TM    5-3805-258-24-1 H  Y  D  R  A  U  L  I  C   S  Y  S  T  E M S  Y  S  T  E  M  S       O  P  E  R  A  T  I  O N When  attachment  valve  spool  (3)  is  in  HOLD position, implement oil goes from inlet passage (7), through passage (12), around tilt valve spool (13) and lift valve spool (14) and through outlet passage ( 15) to the hydraulic tank. The position of the spool holds (blocks) the oil in both ends of the attachment cylinders (or  cylinder) and keeps the cylinders from movement. OPEN or RAISE Operetion When  the  attachment  (third)  control  lever  is moved to OPEN or RAISE position, pilot oil is sent to chamber (1) and attachment valve spool (3) moves to the right. Implement oil is sent from inlet passage (7) to load check valve (6). The load check valve prevents  reverse  oil  flow  which  can  cause  cylinder drift or load loss. Load check valve (6) opens when inlet  pressure  becomes  greater  than  the  force  of spring (5) and the rod end pressure of the attachment cylinders. The oil goes through passage (4) and out passage (11) to the rod end of the attachment cy- linders. This causes the cylinder rods to retract. Oil from the head end of the attachment cylinders comes  through  passage  (10),  around  attachment valve spool (3), through passage (9) and outlet pas- sage (15) to the hydraulic tank. NOTE: When attachment valve spool (3) is fully in OPEN or RAISE position, no implement oil goes to the tilt and lift valve spools. The tilt and lift circuits will not operate. CLOSE or LOWER Operation When the attachment control lever  is moved to CLOSE  or  LOWER  position,  pilot  oil  goes  to chamber (8) and attachment valve spool (3) moves to the left. Oil flow in CLOSE or LOWER position is the reverse of oil flow in OPEN or RAISE position. Implement oil is sent from inlet passage (7) to load check  valve  (6).  The  load  check  valve  opens  when inlet  pressure  becomes  greater  than  the  force  of spring (5) and the head end pressure of the attach- ment cylinders. The oil now goes through passage (4) and out passage (10) to the head end of the attach- ment  cylinders.  This  causes  the  cylinder  rods  to extend. Oil from the rod end of the attachment cylinders comes  through  passage  (11),  around  attachment valve spool (3) through passage (12) and outlet pas- sage (15) to the hydraulic tank. NOTE: When attachment valve spool (3) is fully in CLOSE or LOWER position, pressure oil from the rod end of attachment cylinders is available to the tilt and lift valve spools. The tilt or lift circuits will operate but more slowly than normal. Load Check Valve The operation of load check valve (6) is the same as the load check valves for the lift and tilt circuits. The load check valve prevents reverse oil flow in the attachment  cylinders  which  can  cause  cylinder  drift. RELIEF  VALVES In an implement hydraulic system with a lift and tilt control valve, there are three relief valves: a main relief valve and relief valves for both rod and head ends of the tilt cylinder. In an implement hy- draulic  system  with  an  attachment  (three  spool) control  valve  there  are  two  additional  relief  valves for both rod and head ends of the attachment cyl- inders.  The  main  relief  valve  prevents  too  high  oil pressure in the implement pump section oil system. The  pilot  section  relief  valve  controls  the  pressure in the pilot oil system. This relief valve is in the hydraulic pump. The rest of the relief valves are fastened  directly  to  the  control  valve. Main Relief Valve The main relief valve is in front of all other valves in   the   implement   hydraulic   system.   It   is   a   pilot- operated,  cartridge-type  valve.  The  pressure  setting of the main relief valve is approximately 19000 kPa (2750 psi). M A I N     R E L I E F     V A L V E 1.    Oil    inlet    opening.    2.    Valve.    3.    Pilot    valve.    4.    Spacer.    5. P l u n g e r .    6 .    P l u g .    7 .    O i l    o u t l e t    o p e n i n g .    8 .    P a s s a g e .    9 . S p r i n g . Implement oil is in inlet opening (1), through the orifice in center of valve (2) and into the chamber for spring (9). The oil in the spring chamber is against pilot valve (3) that has the force of a spring to keep it closed when the pressure of the oil in the circuit is less than the setting of the relief valve. 3-108