53 The fundamentals of a hydraulic control system

The effective operation of an automatic transmission relies upon a hydraulic control circuit to actuate the gear changes relative to the vehicle's road speed and acceleration pedal demands with the engine delivering power. Only a very small proportion of a transmission's operating time is spent in performing gear changes. In fact, the hydraulic system is operational for less than 1% of the driving time. The transition time from one gear ratio to the next takes roughly one second or less and therefore the hydraulic control valves must be designed to direct fluid pressure to the appropriate operating pistons which convert the fluid pressure into mechanical force and movement to energize the respective clutches and band brakes instantly and precisely.

An understanding of a basic hydraulic control system can best be considered under the four headings:

1 Pressure supply and regulating valves

2 Speed and load sensing valves

3 Gear shift valves

4 Clutch and brake coupling and hold devices

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