Automatic gearbox (Stepspeed Em-CVT) (Operation): Description and operation

The diagnostic socket is connected to the ECM on an ISO9141 K Line. The system uses a ’P’ code diagnostic strategy and can record faults relating to the gearbox differential speed sensor and the ratio control motor.

The GIU and ECM monitor all inputs and outputs relating to the Em-CVT system. If a fault is detected, a code applicable to that fault is stored in a fault log in the GIU. When TestBook is connected, GIU codes are accessed via the ECM diagnostic strategy.

The MEMS3 ECM is a major component in gearbox control. Functionality of engine sensors should be established to eliminate any ECM related faults from the gearbox diagnostics.

OPERATION

General

The transmission is driven from the engine via a torsion damper bolted to the flywheel. The torsion damper drives the input shaft which in turn drives the planet carrier. Depending on whether forward or reverse is selected, the primary pulley will rotate, transferring torque to the secondary pulley causing the vehicle to move in the required direction.

The steel belt is fitted between the primary and secondary pulleys. Each pulley consists of one fixed sheave and one axially moveable sheave. The moveable pulley sheaves are located diagonally opposite to each other to prevent misalignment of the belt during shift changes.

Each moving pulley sheave is connected to a hydraulic cylinder which is controlled by hydraulic pressure generated by the integral pump running at engine speed. Moving the pulley sheaves together increases their effective diameter and moving them apart decreases the diameter due to the conical faces of each pulley. In this way the gear ratios of the Em-CVT unit are achieved.

The Em-CVT unit has two multiplate wet clutch packs; one for forward and one for reverse. Each pack comprises three friction plates. The clutches are hydraulically controlled which enables the vehicle to move smoothly from standstill irrespective of the throttle position. The clutches are fed hydraulic fluid from the fluid cooler to prevent them overheating.

When the selector lever is in the Park (P) position, a spring and cone operated pawl mechanically locks the secondary pulley, consequentially locking the rear wheels. If ’P’ is selected when the vehicle is moving, the pawl will not engage until the vehicle speed falls to below 4 mph (7 km/h). A rattling sound may be heard if ’P’ is selected when the vehicle is moving.

Driving - Automatic mode

To pull away from a standstill a low ratio is required. The primary pulley is held fully open, reducing its diameter and allowing the belt to seat at the bottom of the pulley. The secondary pulley is held closed, forcing the belt to run in its increased diameter.