Automatic gearbox (Stepspeed Em-CVT) (Description): Description and operation (page 3 of 3)

Differential

Drive from the final reduction gear is transferred to the differential crownwheel. The crownwheel is bolted to the differential case with eight bolts. Drive from the crownwheel is transferred via bevel gears to the drive shafts. The differential is supported on tapered bearings.

Hydraulic pump

The hydraulic pump is located on the opposite side of the gearbox to the planetary gear set. The pump is driven directly from the torsion damper via a shaft which is located through the centre of the input shaft. The shaft is splined to the planet carrier which always rotates at engine crankshaft speed.

The pump has a swept volume of 10.32 cc per revolution and can produce a pressure of up to 40 bar (580 lbf/in²) for the highest torque requirement. The pressurized fluid from the pump is used for gearbox lubrication and transmission control.

The following electrical components are used on the Em-CVT unit:

• Ratio control motor
• Gearbox differential speed sensor
• Park/Neutral switch.

Ratio Control Motor

The ratio control motor operates to match the gear ratio to the target engine speed. In any mode, the motor position is varied to provide the ’kickdown’ function of a conventional automatic transmission. In manual mode, the position of the motor is varied to control the engine speed, fixing the current gear ratio. The motor operates the hydraulic control unit to adjust the primary pulley.

The ratio control motor is located inside the gearbox, on one side of the hydraulic control unit. The motor is connected to the main harness via a circular seven way connector with four connections used for the motor operation. The connector is screwed into the forward face of the gearbox and is secured with a nut.

The ratio control motor is a linear actuator which is controlled by the GIU in response to PWM signals at 500 Hz from the ECM. The motor itself is a bi-polar stepper motor which contains two bobbins which create magnetic fields to move the motor to the required position.

Gearbox Differential Speed Sensor

The gearbox differential speed sensor is located in the rear face of the gearbox adjacent to the road speed transducer. The sensor is connected to the main harness via a fly lead with a three pin connector.

The gearbox differential speed sensor is a Hall effect sensor which uses an 81 tooth target wheel which is the teeth of the gearbox differential crown wheel. The sensor output is read directly by the ECM which calculates an accurate vehicle speed. The sensor output is unaffected by locking of the wheels due to the operation of the differential. The ECM vehicle speed calculation allows the Em-CVT system to calculate the current gear ratio.

Park/Neutral Switch

The park/neutral switch is screwed into the rear face of the gearbox below the LH drive shaft. The switch is connected to the main harness by a four pin connector.

The switch is operated by a cam which also operates the hydraulic control unit within the gearbox. The cam is controlled by the selector lever via a cable to the gearbox. The switch has two positions and performs several functions.