THEORY OF OPERATION - CONTINUED
Braking and hydraulic fan system contains a fixed displacement hydraulic pump. Braking system has priority over
hydraulic fan system.
When engine is started, hydraulic oil is drawn from tank by braking and hydraulic fan pump. Flow of hydraulic oil
into hydraulic fan system is blocked by brakes flow control valve until accumulators are fully charged. Hydraulic
oil does flow from braking and hydraulic fan pump, through filter to orifice, and through check valve. Oil then
flows through shuttle valve to accumulators to charge accumulators. Shuttle valve will direct flow to accumulator
with lowest pressure. This allows both accumulators to charge to equal pressures.
Brakes flow control valve is controlled by a brake charging solenoid valve which is controlled by machine ECM.
Brake charging solenoid valve controls pressure drop across each orifice. Control of brake charging solenoid valve
is based on pressure detected by brake charging pressure sensor. When pressure in accumulators rises above cutout
pressure, brake charging pressure sensor will detect rise. Machine ECM will send a signal to brake charging sole-
noid valve to shift brake charging solenoid valve more open. When brake charging solenoid valve shifts more
open, pilot oil that acts on brakes flow control valve is vented to tank. Pressure drop across orifice will increase.
This pressure drop will cause brakes flow control valve to shift more open. When brakes flow control valve shifts
more open, oil flow from braking and hydraulic fan pump will be directed to fan motor. Check valve will stop oil
flow to accumulators.
Hydraulic oil flow from brake accumulators is blocked at brake control valve. When brake pedal is pressed, brake
control valve routes hydraulic oil from accumulators to rear and front service brakes. This engages service brakes.
Pressure feedback is sensed by each spool in brake control valve. This allows operator to feel application of service
brakes. Due to spring below bottom brake valve, maximum pressure to front brakes is lower than maximum pres-
sure to rear brakes.
Brake accumulators discharge partially when brakes are applied. When accumulator charge pressure falls below
cut-in pressure, machine ECM will send a signal to brake charging solenoid valve to close. When brake charging
solenoid valve is closed, pressure drop across orifice will decrease, causing brake flow control valve to close. This
will allow more oil to flow from pump to accumulators. Oil flow will open check valve. When check valve opens,
oil will flow to accumulators.