CONVERTER DRIVE

Flow from the transmission pump is directed through the transmission filter to the priority valve, the impeller clutch solenoid valve, and the lockup clutch solenoid valve. The priority valve maintains a minimum oil pressure to the impeller clutch solenoid valve and the lockup clutch solenoid valve during transmission shifts. When the transmission pump supply pressure increases above the spring setting of the priority valve, the priority valve opens and oil is directed to the speed manifold and the direction manifold. Also, pump supply oil is directed past the orifice to the inlet port for the 1st and 3rd speed selector spool and the inlet port for the 2nd speed selector spool.

When the operator moves the directional switch and upshift or downshift switch to the 1st speed forward position, the Power Train ECM energizes the impeller clutch solenoid (the impeller clutch solenoid will be energized and then de-energized). The solenoid for the lockup clutch is also de-energized. Then, the No. 5 solenoid is energized first and No. 2 solenoid will be energized next. 3 1 2 5 2 3 1 5 4 Torque Converter Lube - Rear Pump Drive Transmission Control Valve Transmission Pump Sump Transmission Filter Torque Converter Filter Priority Valve Impeller Clutch Modulating Valve Lockup Clutch Modulating Valve TRANSMISSION HYDRAULIC SYSTEM

FIRST SPEED FORWARD

CONVERTER DRIVE

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is ENERGIZED, pilot oil is directed to the upper end of the directional selection spool. The oil pressure overcomes the force of the selector spool spring and the spool shifts downward. The No. 2 clutch will be engaged.

When the oil requirements of the selector and pressure control valve have been satisfied, the remaining oil combines with the oil from the transmission pump (orange). The combined oil flows to the torque converter. Flow continues through the torque converter through the power train coolers to the transmission lubrication circuit.

When the transmission is in NEUTRAL, the Power Train ECM pressurizes the impeller clutch in response to the engine speed. When the engine speed is less than 1100 rpm, the impeller clutch pressure is maintained at a holding pressure of 550 ± 207 kPa (80 ± 30 psi). When the engine rpm increases from 1100 to 1300 rpm, the Power Train ECM increases the impeller clutch pressure from 550 ± 207 kPa (80 ± 30 psi) to 2580 ± 207 kPa (375 ± 30 psi) for one second. The Power Train ECM then reduces the impeller clutch pressure to 2274 ± 207 kPa (330 ± 30 psi). The impeller clutch pressure remains at 2274 ± 207 kPa (330 ± 30 psi) for all engine speeds above 1300 rpm. The torque converter housing and impeller rotate at engine speed.

When the engine rpm decreases from 1300 to 1100 rpm, the Power Train ECM decreases the impeller clutch pressure from 2274 ± 207 kPa (330 ± 30 psi) to 550 ± 207 kPa (80 ± 30 psi). The impeller clutch pressure remains at a holding pressure of 550 ± 207 kPa (80 ± 30 psi) for all engine speeds below 1100 rpm. The low pressure allows the impeller clutch to remain filled without engaging. The torque converter housing rotates with the engine while the torque converter impeller is only partially engaged without transmitting torque.

NOTE: The impeller clutch pressure is reduced because the pressure to the impeller

clutch is reduced after the first second (1/60 of a minute) of engagement to extend the life of the seals and pistons in the impeller clutch. This can be demonstrated by connecting a pressure gauge to the impeller clutch pressure tap and viewing the gauge during a directional shift. Caterpillar Electronic Technician (ET) can also be used to view the impeller clutch pressure and the impeller clutch solenoid valve current during a directional shift.

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When the transmission is shifted from FIRST SPEED FORWARD to SECOND SPEED FORWARD (speed shift), the Power Train ECM de-energizes the No. 5 clutch solenoid, and energizes the No. 4 clutch solenoid. The Power Train ECM also continues to de-energize the impeller clutch solenoid and the lockup clutch solenoid.

When de-energized, the No. 5 clutch solenoid valve interrupts the flow of pilot oil to the speed selector spool and directs the pilot oil to the sump.

When energized, the No. 4 clutch solenoid valve sends oil to the end of the 2nd speed selector spool. The force of the oil pressure overcomes the force of the selector valve spring and moves the spool from its center position. Oil from the inlet passage flows through the orifice, into the No. 4 speed clutch.

The empty No. 4 clutch causes the pressure at P1 and P2 to be less than 375 kPa (55 psi). The decrease in P1 oil pressure allows the pressure differential valve spring to move the differential valve up. When the differential valve moves up, the differential valve opens a passage for oil in the differential valve spring chamber and the load piston cavity to flow to drain. The

transmission control valve then repeats the fill and modulation cycle.

During a speed shift, the Power Train ECM maintains maximum pressure in the impeller clutch. The transmission directional clutch picks up the load after a direction shift.

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