Secondary Braking And Automatic Apply; Parking Brake Circuit; Normal Operation (Key Switch On, Engine Running) - Komatsu 830E-AC Shop Manual

SECONDARY BRAKING AND
AUTOMATIC APPLY
A fundamental function of the brake system is to pro-
vide reserve braking in the event of any single failure.
For this reason, the system is divided into multiple
circuits, each with its own isolation check valve,
accumulator, and circuit regulator. The secondary
system becomes whatever circuit(s) is operable after
a failure. If the failure is a jammed treadle valve, then
the brake lock becomes the secondary system, oth-
erwise, either of the two brake circuits would be the
secondary system.
The brake accumulators (1 & 3, Figure 2-1), (as
described under service brake circuit) perform two
functions; rapid flow for good response and store
energy for secondary braking. The circuit check
valve assures this energy is retained should a failure
occur in brake system supply or an accumulator cir-
cuit.
If a failure occurs in the pump, steering or either
brake accumulator circuit, a low brake pressure
warning light (on the instrument panel) and an audi-
ble alarm (in the cab) will actuate and the vehicle
should be stopped as soon as practical. When the
pressure in one accumulator circuit is less than the
preset level, all the service brakes will be automati-
cally applied. Automatic brake application is accom-
plished by the "Automatic Apply Valve" (PS1, 19),
located in the brake manifold. This valve senses the
lower brake accumulator pressure, and when the
pressure is less than 14 479 ± 520 kPa (2100 ± 75
psi), the valve shifts, operating the brake treadle
valve and applying all the brakes full on.
Regardless of the nature of location of a failure,
sensing the lowest brake accumulator circuit pres-
sure assures two to four full brake applications after
the low brake warning light and buzzer, and before
automatic apply. This allows the operator the oppor-
tunity to safely stop the truck after the warning has
turned on.

PARKING BRAKE CIRCUIT

The parking brake is spring applied and hydraulically
released.
NOTE: Whenever the park brake solenoid is de-
energized, a spring in the solenoid valve will shift the
spool to the position to allow the park brake to be
applied.
J02037
Normal Operation (key switch on, engine run-
ning)
• Directional Control Lever in Park Position
The parking brake solenoid (16) is de-energized.
The oil pressure in the parking brake lines return
to tank and the springs in the parking brake will
apply the brake. The parking brake pressure
switch (23) will close at 8618 Kpa (1250 psi),
completing a path to ground, and illuminating the
parking brake light on the instrument panel.
• Directional Control Lever Not in Park Position
The parking brake solenoid is energized. The
pressure oil is routed from the park brake
solenoid, to the parking brake pressure regulator
(4) (reducer), then to the park brake pistons for
release. The parking brake circuit is protected
against accidental apply by monitoring a wheel
motor speed sensor. The park brake with not
apply until the truck is virtually stopped. This
eliminates brake pad damage.
• If the key switch is turned OFF (directional
control lever in PARK), the park brake will not
apply until vehicle speed is less than 1/3 MPH,
due to the monitoring of the wheel motor speed.
• If loss of hydraulic supply pressure occurs, with
the directional control lever not in PARK, the
parking brake solenoid will still be energized. The
hydraulic supply circuit is still open to the parking
brake pistons. A check valve in the park brake
hydraulic supply circuit traps the oil, holding the
parking brake in the release position.
NOTE: Normal internal leakage in the parking brake
solenoid and the pressure reducing valve may allow
leakage of the trapped oil to return back to tank, and
eventually allow park brake application.
• If 24 volt power to the solenoid is interrupted, the
park brake will apply at any vehicle speed. The
spring in the solenoid will cause it to shift,
opening a path for the oil pressure in the park
brake line to return to tank and the springs in the
parking brake will apply the brake. The parking
brake pressure switch (23) will close, completing
a path to ground, and illuminating the parking
brake light on the instrument panel.
Brake Circuit J2-5