1. The excavator engine shutdown motor has three functions
(1) When the key switch is turned ON, the engine stop motor will move the fuel cut-off lever to the open (start) position.
(2) When the key switch is turned OFF (disconnected), the engine stop motor will move the fuel cut-off lever to the off (fuel cut-off) position.
(3) If the engine oil pressure drops during operation, the oil pressure switch activates the emergency relay, and the emergency relay sends an electric signal to the stop motor after a delay of 3 seconds to order the engine to stop.
2. The key switch starts the engine
(1) When the key is turned to the START position (refer to the figure below), the battery supplies power to both terminals M and ST. Since the terminal M is connected to the engine stop relay signal circuit, the current energizes the engine stop relay. The current from terminal ST is fed to terminal C of the emergency relay, so that the timer can work for 10 seconds after the engine is started. If the engine oil pressure does not reach the normal level within 10 seconds, the timer activates the engine stop motor to stop the engine.
(2) Current flow sequence: battery-fuse-stop motor B-P2 -emergency relay P2', P2-engine stop relay 5~3-engine stop motor A-ground. This current flows through the stop motor, and the named excitation moves the fuel cut-off lever to the open (start) position, thereby starting the engine.
3. The key switch stops the engine
(1) Turn the key switch to OFF (open), that is, cut off the current to the engine stop relay. When the current of the key switch ST is off, the engine stop relay terminals 4 and 3 are connected (refer to the figure below).
(2) Battery-fuse-engine stop motor B-P1-engine stop relay 3, 4-engine stop Motor A-ground. Therefore, the engine stops and the motor rotates clockwise.
(3) When the engine stops and the rotation of the motor exceeds 180°, the fuel supply to the engine starts to be cut off. When the motor rotates to the 360° position, the fuel is completely cut off.
4. Emergency engine shutdown
(1) If some excavator maintenance failure occurs while the engine is running and the engine oil pressure drops, the engine oil pressure switch will respond (close). The emergency relay activates and connects P1 and P2 to open P2.
(2) Then the current flows in the following order: battery-fuse-engine stop motor B-P1-emergency relay Pl, P2- engine stop relay 5, 3-engine stop motor A-ground. This causes the engine motor to rotate clockwise.
(3) When the engine stops, the rotation of the motor exceeds 180. When the motor is turned to 360°, the fuel supply to the engine will be cut off.
