TM 5-3805-294-23-1
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ELECTRICAL SYSTEM - Continued
Starting Circuit
The starting circuit consists of the batteries, the key switch, the starter motor, the starter relay, the starter relay diode,
the security diode, the pilot shutoff switches, and the pilot shutoff diode.
When the key switch is moved to the START position, power is applied from the key switch to the starter relay and
the battery relay. The energized battery relay supplies power to the starter motor. The de-energized starter relay
terminal supplies power to a starter motor terminal, cranking the engine.
HYDRAULIC SYSTEM
Hydraulic Cooler and Fan Operation
Return oil from motors and cylinders is routed into return passages in the control valve by valve spools. From return
passages, return oil flows out of the control valve, through hydraulic oil cooler, and then to the hydraulic reservoir.
The oil cooler bypass valve opens to route return oil around the oil cooler and directly to the hydraulic reservoir when
resistance to flow through the oil cooler becomes high because oil is cold (high viscosity), there is surge of return
oil, or oil cooler becomes plugged. Back pressure in the return passage provides makeup oil to the swing motor.
A hydraulic-powered cooling fan is used to provide air flow for the HYEX cooling package. The fan drive system
consists of a gear type pump, fan speed control valve and fan drive motor.
The fan drive pump is directly driven off the front gear train of the engine. Oil flow to the pump comes from the
hydraulic reservoir. Return oil from the fan drive motor is routed through the hydraulic oil cooler to the return filter in
the hydraulic reservoir.
The fan speed solenoid valve regulates oil flow and pressure buildup in the fan drive system. The more oil flow to
the fan drive motor, the greater the fan speed. How much oil flow is routed to the fan drive motor depends on the
current applied to the fan speed solenoid valve from the ECU. The reversing function reverses the cooling fan to
help clean the machine cooling package of dirt and debris. The reversing function can be initiated manually by the
operator or automatically by the ECU. When the circuit is initiated, the reversing control valve stops the fan and
reverses the motor at full power for 15 seconds.
The ECU uses four parameters to control the current sent to the proportional fan speed solenoid valve. The engine
coolant temperature sensor, the hydraulic oil temperature sensor, the turbocharger compressor inlet temperature
sensor, and the ambient air temperature sensor send system temperatures to the ECU.
When HYEX systems are cold, the ECU sends maximum current to the fan speed solenoid valve. This shifts the
solenoid valve to allow most oil flow to be returned to the hydraulic reservoir and bypass the fan drive motor. The
fan drive motor and cooling fan operate at a low speed.
As the HYEX warms up during operation, the ECU decreases the current to the fan speed solenoid valve. The
decrease in current applied causes the solenoid valve to shift and restrict oil flow through the fan speed solenoid
valve, increasing oil flow to the fan drive motor. As oil flow to the fan drive motor increases, cooling fan speed
increases.
When maximum cooling is required, the ECU supplies no current to the fan speed solenoid valve. With the solenoid
valve de-energized, the fan speed solenoid valve is fully shifted to allow maximum oil flow to the fan drive motor.
The fan drive motor and cooling fan operate at maximum speed.
Pumps
The main hydraulic pumps supply hydraulic oil for all functions including boom, arm, bucket, thumb/attachment,
swing, and travel. The main pump housing contains a pilot pump, pump 1 and pump 2. Pump 1 is on the right side
as you are looking at the pump compartment (toward the front of the HYEX) and pump 2 is on the left side (toward
the rear of the HYEX). The engine drives the pumps through a coupler and a gearbox.
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