Line pressure can be varied according to accelerator position and to vehicle speed that requires additional control over the primary regulating valve.
High line pressure is required to hold a band or apply a clutch securely to prevent slippage when the accelerator is depressed and a high torque is transmitted.
On the other hand, once the vehicle is moving and the transmission is up-shifting, less torque is being transmitted and so a lower value of line pressure will be adequate.
Applying throttle pressure to the spring end of the primary regulating valve lifts line pressure. Applying modulator pressure to the opposite end reduces it.
Line pressure is supplied to the throttle valve at all times and throttle pressure is derived from it.
At closed throttle, the throttle valve is at rest and line pressure is blocked off by a land. Throttle pressure is zero.
When the accelerator is depressed, the movement is transferred through the kickdown valve and its spring to open the throttle valve and the land uncovers the line pressure port.
Fluid at line pressure is now metered into the throttle valve circuits and a T.V. pressure is quickly established.
This extends through the orifice to the valve reaction area and this tends to close the valve against the spring force and minimizes pressure fluctuations.
A balance is established to produce a T.V. pressure which is relative to the accelerator position. T.V. pressure can vary from 0 to approximately 1200 kilopascals.
T.V. pressure is directed back through the circuits to the spring end of the primary regulator valve to assist the spring in opposing the force of fluid coming from the pump and acting on the valves' differential area via the orifice.
Consequently T.V. pressure holds the primary regulator valve down to increase line pressure relative to the extent that the accelerator is depressed.
This ensures clutch and band apply forces will be maintained in step with accelerator position.
