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 C.  Autopilot system electrical signals operate valves which modulate hydraulic pressure to displace a hydraulic piston and provide a rotary output to the respective PCU. Control and position signals are provided by the following components which are installed on each actuator: engage solenoids, transfer valve, linear variable displacement transducer (LVDT), and pressure regulator.
 D.  Engage Solenoids
 (1)  Two engage solenoids are on each autopilot module. Each solenoid is an electrically operated valve (28 volts dc) which, when energized, applies hydraulic pressure within the module. The ARM SOLENOID provides hydraulic pressure to the TRANSFER VALVE and to engage THE DETENT SOLENOID. The detent solenoid provides hydraulic pressure to the detent mechanism. Both solenoids are energized at A/P engagement; however, the detent solenoid is delayed slightly from the arm solenoid. The solenoids are attached to the module with four bolts. Electrical pins mate with wiring within the module when the units are installed. Hydraulic pressure is ported into the units through ports which align when the solenoids are installed. The solenoids are line replaceable units.
 E.  Transfer Valve
 (1)  The transfer valve distributes hydraulic fluid to the actuator piston through two ports. With the AP engaged and no command signal output (null), the hydraulic flow is equal between output ports. An autopilot signal (vdc) torques the transfer valve to provide an unbalanced hydraulic flow through the control ports which in turn caused the actuator piston to displace. Electrical connections and hydraulic fluid is supplied to the transfer valve similar to the solenoid valves. The transfer valve is installed with four bolts. The transfer valve is a line replaceable unit.
 F.  Linear Variable Displacement Transducer (LVDT)
 (1)  The linear variable displacement transducer provides positional information for the actuator piston. The LVDT is excited by 14 volts ac 1800 Hz and provides an ac output signal in proportion to piston position. The LVDT is not line replaceable since it requires mechanical adjustment for nulling to match the actuator.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
VALVE TRANSFER AUTOPILOT  FILTER  SOLENOID DETENT  SOLENOID ARM FILTER 
ENGAGE ORIFICE CONTROL DETENT
PORT PRESSURE 
INPUT SIGNAL A/P  PIPE JET  SPOOL CONTROL  ORIFICES LIMITING RATE  PORT RETURN 
AIL HYD SWITCH PRESSURE  VALVE & RELIEF REGULATOR PRESSURE 
(LVDT) SENSOR POSITION ACTUATOR 
SPRINGS DETENT  OUTPUT CRANK EXTERNAL  PISTONS DETENT OUTPUT CRANK INTERNAL 
Figure 3 Autopilot Actuator Schematic 

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 G.  Pressure Regulator
 (1)  The pressure regulator is in line with the hydraulic passages between the detent solenoid and the detent piston (which locks the actuator piston to the output crank). The regulator bypasses hydraulic fluid to limit the output force (autopilot authority) of the actuator when the unit is backdriven or stalled.
 10. Position and Neutral Shift Sensors__________________________________
 A.  The aileron position sensor, spoiler position sensors, elevator position sensor and neutral shift sensor are dual (two synchros in each unit) and are all identical and interchangeable. The stabilizer position sensors are single synchros and are identical and interchangable. They are three-wire synchros with 14 volts ac 1800 Hz supplied to the excitation winding.
 11. Stabilizer Trim Servo_____________________
 A.  The stabilizer trim servo is located on the stabilizer jackscrew gearbox assembly in the tail cone. The trim servo consists of a nose-up relay, a nose-down relay, a speed change relay, a two-speed three-phase ac motor, a gearbox assembly with an electromagnetic clutch and an output shaft. Stabilizer trim control signals (trim up or trim down) are generated in the engaged A/P computer and are sent to the appropriate relay. When the autopilot is disengaged and flaps are not up, the speed trim system control signals generate stabilizer trim control signals. Actuation of a relay supplies 115-volt ac, three-phase power to drive the servomotor in the desired direction. An additional set of contacts of the relays complete 28 volts dc from the FCC to the electromagnetic clutch solenoid.
　
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