TM 5-3805-258-24-1POWER TRAINTRANSMISSION HYDRAULIC CONTROLSS Y S T E M S O P E R A T I O NWhen the No. 6 clutch is full of oil, the pressure inthe speed clutch circuit starts to increase. The in-crease is felt in the slug chamber of modulation reliefvalve (13) and the chamber at the left end of pressuredifferential valve (14).The oil to pressure differential valve (14) starts tofill the chamber at the left end of the valve spool,through the small orifice.When the pressure in the No. 6 clutch is approxi-mately 380 kPa (55 psi), pressure differential valve(14) starts to move to the right. The movement of thevalve spool opens No. 2 clutch (2) to pump oil. It alsocloses a passage from the chamber behind load piston(11) to the reservoir.When the No. 2 clutch is full of oil, the pressure inthe direction clutch circuit increases. The increase isfelt in the spring chamber of pressure differentialvalve (14). The pressure in the spring chamber andthe force of the springs moves the valve to the leftagainst the speed clutch pressure at the left end of thevalve spool.As the pressure in the speed clutch circuit in-creases, the pressure in the chamber at the left end ofpressure differential valve (14) increases.The increase in pressure moves the valve spool tothe right against the force of the springs. This opensNo. 2 clutch (20) to pump oil.As the pressure in the No. 2 clutch increases, thepressure in the spring chamber of valve (14) in-creases. The increased pressure in the springchamber and the force of the springs, moves the valvespool to the left.This stops the flow of pump oil to the No. 2 clutch.This function continues until the pressure in the No.2 clutch is at its maximum. At this time, the pressurein the spring chamber and the force of the springsmoves the valve spool to the left, until the flow of oilto the clutch is stopped. Now the valve moves to theright and left (meters) to keep a constant pressure inthe No. 2 clutch. This pressure is approximately 380kPa (55 psi) less than the pressure in the speedclutch. This difference is determined by the force ofthe springs of the pressure differential valve.Pump oil also goes through flow control orifice(22) to ratio valve (16) for the torque converter. Itgoes through an orifice in the valve spool and fills theslug chamber.Oil, from the pump, also goes to modulation reliefvalve (13). It fills the chamber around the valvespool. The oil goes through an orifice in the valvespool and opens the poppet valve at the left end of thevalve spool. This lets oil fill the slug chamber at theleft end of the valve spool.The function of modulation relief valve (13) andload piston (11), is to control the rate of the pressureincrease in the speed clutch circuit as follows:As the pressure in the No. 6 clutch increases,modulation relief valve (13) moves to the right andload piston (11) moves to the left. The orifice in thesupply passage to the load piston, causes the oil to goto the area behind the load piston at a specific rate.As the modulation relief valve moves to the right andthe load piston moves to the left, the pressure in No. 6clutch (8) increases gradually. This gradual increasein pressure is known as modulation.The load piston moves more to the left against theforce of its springs, until the area behind the loadpiston is open to a drain passage. At this time, modu-lation stops. As the oil goes out the drain passage, oilcontinues to fill the area behind the load piston. Thiskeeps the load piston in this position. After the pres-sures in the clutches are at their maximum, modula-tion relief valve (13) lets the extra oil go to the torqueconverter.The operation of the load piston and the modula-tion relief valve keeps the system pressure at a con-stant rate.3-46
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