiometer" is used in the system. Voltage through the q-potentiometer is provided by the Yaw Damper system. The Yaw Damper coupler BITE is calibrated with no airspeed inputs to the Digital Air Data Computer. Removal of black boxes and system components requires normal safety and maintenance precautions such as removal of power prior to removal of components and capping of all exposed electrical connectors.

 (6)  
The Yaw Damper system may disengage when electrical power is transferred after a flight where the autopilot or flight director was used during landing. This condition is normal, and no maintenance action is required. If the Yaw Damper system disconnects after power transfer, engage the Yaw Damper system again. Make sure that the Yaw Damper engage light is not illuminated, and that the system powers up normally.
　
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