PR solenoid faults 
PR sol fault  msg. no. 22-21004   PR sol /wrg/YDC  LATCHED: Note 3  TYPE: Note 2  . Pos  Causes: Pressure reducer bypass valve sible
.  Yaw damper coupler 
Troubleshooting: 
.  Verify resistance from D295 socket 31 to ground is 60 ohms. 
.  Verify wiring between D295 socket 31 and D7377 socket 1 at the pressure reducer bypass valve solenoid. 
.  Verify connection to ground from D7377 socket 2 at the pressure reducer bypass valve solenoid. 
.  Inspect electrical connectors for bent or broken pins, contamination, or damage. 
.  If problem persists, replace the pressure reducer bypass valve. 
.  If problem persists, replace the yaw damper coupler. 

YD switch faults
YD sw
msg. no.
FDE:
YD sw
LATCHED:
TYPE: Note
fault 22-21005
YD disenga
/wrg/YDC
Note 3
2
Possible Causes:
.  
Yaw damper engage switch

.  
Yaw damper coupler

.  
Autopilot accessory unit

Troubleshooting:
.  
Verify resistance from D295 socket 14 to ground is less than 2.0 ohms with yaw damper engage switch in the OFF position.

.  
Verify wiring between D295 socket 14, the yaw damper engage switch, and autopilot accessory unit.

.  
Inspect electrical connectors for bent or
broken pins, contamination, or damage.

.  
If problem persists, replace the yaw damper engage switch.

.  
If problem persists, replace the yaw damper coupler.

.  
If problem persists, replace the autopilot
accessory unit.

500 
May 01/99  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  22-12-01 Page 105 

Airplane configuration faults 
APL type error  msg. no. 22-21006  FDE: YD disenga  APL/pgm pin/YDC  LATDHED: YES  TYPE: Note 2  Possible Causes: . Yaw damper coupler replacement . Aircraft wiring . Yaw damper coupler Troubleshooting: . This fault may be present when a yaw damper coupler is initially installed into an aircraft. Perform the yaw damper coupler installation  test, ref AMM12-12-01, page 401 step 4. . If the problem persists, inspect D295 socket 17, D295 socket 53, and YDC electrical connector for bent or broken pins, contamination, or damage. . Verify D295 socket 17 and D295 socket 53 resistance to ground is consistent with aircraft wiring diagrams, summarized; 737 MODEL D295 - 53 D295 - 17 -100/200/200 ADV GROUND OPEN RIDE QUALITY -100/200/200 ADV OPEN OPEN . Perform the yaw damper coupler installation  test, ref AMM12-12-01, page 401 step 4. . If the problem persists, replace yaw damper coupler. 
28 VCD faults 
YDC 28V DC high  msg. no. 22-21007  FDE: YD disenga  28V src /wrg/YDC  LATCHED: NO  TYPE:  Possible Causes: . Excessive 28 VDC voltage applied to yaw damper coupler . Yaw damper coupler Troubleshooting: . Verify voltage to D295 socket 4 is between 22.0 and 29.5 volts DC with the yaw damper DC circuit breaker closed. . If problem persists, replace yaw damper coupler. 
YDC 28V DC low  msg. no. 22-21008  FDE: YD disenga  CB/wire/ 28V src  LATCHED: Note 3  TYPE: Note 2  Possible Causes: . Low or No 28 VDC voltage applied to the yaw damper coupler . Yaw damper coupler Troubleshooting: . Verify yaw damper DC circuit breaker is closed. . Verify voltage to D295 socket 4 is between 22.0 and 29.5 volts DC. . Inspect electrical connectors for bent or broken pins, contamination, or damage. . If problem persists, replace yaw damper coupler. 

500 
22-12-01  May 01/00 
Page 106 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

YDC/solenoid/wiring faults 
YD sol short  msg. no. 22-21009  FDE: YD disenga  YD sol /YDC/wrg  LATCHED: Note 3  TYPE:  Possible Causes: . Rudder power control unit actuator solenoid 
.  Yaw damper coupler 
Troubleshooting: 
.  Verify resistance from D295 socket 20 to ground is 60 ohms. 
.  Verify wiring D295 socket 20 and D291 socket 1 at the rudder power control unit actuator solenoid. 
.  Verify connection to ground from D291 socket 2 at the rudder power control unit actuator solenoid. 
.  Inspect electrical connectors for bent or broken pins, contamination, or damage. 
　
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