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| Illustration 1 | g00278687 |
Dump Valve (Normal Operation) (1) Passage. (2) Slug cavity. (3) Spool. (4) Slug. (5) Spool. (6) Passage. (7) Drain passage. (8) Passage. (9) Orifice. (10) Spring. (11) Spring. (12) Piston. | |
The dump valve assists in pressure reduction within the load piston cavity. Pressure is reduced within the load piston cavity in order to allow the load piston to fully reset. This ensures both full modulation of the clutch pressure and an easy clutch engagement.
When the transmission is in gear, directional clutch pressure P2 from the transmission hydraulic control enters the dump valve through passage (6). This pressure moves spool (5) downward against spring (10). Load piston pressure enters the dump valve through passage (1). Speed clutch pressure P1 from the transmission hydraulic control which is flowing through passage (8) is blocked by spool (5).
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| Illustration 2 | g00278689 |
Dump Valve (Shifting) (1) Passage. (2) Slug Cavity. (3) Spool. (4) Slug. (5) Spool. (6) Passage. (7) Drain passage. (8) Passage. (9) Orifice. (10) Spring. (11) Spring. (12) Piston. | |
When a shift of the transmission is made, directional clutch pressure P2 drops. When directional clutch pressure drops below 1030 kPa (150 psi), spring (10) moves spool (5) upward against the reduced P2 pressure. This opens slug cavity (2) to P1 pressure through passage (8) .
P1 pressure in slug cavity (2) moves spool (3) and piston (12) downward against spring (11). The load piston pressure is opened in passage (1) in order to drain through passage (7). This causes P2 pressure to decrease to a maximum initial pressure of 310 kPa (45 psi).
The pressure of the P1 circuit that is flowing through orifice (9) causes spool (3) to move upward. This blocks the flow of load piston pressure from passage (1) to drain passage (7). Orifice (9) is sized in order to provide a delay in closing the load piston pressure to the drain. When the load piston pressure is blocked from the drain, the function of the modulating relief valve can be performed. When P2 pressure increases to 1030 kPa (150 psi), spool (5) moves downward and the P1 pressure from slug cavity (2) is blocked. The slug cavity is opened to the drain. Spring (11) then pushes piston (12) upward until contact is made with spool (3). This returns spool (3) to the reset position for the next shift.