ACTUATOR
WIRE
BRACKET
SWITCH
LEFT SIDE SHOWN, RIGHT SIDE SIMILAR A
Brake Pedal Bus Switch Location
Figure 9

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MAINTENANCE MANUAL

 C.  When the brakes are operated the brake pedal bus turns, and the control quadrant lever arm on the brake pedal bus pushes against the actuator on the brake-pedal-bus switch. This closes the switch. When the switch closes the autobrake-pedal-disarm logic in the brake control unit is connected to ground and the autobrake disarms.
 D.  A malfunction of the brake-pedal-bus switch can be isolated during the system BITE test using the CMC. The brake-pedal-bus switch, actuator and mounting bracket may then be removed and replaced as a unit. All adjustments to the actuator are done when the actuator and switch are installed on the bracket. Adjustments by line maintenance persons are not necessary. Steps that follow the installation procedure do a test on the switch that was installed.
 12. Brake Torque Sensor (Fig. 10)
___________________
 A.  A brake torque sensor is installed on each brake equalizer rod. The brake torque sensor measures the amount of torque that is applied to the brake equalizer rod by the operation of the brake. The torque is measured using a strain gage resistor bridge that is bonded to the brake equalizer rod. A protective sleeve and conduit recepticle is installed over the strain gages and held with clamps. The resistor bridge is connected to a conduit. The conduit passes through the recepticle and connects to airplane wire bundles at a panel under the truck beam.
 B.  The brake control unit transmits an excitation voltage of 10 vdc to the resistor bridge. With no force on the brake equalizer rod, the stain gage bridge should have an output of 0 vdc. Any change in the force on the brake equalizer rod changes the resistance of the strain gages which changes the output signal of the resistor bridge. Force in one direction causes a negative output voltage and force in the other direction causes a positive output voltage. The output signal is transmitted to the brake control unit and is used to prevent damage to the landing gear structure from too much torque during brake operation.
 C.  The brake control unit monitors the brake torque sensor for these malfunctions:
 -Zero drift
 -No output signal when the brakes are operated
 -An open circuit
 -A short circuit
 (1)  
The brake torque limiter subsystem performs an autozero after takeoff and when requested during system testing. The autozero after takeoff is performed two minutes after the ground to air transition has taken place with the wheel speeds high.

 (2)  
The offset data from the autozero for each brake is stored in non-volatile system BITE memory and is transmitted to the brake brake torque limiter subsystem at power up and until a new autozero has been performed by the brake torque limiter subsystem. If the offset for any one brake torque sensor is greater than |5000 foot-pounds a fault is indicated on EICAS and the offset is set to |5000 foot-pounds.

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL

INNER CYLINDER YOKE
MAIN GEAR TRUCK BEAM ASSEMBLY
BRAKE
SEE A PISTON HOUSING
TORQUE ARM
WING GEAR SHOWN, BODY GEAR SIMILAR
BRAKE TORQUE
SENSOR INSTALLATION
CONDUIT

CONNECTOR
BRAKE EQUALIZER ROD
A

Brake Torque Sensor Location Figure 10
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