Automatic gearbox - CVT (Selector mechanism): Description and operation

The transmission has two multiplate wet clutch packs; one forward and one reverse. Each pack has 3 friction plates and hence 6 friction surfaces. The clutches are controlled so the vehicle can move from rest very smoothly regardless of the degree of throttle opening. During this operation the clutches are cooled by cool fluid fed from the fluid cooler to prevent them overheating.

The transmission takes its control inputs from the selector lever, throttle position, engine speed and output speed. The latter two inputs are derived from pitot tubes mounted in side the transmission casing. The pitot tubes transmit centrifugal pressure generated by the rotation of the primary and the secondary pulleys to the valve block.

Selector mechanism

The vehicle is fitted with a 5 position selector lever. P = Park R = Reverse N = Neutral D = Drive L = Low drive

Selection of one of the above positions activates a selector shaft inside the transmission. The lever and the shaft are connected by a push and pull type cable. A cam fitted to the selector shaft is further connected to the manual valve of the control system and sets it in one of its 5 discrete positions (P, R, N, D, L).

• ’P’ - A spring and cone operated pawl mechanically locks the secondary pulley, thereby locking the front wheels. If the selection is made while the vehicle is moving the pawl will simply rattle ineffectively until the vehicle speed drops below a threshold of approximately 7 km/h.
• ’R’ - The reverse clutch is activated and the forward clutch is inhibited. The reverse clutch holds the annulus of the planetary gear set stationary. The planet carrier drives three pairs of planet gears, forcing the sun wheel to rotate in the opposite direction moving the vehicle in reverse. The car will reverse without upshifting. When the driver selects reverse at a high forward vehicle speed, an inhibitor valve makes sure that the reverse clutch is not engaged unless the vehicle speed has dropped to an acceptable level.
• ’N’ - Both forward and reverse clutches are inhibited. The fluid pump is running at engine speed but the rest of the transmission is standing still.
• ’D’ - The forward clutch is activated and the reverse clutch is inhibited. The epicyclic gear set rotates as one unit, and the engine torque is transmitted directly to the primary pulley which drives the secondary pulley moving the vehicle forward. Can be used for all driving situations from take-off to top speed. The driver simply controls the vehicle speed by means of the accelerator pedal.
• ’L’ - Using the low drive option results in a different shift pattern to the normal drive position. The ’L’ position is best selected for mountain driving, towing conditions and for sporty driving but switching between ’D’ and ’L’ is possible at any speed. Selecting the low drive option provides additional engine braking when the accelerator pedal is released. Top speed can also be achieved in this position.

Starter inhibitor switch

The transmission selector shaft cam is used to activate an inhibitor switch which is connected into the vehicles starter and reverse lamp circuits. The inhibitor switch will only allow the vehicle to be started in ’N’ or ’P’. In ’R’ the reversing lights are switched on.