Shift Control valves are responsible for directing fluid to different passages in the transmission. They can be manually controlled, solenoid controlled or hydraulically controlled. They block hydraulic passages while other lands of the valve open passages.
The manual valve directs line pressure to various passages in the valve body. It is linked to the driver’s selector lever and shifts the transmission into and out of the P, R, N, D, 2, L ranges as directed by the driver. As the valve moves to the right, it exposes passages to line pressure, which will determine the gear selected. The various positions of the valve are maintained by a detent mechanism which also provides feedback to the driver.
The downshift plug is located below the throttle valve. It is actuated by the throttle cam in response to engine throttle movement when the driver presses down on the accelerator, opening t more than 85%. It is used in a governor controlled transmission to enhance downshifting rather than relying on throttle pressure alone to overcome governor pressure and move the shift valve down. The net result is that a downshift occurs at a higher vehicle speed than if relying on throttle pressure alone.
When the throttle is opened 85% or more, the downshift valve moves upward and detent regulator pressure is directed to each shift valve to counter governor pressure. Detent pressure provides added force in addition to throttle pressure and spring tension to move the valve downward against governor pressure. Depending on the vehicle speed, governor pressure may be great enough to allow the 1-2 shift valve and 2-3 shift valve to remain up, whereas the 3-4 shift valve may immediately move downward to cause a 4 to 3 downshift.
Timing Valves are responsible to finesse the quality of transmission shift characteristics. In some cases, the applied clutch is a dual piston application and one is applied before the other. In other cases, the pressure which applies a holding device or forces a shift valve to downshift is reduced to enhance the application.
The downshift timing valve serves to prevent a direct downshift from overdrive to a low gear. This causes the transmission to automatically shift into a higher gear for a moment before selecting the desired gear. This is to avoid shifting shock. In an example shifting from overdrive to second gear, after the line pressure acting on the intermediate shift value is switched from the overdrive brake to the overdrive clutch, it acts on the 2-3 shift valve, causing it to shift from third gear to second.
The Reverse Clutch and Brake Sequencing valves control the timing of the application when shifting reverse. As line pressure increases when shifting reverse, the sequencing valve reduces shift shock. When moving the selector to R, the passage to the outer piston of the direct clutch is blocked by the sequencing valve. As pressure builds and the inner piston begins to apply, the valve moves to compress the spring. Line pressure is applied to the outer piston for full engagement of the clutch.
The accumulators act to cushion shifting shock. These valves are basically pistons located in a bore with a heavy calibrated spring to counter hydraulic pressure. They are located in the hydraulic circuit between the shift valve and the holding device. When the shift valve moves, fluid is directed to the circuit of the holding device. As the piston begins to compress the clutch return springs, pressure in the circuit begins to build. As pressure builds, it acts to load the spring in the accumulator. Pressure in the circuit cannot reach its potential ntil the spring is compressed and the piston is seated. The pressure builds more slowly ad the clutch engagement is softened.
Clutch application can be tailored even more closely by providing hydraulic pressure to the spring side of the accumulator. Line pressure applying the holding device has to overcome spring tension and additional fluid pressure and therefore, higher pressure is exerted on the holding device before full pressure is applied. Hydraulic pressure to the accumulator is controlled by the accumulator control valve, or electronically controlled solenoid.
Pressure Modulating Valves change controlling pressures to tailor operational characteristics of the automatic transmission. Line pressure, throttle pressure and governor pressure, all have an effect on how the automatic transmission operates. Modulator valves further reduce these controlling pressures to finesse the transmission’s operation.
The Accumulator Control Valve modifies line pressure from the pump to the accumulators based on engine load. It reduces shock shift by lowering the back pressure of the direct clutch accumulator and the 2nd brake accumulator when the throttle opening is small. The valve is balanced between throttle pressure and spring tension at its base and metered line pressure at the top of the valve.
Since the torque produced by the engine is low when the throttle opening is small, accumulator back pressure is reduced. This prevents shift shock when the brakes and clutches are applied. Conversely, engine torque is high when the throttle angle is large during moderate to heavy acceleration. Not only is line pressure increased, but throttle pressure acting at the base of the accumulator control valve increases back pressure to the accumulators. Accumulator pressure is increased to prevent slippage when the clutches and brakes are applied.
The Governor Modulator Valve works in conjunction with the cut back valve to reduce engine load at high speed. It modifies governor pressure to the cut back valve as the vehicle speed component.
The Cut Back Valve modifies throttle pressure based on vehicle speed. Lowering line pressure prevents unnecessary power loss at the transmission oil pump during higher speeds.
The Detent Regulator Valve modifies line pressure to the Down Shift Plug during kick down to stabilize the hydraulic pressure acting on the shift valves. Detent pressure provides a pressure in addition to throttle pressure to improve downshift response.
The Low Coast Modulator Valve reduces line pressure from the manual valve to reduce shock when the gear selector is moved to the L position. The low coast modulator pressure pushes the low coast shift valve down and applies the 1st and reverse brake to buffer shock.
To read more about Automatic Transmissions and understanding their components, please view the following links: