POWER STEERING CIRCUIT.

4-15.

PUMP MOTOR CIRCUIT.

with FW RES (a resistance) close, to connect the

resistance in parallel with the travel motor field. This

weakens the field, causing the motor to operate at a

4-16.

The hydraulic pump motor circuit is completed

higher speed. Thus, cruising travel motor speed is

when either the hoist or tilt control is operated from the

established.

neutral position.

This closes the pump switch to

energize the pump motor relay. The completed circuit is

4-11.

PLUGGING CONTROL Plugging is a term

through fuse 7FU, the closed pump switch, the pump

used to define the situation where the direction of

motor and hydraulic oil thermocouples to pump motor

motion of the truck is suddenly reversed. When this

relay IMP. When the relay energizes, its IMP contacts

occurs, the travel motor series field is reversed during

close to apply battery voltage across the pump motor

travel and the armature counter emf reverses polarity

through fuse 11IFU.

and tends to increase the effective system voltage by

4-17.

HORN CIRCUIT.

acting as an additional voltage source in series with the

battery. This would create high braking currents to

circulate around the armature field. Diode I REC,

4-18.

The horn circuit is energized by the operation

across the travel motor, clamps the armature voltage to

of the horn switch. When the switch is closed, battery

a very low value when travel direction is suddenly

voltage is applied across the horn through fuse 8FU to

changed to prevent this armature voltage buildup.

sound an audible alarm.

4-19.

LIGHT CIRCUIT.

4-12.

TURN-OFF PROTECTION.

When silicon

controlled rectifier ISCR is cut-off, a large amount of

energy remains stored in the inductances of the travel

4-20.

The light circuit consists of the spotlight and tile

motor armature and field. This energy, if left to

tail and stoplight. The spot and taillights arc energized

dissipate itself, would produce high voltage transients

simultaneously by the operation of the light switch.

which would appear across the cut-off SCR and lead to

When the switch is closed, battery voltage is applied

the possibility of shorting out the device. Flyback diode

through fuse 5FU directly across the spotlight and also

2REC prevents this by establishing a series path to

across series resistor 3RES and the tail and spotlight.

dissipate this energy through itself, the armature and the

Thus the tail and spotlight illuminates at a reduced

field.

brilliance. When the brake switch is closed, resistance

3RES is bypassed and full battery voltage is applied

4-13.

POWER STEERING CIRCUIT.

directly across the tail and stoplight for full brilliance.

4-21.

HOURMETER CIRCUIT.

4-14.

The power steering circuit is completed when

the master switch relay is energized and the directional

switch is placed either in the forward or reverse position.

4-22.

The hourmeter circuit consists of elapsed time

The completed circuit of the power steering relay IS is

indicator ETI, the two EHP diodes, capacitor IMF and

through fuse 6FU, the steering motor thermocouple, the

fuse 9FU. The hourmeter is placed in the circuit so that

FWD or REV 2 CIRC section of the directional switch,

whenever the travel motor or pump motor circuit is

the ARP pack to the IS relay coil. When the relay is

energized, elapsed time is recorded.

energized, its IS contacts close to apply battery voltage

across the power steering motor through fuse 10FU.

4-4