25.22.
Additional Features of Automatic Transmission
The modern automatic transmission system incorporates a number of features in addition to the basic system described above. These are fitted to make the gear changes smoother and the vehicle more economical, and to provide the driver extra control over the gearbox. Some of the additional features are discussed below.
Modulator Valve.
This type of valve is installed to reduce the pressure in a part of a given line to suit some special operating condition. As an example it is fitted in the throttle line supply to the regulator valve to modulate the rise in throttle pressure acting on the regulator.
Accumulator.
The purpose of a hydraulic accumulator (sometimes called a damper) as applied to an automatic transmission system, is to reduce the rate at which a clutch or brake is actuated. It cushions the shock by delaying the full application of the fluid pressure. An accumulator (Fig. 25.58) uses a spring-loaded piston in a chamber installed in the fluid line between the shift valve and clutch (or brake servo). When the shift valve is opened, the time taken for the fluid to compress the accumulator spring ensures a gradual rise of the line pressure.
