Buick Regal 1997-2000

Understanding The Automatic Transaxle


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.

System Operation

The 4T60-E and 4T65-E transaxles used in the vehicles covered by this manual 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.

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.

The 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.

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.

The 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.

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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.

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

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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 product, 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.

The Servos & 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.

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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.

The Torque Converter

The torque converter has several main functions: