GASKET 3T1873 - Caterpillar

Engine Running, Transmission in Neutral
When the transmission is in neutral speed selector spool (1) and directional selector spool (5) are in the positions shown in the illustration.Oil from the pump enters the valve group and flows to No.4 clutch. The positions of the speed and directional spools open No.1, No.2, No.3, No.5 and No.6 clutches to sump. Pump oil also flows to modulation relief valve (7), torque converter inlet pressure valve (6), pressure differential/safety valve (4) and load piston (3). Oil flowing to the pressure differential/safety valve (4) passes through orifice (8) to load piston (3).With the transmission control in neutral, orifice (9) cannot allow oil to flow to sump because of the position of the speed selector spool. When the pressure differential/safety valve has moved a small amount, orifices (9) are blocked by the valve body and the left end of the spool starts to uncover a drain passage. This prevents the spool from moving any further. This is termed the SET position.Oil flowing to modulation relief valve (7) flows around the valve spool and through an orifice to act against the integral poppet valve. The poppet valve unseats allowing oil to fill the slug chamber in the spool. At the same time oil flows through an orifice to the right end of load piston (3). The position of pressure differential/safety valve (4) blocks the tank passage, allowing pressure to build up against the face of the load piston.When No.4 clutch is full of oil the speed clutch pressure starts to rise. The increasing pressure is transmitted to the slug chamber of modulation relief valve (7). This moves the modulation relief valve to the right, allowing excess flow to pass into the torque converter line. Increasing speed clutch pressure is also transmitted to the right end of load piston (3), which moves the load piston to the left. The orifice in the passage leading to the load piston causes a short delay between the pressure rise at modulation relief valve and at the load piston.
Engine Running, Transmission in Neutral
1. Speed Selector School 2. Valve Body 3. Load Piston 4. Pressure Differential/Safety Valve 5. Directional Selector Spool 6. Torque Converter Inlet Pressure Valve 7. Modulation Relief Valve 8. Orifice 9. OrificeAs the load piston moves to the left the resistance to motion of the modulation relief valve increases. This allows the pressure in No.4 clutch to increase further. This gradual increase in pressure is known as modulation and prevents shock loading of the transmission by allowing the clutches to pick up the load gradually. Eventually load piston (3) moves so far to the left that the chamber at the right end is opened to tank. Movement of the load piston then stops and the piston is held in a balanced position. The modulation relief valve also takes up a balance position with excess oil flowing to the torque converter circuit. At this point the speed clutch pressure has reached its correct value.Torque converter inlet pressure valve (6) controls