The Duties that a servo performs in an automatic transmission are as such:
1. A hydraulic servo-mechanism for an automatic of a vehicle, is specifically designed for applying a band brake to engage a particular gear range of the appropriate band brake to a drum, where the band has an apply side pin or lug in the case and where the hydraulic servo includes a rod/pin for selectively positioning and holding the rod/pin in one of two positions including an apply position for applying the band brake, a standby position in which the pin is retracted and in a fully released position from the apply position. A retracted position in which the rod is fully backed into the release direction.
FYI: The T-700R4 is a perfect example of an automatic transmission with a classic servo assembly.
2. A hydraulic servo-mechanism, the application of the band brake achieves, for the automatic transmission, at least a first gear stage, a second gear range close to the first gear stage, and a third gear stage interposing said second gear stage between itself and said first gear stage, and wherein the rod of said hydraulic servo is set to the apply position for establishing said first gear stage, said standby position at said second gear stage, and a retracted position at the third gear stage.
3. A hydraulic servo-mechanism according the hydraulic servo has an apply oil chamber for receiving a first oil pressure for pushing said rod in an apply direction, and a return oil chamber for receiving a second oil pressure for pushing the rod/pin in the release direction, and wherein therod is set to the apply position by supplying the first oil pressure to the apply oil chamber, then the standby position by releasing the second oil pressure from the return oil chamber and the first oil pressure from apply oil chamber, and retracted position by supplying the second oil pressure to the return oil chamber.
Simply put, the pin in the servo assembly extends to squeeze the band in order to change from a particular gear range, then releases for perhaps a clutch pack to produce the next gear range. Sometimes the servo and clutch drum combine to make a gear range too. This on off procedure which is controlled by the valve body and is constantly active during your driving experience.
Make sure to come back for our final article on servo assembly’s presented by GotTransmissions.com Blog.
Read Part-1: What is a transmission hydraulic servo assembly?