55 Basic four speed hydraulic control system

A simplified hydraulic control system for a four speed automatic transmission will now be examined for the reader to obtain an appreciation of the overall function of the hydraulic computer (control) system.

With the manual valve in D, drive position, fluid is delivered from the oil pump to the pressure regulating valve. It then divides, some being delivered to the torque converter, the remainder passing out to the manual valve as regulating pressure (more commonly known as line pressure). Line pressure from the manual valve is then channelled to the forward clutch, which is energized, and to the overdrive band servo on the applied side. At the same time, line pressure from the pressure regulating valve passes through the 3-4 shift valve where it is directed to energize the drive clutch and to the released side of the overdrive band servo, thus preventing the engagement of the band. Line pressure is also directed to both the governor valve and to the vacuum throttle valve. The reduced pressure output from the governor valve which is known as governor pressure is directed to the end faces of each of the three shaft valves, whereas the output pressure from the throttle valve, known as throttle pressure, is conveyed to the spring end of the 2-3 and 3-4 shift valves. On the other hand, the 1-2

shift valve spring end is subjected to line pressure from the manual valve.

Whilst the transmission is in drive first gear the one way clutch will engage, so preventing the reverse planetary carrier from rotating (not shown in hydraulic system).

With the manual valve still in D, drive position, hydraulic conditions will be similar to first gear, that is, the overdrive and forward clutches are engaged, except that rising vehicle speed increases the governor pressure sufficiently to push the 1-2 shift valve against both spring and line pressure end loads. As a result, the 1-2 shift valve middle land uncovers the line pressure supply passage feeding from the manual valve. Line pressure is now directed to the second gear band servo on the applied side, energizing the second gear brake and causing both the forward and reverse sun gears to hold.

If there is a reduction in vehicle speed or if the engine load is increased sufficiently, the resulting imbalance between the spring and throttle pressure load as opposed to governor pressure acting on the 1-2 shift valve at opposite ends causes the shift valve to move against the governor pressure. Consequently the hydraulic circuitry will switch back to first gear conditions, causing the transmission to shift down from second to first gear again.

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