Autobrake Issue

Working a repeat chronic AUTOBRAKES issue with the maintenance message "32290 - AUTOBRAKE VALVE MODULE CONTROL VLV OR CONTROL VALVE PRESS SW (BSCU)." 

Numerous parts had been replaced, including BSCU (multiple times) and the A/B hydraulic valve module. 

Like most Boeing, the 747 A/B hydraulic valve module has a control valve and solenoid valve. Two pressure switches are installed to monitor the operation of each valve. The maintenance message is pointing towards a failure of either the control valve solenoid or it's related pressure switch. Since the entire valve module had been previously replaced, I accomplished wiring checks between the BSCU and Control Valve Solenoid / Control Valve Px Sw. 

 

The control valve wiring checked good. The pressure switch circuit was also checking good, until I megged pins 2-3 at DM439D. Checking the prints, I noticed that pressure = open and low pressure = continuity back to BSCU. A short between the two pins would incorrectly signal "low" pressure to the BSCU even if the control valve/solenoid are fully operational. After the wires were de-pinned from the connector, the short was no longer present. Looks like hydraulic contamination resulted in the short. 

 
 

Great find.

Was plug replacement required?