SYSTEM / Hydraulic System Emergency Boom Lower Circuit (Refer to the Others (Upperstructure) / COMPONENT OPERA- TION.) The accumulator is provided between pilot pump and pilot shut-off solenoid valve for boom lower at emergency. The accumulator accumulates pressure oil from the pilot pump. When the engine stops suddenly and the control lever is operated with the pilot shut-off lever in UNLOCK position, pressure oil from the accu- mulator flows to each spool in the control valve through each pilot valve. Therefore, as the spool in operated control valve is operated, the circuit between main pump and actuator is connected through the control valve spool.
As pressure oil between control valve and actua- tor flows to the hydraulic oil tank through the con- trol valve spool, pressure between control valve and actuator decreases. Consequently, when the engine stops suddenly, by operating the control lever (boom lower) right away, the front attachment can be lowered onto the ground (due to own weight). NOTE: Immediately after the engine stops (the key switch is turned OFF), the battery relay is kept ON for a specified time due to load damp relay operation.
(Refer to the Surge Voltage Prevention Circuit in Electrical System group / SYSTEM.) Therefore, elec- trical current from the battery (fuse #4) flow to the pilot shut-off solenoid valve and the pilot shut-off solenoid valve is kept ON. NOTE: Immediately after the engine stops, by op- erating all the control levers, any pressure in the circuit between control valve and actuator can be released. T2-4-12