Section I. ELECTRICAL SYSTEM
stages within the control unit.
3-1. Description of Pulse Width Modulation
d. Hydraulic Pump Motor. The hydraulic pump
a. Pulse width modulation is a method of providing
motor is kept off except when one of the three hydraulic
stepless acceleration through control of drive motor
control levers (lift, tilt, or side shift) is operated.
power. This control is achieved by supplying the motor
Operation of a lever closes the associated microswitch
with pulses of power. Supplying power in pulses
contacts, thereby energizing the hydraulic pump (P)
provides controlled regulation of the power flow from the
contactor. This closes the P contacts to energize the
battery. The regulation of power flow from the battery
hydraulic pump motor. Thermal relays are mounted on
permits the system to operate at peak efficiency (near
the hydraulic pump motor and reservoir. Should the
100 %). The power flow is controlled by a smoothly
system overheat, the circuit to the P contactor is opened
variable speed control.
to de-energize the hydraulic pump motor.
b. The lift truck is powered by a direct current motor
e. Power Steering Pump Motor.
which is driven by pulses occuring at a 120 per second
steering pump contactor (PS is energized whenever
rate. The regularity of the pulses, their fast rate, and the
the seat is occupied and the key switch is turned on.
inertia of the motor provide smooth operation.
This closes the PS contacts, causing the power steering
3-2. Schematic Diagram Description (fig. 3-1.)
pump motor to operate. A thermal relay, mounted on the
a. General. Battery power is continually connected
motor, opens if the motor overheats. This de-energizes
to the horn and lights circuits and to the seat switch
the PS contactor and thereby interrupts power to the
contacts through the emergency cutout contactor. If the
seat is occupied and emergency stop switch released,
f. Direction Control.
the switch contacts are closed and the emergency cutout
(1) With either the forward (F) or reverse (R)
(EC) contactor is energized. The EC contacts close and
direction selected with the directional control switch, a
apply power through the 400 ampere and 35 ampere
circuit is completed to energize the control relay (CR).
fuses, enabling operation of the truck.
Assuming forward operation, the circuit is through the
closed contacts of the speed interlock switch, the
Figure 3-1 Electrical system schematic diagram.
normally closed CR contacts, the CR coil, a diode of the
(Located in back of manuals)
bridge block, and the directional control switch. The
switch terminates at terminal 35 of the protective circuit,
b. Emergency Cutout. Should an emergency arise
which provides a voltage close to battery positive as long
where it is necessary to cut power, the emergency stop
as the circuit does not detect a failure.
pushbutton on the contactor unit cover is depressed.
(2) When the accelerator pedal is depressed,
This de-energizes the EC contactor, thus opening the EC
the speed interlock switch contacts are allowed to return
This removes power from all but the
to normal. The F contactor is energized through terminal
emergency cutout circuit.
12, a pair of now-closed CR contacts, the normally
c. Operating Power. When the key is inserted and
closed speed interlock switch contacts, the brake
turned in the control panel, the key switch contacts close
interlock switch and the directional control switch. This
to apply operating power to several circuits, as follows:
operates one pair of F contacts at the drive motor field
(1) Hydraulic pump (P) contactor.
winding. The winding is thus connected to carry current
(2) Power steering pump contactor (PS).
in the forward direction. The brake interlock switch
(3) Control relay (CR), forward (F) contactor,
opens when the brake is depressed. This prevents
and reverse (R) contactor.
application of drive power and braking at the same time.
(4) Power on lamp.
g. Speed Interlock. The speed interlock switch
(5) Protective circuit and control unit. The
prevents the truck from being started at a high motor
resistor assembly provides load resistances for transistor