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    GM Century/Lumina/Grand Prix/Intrigue 1997-2000

    Understanding the Automatic Transaxle

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    The automatic transaxle allows engine torque and power to be transmitted to the front wheels within a narrow range of engine operating speeds. It will allow the engine to turn fast enough to produce plenty of power and torque at very low speeds, while keeping it at a sensible rpm at high vehicle speeds (and it does this job without driver assistance). The transaxle uses a light fluid as the medium for the transaxle of power. This fluid also works in the operation of various hydraulic control circuits and as a lubricant. Because the transaxle fluid performs all of these functions, trouble within the unit can easily travel from one part to another. For this reason, and because of the complexity and unusual operating principles of the transaxle, a basic understanding of the basic principles of operation will simplify troubleshooting.

    TORQUE CONVERTER



    The torque converter has several main functions:



    It allows the engine to idle with the vehicle at a standstill, even with the transaxle in gear.
     
    It allows the transaxle to shift from range-to-range smoothly, without requiring that the driver close the throttle during the shift.
     
    It multiplies engine torque to an increasing extent as vehicle speed drops and throttle opening is increased. This has the effect of making the transaxle more responsive and reduces the amount of shifting required.
     
    The torque lock-up feature reduces slippage when at cruise speed, increasing efficiency and fuel mileage.
     

    The torque converter is a metal case, generally shaped like a sphere flattened on opposite sides. It is bolted to the engine flywheel, rotating at engine speed.

    The case contains three sets of blades. One set is attached directly to the case. This set forms the torus or pump. Another set is directly connected to the output shaft, and forms the turbine. The third set is mounted on a hub which, in turn, is mounted on a stationary shaft through a one-way clutch. This third set is known as the stator.

    A pump, which is driven by the converter hub at engine speed, keeps the torque converter full of transaxle fluid at all times. Fluid flows continuously through the unit to provide cooling.

    Under low speed acceleration, the torque converter functions as follows:



    Click image to see an enlarged view

    Fig. The torque converter housing is rotated by the engine's crankshaft by means of the flywheel, and turns the impeller. The impeller then spins the turbine, which gives motion to the turbine shaft, driving the gears

    The torus is turning faster than the turbine. It picks up fluid at the center of the converter and, through centrifugal force, slings it outward. Since the outer edge of the converter moves faster than the portions at the center, the fluid picks up speed.

    The fluid then enters the outer edge of the turbine blades. It then travels back toward the center of the converter case along the turbine blades. In impinging upon the turbine blades, the fluid loses the energy picked up in the torus.

    If the fluid was now returned directly into the torus, both halves of the converter would have to turn at approximately the same speed at all times, and torque input and output would both be the same.

    In flowing through the torus and turbine, the fluid picks up two types of flow, or flow in two separate directions. It flows through the turbine blades, and it spins with the engine. The stator, whose blades are stationary when the vehicle is being accelerated at low speeds, converts one type of flow into another. Instead of allowing the fluid to flow straight back into the torus, the stator's curved blades turn the fluid almost 90° toward the direction of rotation of the engine. Thus the fluid does not flow as fast toward the torus, but is already spinning when the torus picks it up. This has the effect of allowing the torus to turn much faster than the turbine. This difference in speed may be compared to the difference in speed between the smaller and larger gears in any gear train. The result is that engine power output is higher, and engine torque is multiplied.

    As the speed of the turbine increases, the fluid spins faster and faster in the direction of engine rotation. As a result, the ability of the stator to redirect the fluid flow is reduced. Under cruising conditions, the stator is eventually forced to rotate on its one-way clutch in the direction of engine rotation. Under these conditions, the torque converter begins to behave almost like a solid shaft, with the torus and turbine speeds being almost equal.

    Since some slippage was inherent in former torque converter design, in recent years vehicle manufacturers have gone to a Torque Converter Clutch (TCC) to "lock-up" the sections of a torque converter at certain speeds. This eliminates much of slippage, providing greater efficiency of operation and better fuel mileage. The Torque Converter Clutch used on the vehicles covered by this guide is controlled by an electronic solenoid. The rate of apply/release is controlled by an electronic Pulse Width Modulation solenoid valve to avoid lack of smoothness found in older TCC designs.

    PLANETARY GEARBOX



    The ability of the torque converter to multiply engine torque is limited. Also, the unit tends to be more efficient when the turbine is rotating at relatively high speeds. Therefore, a planetary gearbox is used to carry the power output of the turbine to the driveshaft.



    Click image to see an enlarged view

    Fig. Planetary gears work in a similar fashion to manual transaxle gears, but are composed of three parts

    Planetary gears function very similarly to conventional transaxle gears. However, their construction is different in that three elements make up one gear system, and, in that all three elements are different from one another. The three elements are: an outer gear that is shaped like a hoop, with teeth cut into the inner surface; a sun gear, mounted on a shaft and located at the very center of the outer gear; and a set of three planet gears, held by pins in a ring-like planet carrier, meshing with both the sun gear and the outer gear. Either the outer gear or the sun gear may be held stationary, providing more than one possible torque multiplication factor for each set of gears. Also, if all three gears are forced to rotate at the same speed, the gearset forms, in effect, a solid shaft.



    Click image to see an enlarged view

    Fig. Planetary gears in the maximum reduction (low) range. The ring gear is held and a lower gear ratio is obtained



    Click image to see an enlarged view

    Fig. Planetary gears in the minimum reduction (drive) range. The ring gear is allowed to revolve, providing a higher gear ratio

    Most automatics use the planetary gears to provide various reductions ratios. On the transaxles used in the vehicles covered by this guide, both bands and multiple disc wet clutches are used to hold various portions of the gearsets to the transaxle case or to the shaft on which they are mounted. Shifting is accomplished, then, by changing the portion of each planetary gearset which is held to the transaxle case or to the shaft.

    SERVOS AND ACCUMULATORS



    The servos are hydraulic pistons and cylinders. They resemble the hydraulic actuators used on many other machines, such as bulldozers. Hydraulic fluid enters the cylinder, under pressure, and forces the piston to move to engage the band or clutches.



    Click image to see an enlarged view

    Fig. Servos, operated by pressure, are used to apply or release the bands, to either hold the ring gear or allow it to rotate

    The accumulators are used to cushion the engagement of the servos. The transaxle fluid must pass through the accumulator on the way to the servo. The accumulator housing contains a thin piston which is sprung away from the discharge passage of the accumulator. When fluid passes through the accumulator on the way to the servo, it must move the piston against spring pressure, and this action smoothes out the action of the servo.

    HYDRAULIC CONTROL SYSTEM



    The hydraulic pressure used to operate the servos comes from the main transaxle oil pump. This fluid is channeled to the various servos through the shift valves. There is a manual shift valve which is operated by the transaxle selector lever and an automatic shift valve for each automatic upshift the transaxle provides.


    NOTE
    All of the automatic transaxles on the GM W-Body vehicles are electronically controlled. Electrical solenoids are used to better control the hydraulic fluid. The solenoids are regulated by the Powertrain Control Module (PCM).

    Only the 4T60-E transaxle uses a vacuum modulator. The slightly more heavy-duty 4T65-E does not. A vacuum modulator responds to the engine's manifold vacuum. In this way, the clutches and bands will be actuated with a force matching the torque output of the engine. The modulator valve helps adjust line boost pressure and affects the 1-2 accumulator valve, a secondary 1-2 accumulator valve, as well as both the 2-3 and 3-4 accumulator valves. A vacuum modulator can be checked with a hand vacuum pump. The modulator should hold approximately 5 in. Hg of vacuum for 30 seconds.

    SYSTEM OPERATION



    The 4T60-E and 4T65-E transaxles used in the vehicles covered by this guide are fully automatic front wheel drive transaxles. They provide four forward ranges including overdrive.

    You can operate the transaxle in any one of the seven following modes:



    P-PARK position prevents the vehicle from rolling either forward or backward. For safety reasons, use the parking brake in addition to the park position.
     
    R-REVERSE allows the vehicle to be operated in a rearward direction.
     
    N-NEUTRAL allows the engine to be started and operated while driving the vehicle. If necessary, you may select this position to restart the engine with the vehicle moving.
     
    OD-OVERDRIVE is used for all normal driving conditions. OVERDRIVE provides four gear ratios plus a converter clutch operation. Depress the accelerator to downshift for safe passing.
     
    D-DRIVE position is used for city traffic, hilly terrain and trailer towing. DRIVE provides three gear ranges. Depress the accelerator to downshift.
     
    2-Manual SECOND provides acceleration and engine braking. You may select this range at any vehicle speed, but, depending on the model, the transaxle will not downshift until the vehicle speed drops below a certain speed (approximately 60 mph).
     
    1-Manual LOW provides maximum engine braking. You may select this range at vehicle speeds under 40 mph. but, depending on the model, the transaxle will not downshift until the vehicle speed drops below a certain speed. Manual LOW is not available on some models.
     

    When an automatic transaxle not operating properly, it is likely influenced by one, or a combination of the following items:



    Fluid level too low or two high.
     
    Engine performance problems.
     
    Manual linkage adjustment.
     
    Internal fluid leaks.
     
    Electrical problems.
     
    Transaxle or other mechanical components
     
    Vacuum Modulator, if equipped (4T60-E transaxles).
     

    If noise or vibration is noticeable in PARK and NEUTRAL with the engine at idle, but is less noticeable as RPM increases, the cause may be from poor engine performance. A noise or vibration that is noticeable when the vehicle is in motion, MAY NOT be the result of the transaxle. Inspect:



    Check the tires for uneven wear, imbalance, mixed sizes or mixed radial and bias ply tires.
     
    Check the suspension for alignment and worn out components or loose fasteners.
     
    Check the engine and transaxle mounts for damage and loose fasteners.
     
    Check the transaxle case mounting holes for missing bolts, stripped threads and cracks.
     
    Check the flywheel for missing or loose bolts, crack and for imbalance.
     
    Check the torque converter for missing or loose bolts or balance weights.
     

     
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