Automatic Transmission/Transaxle: Description and Operation

These transmissions, see images, are fully automatic units consisting primarily of a three-element hydraulic torque converter and a compound planetary gear set. On 200C models the torque converter includes a converter clutch.

Three multiple disc clutches, a roller clutch and a band provide the friction elements required to obtain the desired function of the compound planetary gear set The compound planetary gear set provides three forward speeds and reverse.

The torque converter couples the engine to the planetary gears through oil and provides torque multiplication. It consists of a pump or driving member, a turbine or driven member, a converter clutch and a stator assembly. The stator is mounted on a one-way roller clutch which allows the stator to turn clockwise but not counter-clockwise.

The torque converter housing is filled with oil and rotates at engine speed. The converter pump is an integral part of the converter housing, therefore the pump blades rotating at engine speed set the oil within the converter into motion and direct it to the turbine causing the turbine to rotate. As the oil passes trough the turbine it travels in such a direction that if it were not redirected by the stator it would strike the rear of the converter pump blades and impede its pumping action. Therefore at low turbine speeds, the oil is redirected by the stator to the converter pump in such a manner that it actually assists the converter pump to deliver power or multiply engine torque. As turbine speed increases, the direction of the oil leaving the turbine changes and flows against the rear side of the stator vanes in a clockwise direction. Since the stator is now impeding the smooth flow of oil, its roller clutch releases and it revolves freely on its shaft Once the stator becomes inactive, there is no further multiplication of torque within the converter. At this point the converter is acting as a fluid coupling since the converter pump and turbine are being driven at about the same speed, or at a one-to-one ratio.

The converter clutch is splined to the turbine assembly, and when in operation, applies against the converter cover, providing a mechanical direct drive coupling of engine to transmission planetary gears. When the converter clutch is released the assembly operates as a normal torque converter.

The converter clutch apply and release is controlled by the position of the converter clutch apply valve located in the oil pump cover.

The apply valve is controlled by a solenoid. When vehicle speed in drive range, third gear reaches approximately 30-45 mph (actual mph will vary according to tire size and axle ratio), the governor pressure switch closes, completing the ground circuit and energizes the solenoid.

To further aid the apply and release of the converter clutch during various driving situations, several controls have been incorporated into the electrical/vacuum system. These controls are located outside of the transmission and perform the following functions.

Not all components are used on all models.

1. Brake release switch - Releases the converter clutch when brakes are applied.

2. Thermal Vacuum Valve - Prevents converter clutch from applying at low coolant temperatures.

3. Low Vacuum Switch - Releases converter clutch when engine vacuum drops to a preset level during moderate acceleration, prior to detent or part-throttle downshift and during zero throttle coastdown, using a ported vacuum signal.

4. Vacuum Relay Valve - Prevents ported vacuum from reaching the low vacuum switch at low engine coolant temperatures (when vacuum to the EFE stops, vacuum from the EGR port of Transmission Converter Clutch can pass through relay valve to low engine vacuum switch).

5. Vehicle Speed Sensor - Provides vehicle speed information to the ECM. Speed must be above a specified calibrated value before the clutch will apply.

6. Coolant Temperature Sensor - Similar to the Thermal Vacuum Valve, this sensor prevents clutch engagement before the engine reaches a specified temperature.

7. Throttle Position Sensor - This sensor provides throttle angle information to the ECM. The ECM uses this information to release the clutch when the vehicle is accelerating or decelerating at a specific rate.

8. Gear Select Switch - This switch informs the ECM as to what gear the transmission is in. The ECM uses this information to vary the conditions under which the clutch applies or releases.

The hydraulic system in this transmission is pressurized by a gear type pump to provide the working pressures required to operate the friction elements and automatic controls.