jcb JS300 Service Manual page 694

Boom Up
K Fig 6. ( T
For schematic,
Servo pressure from the hand controller 48 enters the
servo shuttle valve 46 at port A3 and is distributed to:
1 Boom up pressure switch 45 at port S2
2 Port B3 to the cushion control valve 49 at port F and
exits at port H, to the main control valve 14 at port Pa8
to select both boom (1), boom (2) spools and upper
pilot pressure switch 52.
3 Port C1 to port Pi2 on the main control 14 valve to
operate boom over bucket priority spool valve BBV.
Flow from pump 2 is stopped by the Boom (1) spool and is
diverted via the parallel working passage of the boom (1)
spool. The oil passes through the spool and merges with
the flow from pump 1. Flow from pump 1 in the neutral
gallery is stopped by the Boom (2) spool and is diverted via
the parallel working passage of the boom (2) spool. The oil
passes through the spool and merges with the flow from
pump 2.
The pressure lifts the boom holding valve BHV off it's seat
allowing oil to leave the main control valve at port A8 to
enter the hose burst protection valve (HBPV) 12 (if fitted)
and the boom rams 16.
Exhaust oil from boom rams 16 enters the main control
valve 14 at port B8 and crosses the boom (1) spool to tank.
Pressure is lost on both negative control ports Ps1 and
Ps2, which is sensed at the pumps, which comes on full
flow.
E-33
E-34).
Hydraulic Operation and Schematics
9813/5100-1
Section E - Hydraulics
Hydraulic System
E-33