Figure 2-9.—Planetary gear system.Power can be transmitted through the planetarygearset in various ways. A shaft from the engine maybe connected to drive the sun gear. It may be connectedto drive the planet carrier or the shaft may be connectedto drive the ring gear. The propeller shaft may also beconnected to anyone of these members; however, powercan be transmitted in the planetary gear system onlywhen (1) the engine is delivering power to one of thethree members, (2) the propeller shaft is connected toone of the other members, and (3) the remainingmember is held against rotation. All three conditionsmust be satisfied for power to be transmitted in thesystem. Automatic transmissions provide for holding amember through hydraulic servos and spring pressure.Automatic Transmission OperationMost automatic transmissions are basically thesame. They combine a fluid torque converter with aplanetary gearset and control the shifting of theplanetary gear with an automatic hydraulic controlsystem. The fluid torque converter is attached to theengine crankshaft and serves as the engine flywheel.This design means that when the engine runs, enginepower flows into the converter and drives the converteroutput (turbine) shaft. There is no neutral in the torqueconverter. Neutral is provided in the planetary gearsetby the release of bands and clutches.The transmission automatically multiplies andtransmits engine torque to the drive shaft as drivingconditions demand. The speeds at which the couplingpoint and the gearshifts occur are controlled partially bythe operator. The operator has only a partial control inthe D-drive position, because the transmission in theD-drive position shifts the planetary gearset into thehigher gears to prevent engine overspeeding regardlessof throttle position.The operation of automatic shift vehicles is quitesimple; however, it is imperative that the professionaloperator learn to operate them smoothly and properly.In vehicles equipped with automatic transmissions,initial gear selection is controlled with a selector lever.When in drive (D or DR), shifting from drive to low (L)and returning to drive is controlled automatically by theengine speed.Most vehicles have four or five of the followingselector positions.P-PARK POSITION.— On light vehicles, such assedans and pickups, this position is used for locking thetransmission so the vehicle cannot roll while parked. Insome heavier vehicles, the park position does not lockthe transmission. In vehicles with a park position, theengine should be started from the park position.N-NEUTRAL POSITION.— Engines of vehiclesnot equipped with a P-park position are started from theN-neutral position.In this position, the engine isdisengaged from the drive shaft of the vehicle.D-DRIVE POSITION.— With the shift lever at Dor DR, the vehicle moves forward as you depress theaccelerator. After starting the engine in neutral or parkposition, step on the brake and change the selector to Dor DR for forward movement. To avoid prematureforward movement, keep pressure on the brake while inthe drive position until you are ready to place the vehiclein motion. Without further operator action, thetransmission automatically shifts to higher gears asspeed increases.L-LOW or POWER POSITION.— T hetransmission will not shift automatically to higher gearratios when the lever is in the low position. The lowposition is used when negotiating steep grades andrough terrain or when the braking power of the engineis required. When low range is no longer needed,release the accelerator temporarily and move the shiftlever to the drive position for normal gear progression.In the drive position, the low range is engagedautomatically when engine speed is reduced. If theaccelerator is suddenly fully depressed, the low rangebecomes engaged. (This procedure may be used toprovide a sudden burst of speed for passing.) When apredetermined engine speed has been attained, thetransmission automatically returns to driving range.2-7
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