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?

Thanks Mark. Yea, the plug had to be replaced.