VALVE GP 7T9388 - Caterpillar

Operation of the Modulation Relief Valve
Pump oil that flows to the pressure differential valve also flows through an orifice at the right end of load piston (4). The right end of the load piston is closed to the reservoir by the position of pressure differential valve (7). The flow of oil to load piston (4) is restricted by the orifice.When No. 3 clutch is full of oil, the pressure in the speed clutch circuit starts to increase. The increase is felt in the slug chamber at the left end of modulation relief valve (3) .When the pressure in the speed clutch circuit is at the initial setting of the modulation relief valve, the modulation relief valve moves to the right. The movement to the right allows extra oil to flow to the lubrication circuit.The pressure that is felt by the modulation relief valve is also felt on the right end of load piston (4). The orifice in the supply passage to the load piston causes a short delay in the pressure increase at the right end of the load piston.The pressure increase at the end of the load piston moves the piston to the left. The movement of the piston to the left increases the force of the springs. The increased force of the springs moves modulation relief valve (3) to the left also. The movement to the left causes a pressure increase in the slug chamber. The increased pressure moves the modulation relief valve back to the right.The clutch pressure increases gradually because of the left and right movement of the modulation relief valve, and the movement to the left of the load piston. The gradual increase in pressure is called modulation.The movement to the left of the load piston stops when the load piston moves to the drain passage. At this time, the modulation stops. As oil flows out of the drain passage, oil comes through the supply passage to the load piston chamber. This keeps the load piston in position without any further movement.Pressure in the system will be limited by the spring force on the modulation relief valve. At this time, the valve is opened in order to allow oil flow to the lubrication circuit.Operation of the Lube Relief Valve
Lube relief valve (6) controls the maximum oil pressure in order to lubricate the transmission. Preventing damage to the components in a cold engine start (cold oil) is the primary purpose of the lube relief valve.The pressure that is caused by the restrictions to oil flow is felt against the left end of the valve spool. The whole diameter of the valve spool is pushed by the pressure.Pressure from the speed clutch circuit is felt in the slug chamber of the valve spool. Only the diameter of the slug is pushed by the pressure.The pressure that is needed for moving the valve to the right is less than the pressure needed for moving the valve to the left. Pressure on the left end of the