Chapter 3. REPAIR INSTRUCTIONS

CHAPTER 3

REPAIR INSTRUCTIONS

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.

The power

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

motor.

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

contacts.

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

3-1