Discrepancy: Yaw damper disengages with Fault code 22-21006
Possible Fix: Rudder PCU or wiring
Tip: If during Yaw damper system test, the Sweep tests fails. Test the Servo loop system, this is the solenoid, the transfer valve and LVDT feedback. If that test is good you can go to the Rudder PCU and verify by watching the PCU that it engages and attempts to move during the sweep test if not. Verify the solenoid engage ciccuit, (D291 pin 1 and 2).
Reason/Explanation: During the Sweep test the #1 SMYD will send a electrical command to the transfer valve to displace the PCU then look for a feedback of LVDT position in the amount that is expected in movement. The FIMM procedure instructs you to “check the wiring after you have replaced the PCU, if the problem persists”. Since you will be at the PCU when you go to replace it you might want to verify solenoid engagment and movement before you take the time to change it.
Applicable Reference: FIMM 22-23-task 809, AMM 22-23-00-710-802, WDM 22-23-12
Thanks to Tom S. for his input !