(h)
L ADC, R ADC - The yaw damper module has determined that the data from the indicated air data computer is invalid.
(i)
ADC DIFF - The yaw damper module has detected an invalid input from an air data computer but is unable to determine which one is at fault.
(j)
AIRGND 1, AIRGND 1/2, AIRGND 2 - The yaw damper module has determined that the indicated air/ground discrete is invalid.
(k)
AIR DIFF - The yaw damper module has detected an invalid air/ground discrete but is unable to determine which air/ground relay is at fault.
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757 MAINTENANCE MANUAL
(l)
26 VAC - The yaw damper module has detected the loss of or the invalid level of the 26V RMS reference voltage.
(m)
HYD SWITCH - The hydraulic pressure high signal agreed with the hydraulic pressure low signal for more than 2 seconds.
(n)
L IRU, C IRU, R IRU - The yaw damper module has determined that the data from the indicated inertial reference unit is invalid.
(o)
IRU OFF - The IRUs are not on or no data is transmitted.
(p)
IRU NCD - IRUs are on but no data or bad data is transmitted.
(q)
IRU DIFF - The yaw damper module has detected an invalid input from an inertial reference unit but is unable to determine which one is at fault.
(r)
LVDT - The yaw damper module has detected that the LVDT common mode voltage is low. During the preflight test, an LVDT fault is recorded if the common mode voltage is low and a position test fault is detected (valid high hydraulic pressure must be present).
(s)
YD ACT - During normal operation, a yaw damper actuator fault occurs when the yaw damper module detects a servo loop fault. During the preflight test, a yaw damper actuator fault is recorded if a position test fault occurs. To record a YD ACT fault, valid high hydraulic pressure must be present and the LVDT common mode voltage must be normal.
(t)
YD TEST - Failure of PFT (Pre-Flight Test).
(u)
YDM - A yaw damper module fault is recorded and latched with the following faults: power up test fault, preflight test fault, A/C type code input fault, control or monitor channel command coincidence fault, or a computing channel fault. A YDM fault is also recorded when either control or monitor channel detected auto-disengage relay faults.
(v)
AIRPLANES WITH MODAL SUPPRESSION ACCELEROMETER; YDM fault is also shown when there is a loss of excitation voltage for the accelerometer. 1) If no YDM fault exists, the YDM fault latch resets for the
following conditions: a) Successful completion of power up test or maintenance reset on ground. b) Power up reset, manual engage or YDM autoreset in air or on ground.
E. Troubleshooting Techniques
(1) Autoflight BITE can reduce troubleshooting time and eliminate unnecessary replacement of LRUs if used properly.
(a) Always check flight faults, record all faults and related diagnostic.
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(b)
If there are no current flight faults, perform ground test 30 and record all faults and associated diagnostic codes.
(c)
If no flight fault exists for a particular flight squawk, perform an associated ground test.
(2)
Interface faults should be corrected first, and associated ground tests rerun to ensure a faulty LRU is not being masked by another fault that renders the interface inoperative.
(a)
If an interface fault cannot be isolated to a specific LRU or wire, redundant LRUs can be swapped and the fault tracked to relate it to an LRU.
(b)
AIRPLANES WITHOUT YDM -122 AND SUBSEQUENT YDM DASH NUMBERS; the faultballs and other fault indicators not related to autoflight BITE can be checked and individual LRU self-tests run.
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