757 MAINTENANCE MANUAL
 (1)  
The module contains four identical valves. Each valve is independent and pressure operated. Two modules are located one each at left and right main wheel well ceiling, just forward of the main gear uplock assembly. Each valve shuttles pressure between normal and alternate systems.

 (2)  
The valve consists of an LRU valve assembly and an LRU filter. The valve has a manual override plug feature. The filter prevents system from contamination.

 (3)  
The basic three-way, two position shuttle valve consists of a normal (input) port, an alternate (input) port and a brake (output) port. Under normal operation, the normal port connects to the brake port. If hydraulic system switches from normal to alternate, a detented slide in the valve moves to block the normal port. This slide shift allows fluid flow from the alternate port to the brake port. In this manner, one input port always connects to the brake port while the other is blocked.

 (4)  
In the event of shuttle valve failure, the slide plug on the face of the valve is removed. A flight dispatch plug (a fly-away ground maintenance tool consisting of a small threaded plug) is installed in its place. The installed plug forces the slide in the valve to shift, thus blocking the normal port and opening the alternate port. This condition remains with the plug installed. The plug has no moving parts and is equipped with a ring to allow an indicator tag to be tied to it while being used.

 I.  Antiskid Wheel Speed Transducers (Fig. 8)________________________________
 (1)  
The transducer, a speed sensing device on each main gear wheel, contains only one moving part, a rotor which rotates inside a fixed stator. The stator attaches to a support inside the main wheel axle. The rotor, thru a four-arm dog rigidly attached on the rotar shaft, couples to the transducer drive in the hubcap. The drive, consisting of a bellows-type coupling and related mounting hardware inside the hubcap, turns the rotor when the wheel rotates. The dog/bellows coupling allows removal of the wheel and hubcap without disassembly of a bolted joint. Both transducer and drive are LRUs.

 (2)  
The stator comprises a permanent magnet, a 150-tooth soft carbon steel pole piece, and a pickup coil. The magnet sets up a magnetic field around the coil, whereby the mating 150-toothed rotor produces dynamic discontinuities when rotated. Turning of the rotor provides field changes as the mating teeth come in and out of alignment to produce a series of voltage pulses (150 times per turn). The voltage, related to the speed of the wheel, provides the control unit with wheel speed data.

 J.
Autobrake Module (Fig. 9)

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757
MAINTENANCE MANUAL

FWD  SEE  A  AXLE NUT (REF)  TRANSDUCER ASSEMBLY 
SUPPORT (REF) 
HUBCAP ASSEMBLY (REF)  TRANSDUCER DRIVE (REF)  WHEEL AXLE (REF) WHEEL (REF) 
DOG (REF) 

A

Antiskid Wheel Speed Transducer
Figure 8

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757
MAINTENANCE MANUAL

 (1)  
The autobrake module is connected to the normal brake lines, and is located on the forward bulkhead of the left main gear wheel well. The module contains an electric hydraulic pressure control servo valve (EHSV) (hereinafter called the pressure control valve), an upstream three-way solenoid shutoff valve (hereinafter called the solenoid valve), and two pressure switches, located one each at the outputs of the solenoid valve and the servo valve. The module is an LRU as the valves and switches are LRUs. Solenoid valve, pressure control valve and pressure switches can be replaced without removing the module from the airplane.

 (2)  
The module develops brake pressure in response to selected deceleration for all required autobrake functions. The solenoid valve provides on-off control of hydraulic power to the valve module, and the pressure control valve controls output pressure from the module as commanded by the control unit. Pressure switches on the module monitor the pressure outputs from the solenoid valve and the pressure control valve and provide the logic to the control unit.

 K.
Autobrake Module Operation (Fig. 10)

MOUNTING BRACKET LBL 52.80
(REF) (REF)
AUTOBRAKE MODULE
SEE A
LEFT MAIN GEARWHEEL WELL FORWARD BULKHEAD REAR SPAR STRUCTURE AT STA 1040 (REF)
FWD INBD
A
Autobrake Module
Figure 9

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