Continuously Variable Transmission/Transaxle: Description and Operation
GENERAL DESCRIPTION TRANSAXLEThe VT25-E is a fully automatic, continually variable, front wheel drive transaxle. It consists primarily of a four-element torque converter, one planetary gear set, a hydraulic pressurization and control system, two variable drive pulleys, two friction clutches and a differential assembly.
The four-element torque converter contains a pump, a turbine, a pressure plate splined to the turbine, and a stator assembly. The torque converter acts as a fluid coupling to smoothly transmit power from the engine to the transaxle. It also hydraulically provides additional torque multiplication when required. The pressure plate, when applied, provides a mechanical "direct drive" coupling of the engine to transaxle.
The planetary gear set provides reverse. Changing gear ratios is fully automatic and is accomplished through the use of a transaxle control module (TCM). The TCM receives and monitors various electronic sensor inputs and uses this information to control the transaxle ratios at the most optimum time.
TRANSAXLE INPUTS
Transaxle Range Switch Assembly
The transaxle range switch assembly is a sliding contact switch attached to the manual shift shaft inside the transaxle case. The five inputs to the TCM from the transaxle range switch assembly indicate which position is selected by the transaxle selector lever. This information is used for engine controls as well as determining the transaxle ratio control patterns. The state of each input is available for display on the Scan tool.
Automatic Transaxle Input and Output (Shaft) Speed Sensor Assembly
The input and output speed sensor assembly is mounted inside the transaxle, under the control valve body cover. Both sensors are variable reluctance magnetic pickups and are wired into the same connector, which plugs into the control solenoid valve assembly. The sensors consist of a permanent magnet surrounded by a coil of wire.
Transaxle Fluid Temperature (TFT) Sensor
The TFT sensor is part of the lead frame (1) assembly, which is attached to the control solenoid valve assembly. The TFT sensor is a resistor, or thermistor (2) which changes value based on temperature. The sensor has a negative-temperature coefficient. This means that as the temperature increases, the resistance decreases, and as the temperature decreases, the resistance increases. The TCM supplies a 5-volt reference signal to the sensor and measures the voltage drop in the circuit. When the transaxle fluid is cold, the sensor resistance is high and the TCM detects high signal voltage. As the fluid temperature warms to a normal operating temperature, the resistance becomes less and the signal voltage decreases. The TCM uses this information to maintain shift quality and torque converter clutch apply quality over the operating temperature range.
TRANSAXLE OUTPUTS
Pressure Sensor
The pressure sensor is used to monitor line pressure and feedback to the TCM with an analog signal of 0-5 volts. The pressure sensor is fed with tier 2 feed fluid pressure, which nominally ranges from 70 psi to 850 psi (483 to 5861 kPa).
Line Pressure Control Solenoid Valve
The line pressure control solenoid valve (normally high, 3-port linear pressure control solenoid) is a precision electronic pressure regulator that controls transaxle line pressure based on current flow through its coil windings. As current flow is increased, the magnetic field produced by the coil moves the solenoid's plunger further away from the exhaust port. Opening the exhaust port decreases the output fluid pressure regulated by the line pressure control solenoid valve, which ultimately decreases line pressure. The TCM controls the line pressure control solenoid valve based on various inputs including throttle position, transaxle fluid temperature and gear state.
Ratio Control Motor
The ratio control motor is a linear position device, which changes transaxle ratio by accurately controlling the position of the variable ratio control valve in order to regulate primary feed fluid flow. This component has a total nominal travel of 22.0 mm (0.87 inch).
Neutral Idle/TCC On-Off Solenoid Valve
The neutral idle/TCC ON-OFF solenoid valve is a normally closed, 3-port, ON/OFF solenoid that the TCM controls to apply and release the converter clutch. When de-energized, actuator feed limit fluid pressure is blocked from pressurizing the TCC/NI enable signal fluid circuit. Without TCC/NI enable signal fluid pressure at the end of the TCC control valve, spring force holds the valve in the released position.
Neutral Idle/TCC Clutch Control Solenoid Valve
The neutral idle/TCC clutch control solenoid valve is a normally low, 3-port linear pressure control solenoid used to control the apply and release of the forward and reverse clutches, and the torque converter clutch. The TCM operates the solenoid with a negative duty cycle at a fixed frequency of 32 Hz to control the rate of clutch apply/release. The solenoid's ability to "ramp" the clutch apply and release pressures results in a smoother clutch operation.