1.18.5. Engine Controls and Indicators Circuit.

TM 5-3895-373-20 1.18.5. Engine Controls and Indicators Circuit. The engine controls consist of the high temperature shutdown circuit and high temperature manual override button. The indicator circuits service all the individual gauges on the operator gauge panel. The engine can be stopped in an emergency using any of the emergency stop switches on the operator switch panel or either screed control panel. High engine temperature can also shut down the engine when the temperature exceeds the normal operating range. The engine starting circuit supplies 12 VDC to the high temperature shutdown sensor. When engine temperature is less than 220°F (105°C), the 12 VDC is passed through the temperature shutdown sensor and DCA engine test switch to energize the fuel shutoff solenoid. When the coolant temperature reaches 214°F (101°C), the high temperature shutdown sensor engages, illuminating the high temperature warning light on the operator gauge panel. When the coolant temperature reaches 220°F (105°C), the high temperature shutdown sensor switch will open, removing the 12 VDC to the engine fuel shutoff solenoid and shutting down the engine. By pressing and holding the high temperature manual override button on the operator gauge panel, 12 VDC from CBI is routed to the engine fuel shutoff solenoid. This will allow the engine to start and operate as long as the override button is held. All engine instruments are located on the operator gauge panel. The voltage for all the instruments is supplied from the "ON" position of the ignition switch. Each gauge, with the exception of the voltmeter, is controlled by a separate transmitter. The voltmeter uses the power directly from the ignition switch. Each individual instrument circuit is discussed in the following paragraphs. The fuel gauge receives a signal from the transmitter located in the fuel tank. A floating arm is attached to a variable resistor. As the fuel goes down, the arm drops toward the bottom of the tank. This drop causes an increase in the resistance of the variable resistor. The increase is sensed by the fuel gauge, setting the position of the fuel gauge needle. The fuel gauge reads from 0 to I (indicating full). The voltmeter uses the 12 VDC from the ignition switch as input. The voltage is applied to a square wave oscillator. The amplitude of the square wave is measured and used to position the needle within the voltage gauge. The gauge reads from 8 to 16 volts. The hourmeter and the tachometer receive a common AC signal input from the alternator. The hourmeter uses the input to power a motor that is gear-coupled to a counter. The readings are in hours and tenths of an hour. The tachometer input is applied to a counter that counts the number of alternating cycles of AC inputs. The number is used to position the dial on the tachometer. The tachometer indicates from 0 to 4000. The coolant temperature and hydraulic oil temperature sensors are moving coil-type transmitters. As the temperature increases, a metal cylinder expands. This expansion is detected by a coil surrounding the metal cylinder and is sent to the gauge as a change in impedance. The change is measured in the gauge and displayed as a needle movement. Both temperature gauges read from 100° to 250°F (38° to 121°C). The engine oil pressure transmitter is a diaphragm-type sensor. As the pressure changes, the position of the diaphragm changes. A change in the diaphragm position is sensed by a coil that surrounds the diaphragm. The output of the coil is transmitted to the gauge and converted into a needle movement. The engine oil pressure gauge reads from 0 to 150 psi (0 to 1034 kPa). 1-28