Valve body inputs

Basically, the control system is designed to control the pulleys and the clutches in the following three ways: 1. Primary pressure is supplied to the primary pulley to control the correct transmission ratio for all driving conditions. 2. Secondary pressure is supplied to the secondary pulley to ensure that there is always adequate clamping force onto the belt for all load conditions. 3. The clutch control consists of:

• Selection of the correct clutch (forward or reverse): clutch selection
• Control of the operation needed for take off: clutch engagement

Oil is supplied, under pressure, by the engine driven gear pump. The oil enters the pump through a suction filter in the sump. CVT fluid is used in order to obtain the correct clutch engagement characteristics.

The inputs used in the valve body are listed in the table titled 'Valve body inputs':

Valve body inputs

The engine speed and the drive mode are used for controlling the clutch engagement.

The speed dependent signals are provided by two pitot systems. Each consists of a pitot chamber and a pitot pipe. The pipe is stationary, while the chamber, which is filled with oil, is rotating at the speed to be measured.

A hydraulic pressure, proportional to speed, is detected at one end of the pipe as the other end is dipped in the rotating oil ring inside the pitot chamber. These pressures act on the primary and secondary valves inside the control unit, influencing transmission operation.

The same effect is obtained by detection of the transmission ratio. The clamping force can be decreased when the transmission up-shifts as the drive belt will run on a smaller radius around the secondary pulley. With the drive belt running in an overdrive situation, a lower secondary pressure still creates enough clamping force to transmit the torque. The sensing shoe acts directly onto the secondary valve spring within the hydraulic control unit, controlling secondary pressure.

A manually operated valve is connected to the selector lever and determines the drive mode of the transmission. For safety reasons, starting the engine is only possible in neutral or in the park position. An inhibitor switch prevents the starter motor from operating in all the other positions.