BLOCK 7D2844 - Caterpillar

Illustration 5 g00280019
Position of the Load Piston and of the Modulating Relief Valve during Dumping (Top Figure) and during the Start of Modulation (Bottom Figure)
(7) Check valve. (10) Modulating relief valve. (17) Load piston.
This movement of modulating relief valve (10) and load piston (17) causes the clutch pressure to gradually increase. This gradual increase in pressure is known as modulation.After load piston (17) has moved by two-thirds of the total movement, the load piston has moved enough to close passage (13). After passage (13) is closed, the P3 circuit is closed to the drain through load piston spring cavity (15). The movement of the load piston to the left stops when the load piston moves to the drain passage. The modulation now stops. As oil comes through screen orifice (26) to load piston cavity (25), oil exits through the drain passage. This maintains load piston (17) in position without any further movement. Load piston (17) and load piston springs (14) work with modulating relief valve (10) in order to control the pressure of P2 oil. Pressure in the system will be limited by the final spring force on the load piston. The modulating relief valve now meters the excess oil flow to the torque converter circuit.Transmission in the FOURTH SPEED FORWARD position with a Running Engine
Illustration 6 g00280027
The Transmission Control Valve is in the FOURTH SPEED FORWARD position. The Engine is Running.
(1) Selector spool and slug (FORWARD HIGH). (2) No. 1 solenoid (FORWARD HIGH). (10) Modulating relief valve. (17) Load piston. (25) Load piston cavity. (28) Selector spool and slug (THIRD SPEED). (29) No. 6 solenoid (THIRD SPEED). (AA) P1 Speed clutch pressure. (BB) P2 Direction clutch pressure. (CC) P3 Converter inlet pressure. (DD) Load piston pressure. (EE) Drain.
Note: In illustration 6, all of the passages with no pattern are open to the drain.When the transmission direction and speed control lever is in FOURTH SPEED FORWARD position, both No. 1 solenoid (2) and No. 6 solenoid (29) are engaged.Pressure oil through No. 6 solenoid (29) has moved selector spool (28) for the third speed clutch to the right. This allows P1 pressure oil to flow to the third speed clutch. The oil engages the third speed clutch.Pressure oil through No. 1 solenoid (2) has moved selector spool (1) for the forward high clutch to the right. This allows P2 pressure oil to flow to the forward high clutch.Modulation has occurred. (Modulation is described in the text for shifting from NEUTRAL position to FIRST SPEED FORWARD position.) Modulating relief valve (10) has moved to the left. The modulating relief valve is metering oil to the torque converter circuit. Load piston (17) has also moved to the left. The load piston is metering the pressure in load piston cavity (25) to the drain.P1 pressure and P2 pressure are stabilized and the transmission is in steady state operation.Transmission in the THIRD SPEED REVERSE position with a Running Engine
Illustration 7 g00280050
The Transmission Control Valve is in the THIRD SPEED REVERSE position. The Engine is Running.
(4)