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BRAKE LOCK CIRCUIT
The primary function of the brake lock is to provide a
means for the operator to hold the vehicle while at the
shovel or dump. The brake lock only applies the rear
service brakes. It may also provide a second means to
stop the truck in the event the primary means (brake
valve) malfunctions.
WARNING CIRCUIT
The brake warning circuit is equipped with a low brake
pressure warning light (on the overhead display panel)
and an audible alarm (in the cab) to alert the operator
of low brake pressures. Several electrical sensors, a
relay and delay timer are used to detect brake system
problems. (See Figure 2-1 for the following component
references.)
• Pressure sensor, system supply pressure
Located on the pump pressure sensing manifold.
When system supply pressure drops below 2300
psi (15.8 MPa), the low steering pressure light, low
brake pressure light and buzzer will turn on.
• Pressure sensor, low brake pressure (20)
Located on the brake manifold. When the accu-
mulator with the lower pressure falls below 1850
psi (12.7 MPa), the low brake pressure light and
buzzer will turn on.
• Differential pressure switches (8)
Located on the dual controller valve (foot treadle)
and on each dual relay valve. During brake appli-
cation, if the difference in brake apply pressure
between the front and rear circuits is greater than
a preset level, the differential pressure switch lo-
cated on the dual controller valve will turn on the
low brake pressure light and buzzer.
If the difference in brake apply pressure between
the left and right circuits of the front or rear wheel
brakes is greater than a preset level, the differen-
tial pressure switch located on the dual relay valve
at either the front or rear wheels will turn on the
low brake pressure light and buzzer.
The pressure differential switches complete a
path to ground in order to turn on the low brake
pressure light and buzzer. The differential pres-
sure switches provide detection of faults such as
a brake line rupture, poor brake valve tracking,
line blockage, excessive brake displacement or
air trapped in the system.
J2-4
By turning on the dash mounted toggle switch, a sole-
noid valve (14, Figure 2-1) and pressure reducing valve
(13) will apply unmodulated pressure oil at 2000 psi
(13.8 MPa) to fully actuate the rear brakes. A shuttle
valve (11) in the rear brake line provides the inde-
pendence from the brake treadle valve for brake appli-
cation.
• Brake Lock Degradation Switch (23)
Located on the junction block (26) in the hydraulic
components cabinet. When the brake lock switch
is turned on, the brake lock solenoid (14) and
brake warning relay are energized. The brake
warning relay switches the electrical connection
from the differential pressure switch to the brake
lock degradation switch (23). If the brake lock
apply pressure is less than 1000 psi (6.9 MPa), a
path to ground will be completed and the low
brake pressure light and buzzer will turn on.
• Brake Warning Relay (28)
Located on the RH wall of the hydraulic compo-
nents cabinet. When the brake lock switch is
turned on, the brake warning relay is energized
and switches the electrical connection from any
of the differential pressure switches to the low
brake lock degradation switch. When the brake
lock switch is turned off, the relay is de-energized
and switches the connection from the brake lock
degradation switch to the differential pressure
switches.
• Brake Warning Delay Timer (27)
Located on the RH wall of the hydraulic compo-
nents cabinet. The delay timer is connected in
series between the low brake pressure light/buzz-
er and the brake warning relay. If any of the
differential pressure switches or the brake lock
degradation switch completes a path to ground,
the delay timer will not complete the circuit for 1.2
seconds. This will allow sufficient time for the
hydraulic brakes to reach the proper pressures
after actuation to avoid false warnings.
Brake Circuit
J02027
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Troubleshooting
