LEVER LOCK 
LEVER  SOLENOID 
LOCK 
FWD 
A 
SWITCH  POSITION 
ACTUATOR  SWITCH 
CONTROL 
LEVER 

SWITCH MOUNTING BOLTS
FWD GUIDE
B
Control Lever Position Swich
Figure 4

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DOWNLOCK SENSOR ACTUATOR
LOCK
STRUT
SEE B
SEE A
MOUNTING BRACKET
FWD INBD DOWNLOCK SENSOR
FLEXIBLE CONDUIT 
UPLOCK HOOK 
UPLOCK SENSOR 
UPLOCK SENSOR ACTUATOR 
FWD  OUTBD  B 
Figure 5 Main Gear Sensors 

SENSOR LEADS
SIDE STRUT
INBD
A
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A
737-300/400/500MAINTENANCE MANUAL
 D.  The nose gear down sensor is located on a bracket mounted on the side of the nose wheel well above the left nose gear trunion pin. The actuator is mounted on the trunnion assembly so that the rotation of the tunnion to the down position brings the actuator into proximity with the sensor causing it to conduct. This, in conjunction with the nose gear lock sensor, provides a path to ground for the nose gear green light indicating that the nose gear is in the down and locked position.
 5.  Operation_________
 A.  Control of the indicator and warning lights with respect to landing gear and control lever position is accomplished by solid-state circuits in the landing gear electrical module (Fig. 7). With the landing gear down and locked, the green indicator lights will come on. When the control lever is placed in UP position, the landing gear will unlock in preparation to retract and the green indicator lights will go off while the red warning lights will come on. The red warning lights will remain on while the gear are in transit until the gear position agrees with the control lever position (gear up and locked) at which time the lights will go off. With all landing gear up and locked, all warning lights will be off.
 B.  With the landing gear up and locked, the indicator and warning lights will be off if the landing gear control lever is in UP or OFF position. When the control lever is placed in DN position, the red warning lights will come on and remain on while the gear unlock and extend to down and locked. As the gear lock in down position, the red warning lights will go off and the green indicator lights will come on and remain on until power is removed from the circuit.
 C.  If the flaps are extended beyond the minimum approach and climb configuration before the landing gear are down and locked, the flap landing warning switch will close to complete the circuit through a solid-state circuit in the landing gear electrical module to sound the warning horn. When the airplane configuration is changed either by extending the gear or retracting the flaps, a circuit is opened through the solid-state circuit to silence the warning horn.
 D.  A steady warning horn is provided to alert the flight crew whenever the airplane is in a landing configuration and any gear is not down and locked. The landing gear warning horn is activated by forward thrust lever and flap position as follows:
 (1)  
Flaps 1 through 10:

 (a)  
Either or both forward thrust levers between idle and approximately 10 degrees thrust lever angle.

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