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757 MAINTENANCE MANUAL
 (b)  
AIRPLANES WITHOUT ETOPS; All air/ground relays operate on 28v dc through the respective landing gear AIR/GND SYS 1, POS SYS 1, AIR/GND SYS 2, or POS SYS 2 circuit breakers on overhead circuit breaker panel P11.

 (c)  
All relays energize either with the airplane in air or ground mode. The relays energize when the PSEU converts the landing gear proximity sensor input signals into ground output signals to the relays.

 (2)  
Logic

 (a)  
When the target of the sensor is in proximity, a logic 0 is at the PSEU prox card output. When the target of the sensor is away from proximity, a logic 1 is at the prox card output. The output of prox card is applied to the PSEU logic card which, thru its logic circuits, provide a logic 1 or logic 0 to the drivers of the PSEU driver card. All drivers require a logic 1 input to provide a ground output signal to the relays.

 (3)  
EICAS

 (a)  
The NOSE A/G DISAGREE message on the EICAS display indicates system 1 and 2 air/ground signal output disagreement. The disagreement may be caused by malfunction of sensor (nose gear compressed), PSEU or associated wiring in the system. If the air/ground relay itself is faulty, the above message will not be shown on EICAS display since EICAS inputs for this message come directly from the PSEU .

 (b)  
The AIR/GND DISAGREE message on the EICAS display indicates system 1 and 2 air/ground relay disagreement. The disagreement may be caused by malfunction of sensor (main gear truck tilted), PSEU, associated wiring in the system or a faulty relay. EICAS computer receives a ground signal from one or more air/ground relays rather than the PSEU, therefore a faulty relay may cause EICAS input disagreement and subsequent message display.

 B.
Control

 (1)  
The air/ground relay operates automatically whenever it is
 energized. It has no operating controls or adjustments.

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757 MAINTENANCE MANUAL
 AIR/GROUND RELAY SYSTEM - MAINTENANCE PRACTICES
_______________________________________________
 1.  General
_______
 A.  This procedure contains the data necessary to do seven tasks. The tasks are:
 (1)  Prepare the safety systems for air mode simulation.
 NOTE: This task is also referred to as the "Flight Mode
____ Simulation Deactivation" procedure.
 (2)  
Put the air/ground relay system in the air mode.

 (3)  
Put the air/ground relay system in the ground mode.

 (4)  
Put the safety systems back to their initial conditions.

 (5)  
The removal of the air/ground relays.

 (6)  
The installation of the air/ground relays.

 (7)  
Do the test for the air/ground relays.

 B.  
Use tasks 1 through 4 to put the airplane in the air mode (for systems that get an air mode signal from the air/ground relay system).

 (1)  
The first task (Prepare safety-sensitive systems for air mode simulation) is done to prevent the operation of some systems. You can cause damage to these systems or cause injury when an air mode signal is supplied to these systems.

 (2)  
The second task (Put the air/ground relay system in the air mode) is to supply an air mode signal to the systems on the airplane.

 (3)  
The third task (Put the air/ground relay system to the ground mode) removes the air mode signal from the systems on the airplane.
　
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