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).
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