TM 5-3805-258-24-1S Y S T E M S O P E R A T I O NPOWER TRAINTRANSMISSION HYDRAULIC CONTROLSWhen the transmission selector lever is in NEU-TRAL, direction selection spool (17) closes the pas-sage between the larger orifice in valve spool (14)and the reservoir. This lets the pressure, in thechamber at the left end of valve spool (14), increase.The increase in pressure moves the valve to the rightagainst the force of the springs. The movement of thevalve spool closes a passage from the area behind theright end of load piston (11) and the reservoir. At thistime, pressure differential valve (14) is in the posi-tion shown in the schematic. This lets the pressure inthe system increase.As the pressure in the chamber at the left end ofpressure differential valve (14) increases, the valvemoves more to the right. This opens the directionclutch circuit to pump oil. It also closes the right endof valve (14) to the reservoir. The pressure in thedirection clutch circuit increases. The increase is feltin the spring chamber of valve (14). When the pres-sure in the direction clutch circuit is at its maximum,the pressure in the spring chambers, plus the force ofthe springs, moves valve (14) to the left. The valvemoves to the left until the flow of pump oil to thedirection clutch circuit is stopped. At this time, themovement of the valve spool stops. Now the valvespool moves to the right and then to the left, (meters)to keep a constant pressure in the direction clutchcircuit.Oil, from the pump, also goes to modulation reliefvalve (13). It fills the chamber around the modula-tion relief valve. The oil goes through an orifice in thevalve spool and opens the poppet valve at the left endof the valve spool. This lets oil fill the slug chamber atthe left end of the valve spool.When the No. 3 clutch is full of oil, the pressure inthe speed clutch circuit starts to increase. The in-crease is felt in the slug chamber at the left end ofmodulation relief valve (13). When the pressure inthe speed clutch circuit is at the initial setting of themodulation relief valve, the modulation relief valvemoves to the right. This lets extra oil go to the torqueconverter.At the same time, pump oil also goes through anorifice to the area between the right end of loadpiston (11) and the cover on the selector and pressurecontrol valve body. This area is closed to the reservoirby the position of differential valve (14). The amountof flow to the area behind load piston (11) is restrict-ed by the orifice. The pressure felt by the modulationrelief valve, because of the increase in pressure in thespeed clutch circuit, is also felt behind load piston(11). The orifice in the supply passage causes the oilto go to the area behind the load piston at a specificrate. As the modulation relief valve moves to theright, the load piston moves to the left. This causesthe pressure in No. 3 clutch (19) to increasegradually.This gradual increase in pressure is known asmodulation. The load piston moves more to the left,against the force of its springs, until the area behindthe load piston is open to a drain passage. At thistime, modulation stops. As the oil goes out the drainpassage, oil continues to fill the area behind the loadpiston. This keeps the load piston in a position with-out any further movement.The operation of the load piston and the modula-tion relief valve keeps the system pressure at a con-stant rate.3-44
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