For efficient operation of the transmission, the bands and clutches must be released and applied at the proper time. It is the responsibility of the hydraulic control system to control the hydraulic pressure being sent to the different reaction members. Central to the hydraulic control system is the valve body assembly. This assembly is made of two or three main parts: a valve body, separator plate, and transfer plate. These parts are bolted as a single unit to the transmission housing. The valve body is machined from aluminum or iron and has many precisely machined bores and fluid passages. Various valves are fitted into the bores and the passages direct fluid to various valves and other parts of the transmission. The separator and transfer plates are designed to seal off some of these passages and to allow fluid to flow through specific passages.

The purpose of a valve body is to sense and respond to engine and vehicle load, as well as to meet the needs of the driver. Valve bodies are normally fitted with three different types of valves: spool valves, check ball valves, and poppet valves. The purpose of these valves is start, stop, or use movable parts to regulate and direct the flow of fluid throughout the transmission.

Oil Flow

The source of fluid flow through the transmission is the oil pump. Three types of oil pumps are used in an automatic transmission: the gear-type, rotor-type, and vane-type.< Oil pumps are driven by the pump drive hub of the torque converter; therefore, whenever the torque converter cover is rotating, the oil pump is driven. The oil pump creates fluid flow throughout the transmission. The valve body regulates and directs the fluid flow to meet the needs of the transmission.

Oil pumps are capable of creating excessive amounts of pressure, which can damage the transmission. Therefore, the transmission is equipped with a pressure regulator valve.

Pressure Regulator Valve

The pressure regulator valve is usually located in the valve body. It maintains basic fluid pressure. Pressure regulating valves are typically spool-type valves that toggle back and forth in their bore to open and close an exhaust passage. By opening the exhaust passage, the valve decreases the pressure of the fluid. As soon as the pressure decreases to a predetermined amount, the spool valve moves to close off the exhaust port and pressure again begins to build. The action of the spool valve regulates the fluid pressure.

Pressure Boosts

When the engine is operating under heavy load conditions, fluid pressure must be increased to increase the holding capacity of a reaction member. This is accomplished by sending pressurized fluid to one side of the pressure regulator's spool valve. This pressure works against the spool valve's normal movement to open the exhaust port and allows pressure to build to a higher point than normal.

Engine load can be monitored electronically through the use of various electronic sensors that send information to an electronic control unit, which in turn controls the pressure at the valve body. Load can also be monitored by throttle pressure. Throttle pedal movement moves a throttle valve in the valve body via a throttle cable. When the throttle pedal is opened, the throttle valve opens and applies pressure to the pressure regulator. This delays the opening of the pressure regulator valve, which allows for an increase in pressure. When the driver lets off the throttle pedal, the pressure regulator valve is free to move and normal pressure is maintained.

Many transmissions have been equipped with a vacuum modulator, which uses engine vacuum to change transmission pressure.< The vacuum modulator allows for an increase in pressure when vacuum is low and decreases it when vacuum is high. When high vacuum is present at the modulator, the pressure regulator works normally and maintains normal pressure. However, when the vacuum is low, the modulator allows pressurized fluid to enter onto the side of the spool valve in the pressure regulator, which allows for an increase in pressure.

Governor Assembly

The governor assembly, located on or driven by the transmission's output shaft, senses road speed and sends a fluid pressure signal to the valve body to either upshift or downshift. When vehicle speed is increased, the pressure developed by the governor is directed to the shift valve. As the speed (and therefore the pressure) increases, the spring tension on the shift valve is overcome and the valve moves. This action causes an upshift. Likewise, a decrease in speed will result in a decrease in pressure and a downshift.

Although the governor sends a signal that will force an upshift, engine load may cause a delay in the shift. This allows for operation in a lower gear when there is a heavy load and the vehicle needs the gear reduction. During heavy load operation, the governor pressure must be strong enough to overcome the high throttle pressure plus the spring tension on the shift valve before it can force an upshift. Because of this, the transmission will remain in a particular gear range until a higher-than-normal engine speed is reached.

Kickdown Valve

The valve body is also fitted with a kickdown circuit, which provides a downshift when the driver requires additional power. When the throttle pedal is quickly opened wide, throttle pressure rapidly increases and directs a large amount of pressure onto the kickdown valve. This moves the kickdown valve, which opens a port and allows mainline pressure to flow against the shift valve. The spring tension on the shift valve, the kickdown pressure, and throttle pressure will push on the end of the shift valve, causing it to move to the downshift position. This forces a quick downshift.

Shift Feel

All transmissions are designed to change gears at the correct time, according to engine speed and load and driver intent. However, transmissions are also designed to provide for positive change of gear ratios without jarring the driver or passengers. If a band or clutch is applied too quickly, a harsh shift will occur. "Shift feel" is controlled by the pressure at which each reaction member is applied or released, the rate at which each is pressurized or exhausted, and the relative timing of the apply and release of the members.

To improve shift feel during gear changes, a band is often released while a multiple-disc clutch is being applied. The timing of these two actions must be just right or both components will be released or applied at the same time, which would cause engine flare-up or driveline shudder. Several other methods are used to smoothen gear changes and improve shift feel.

Clutch packs sometimes contain a wavy spring-steel separator plate that helps smoothen the application of the clutch. Shift feel can also be smoothened out by using a restricting orifice or an accumulator piston in the band or clutch apply circuit. A restricting orifice in the passage to the apply piston restricts fluid flow and slows the pressure increase at the piston by limiting the quantity of fluid that can pass in a given time. An accumulator piston slows pressure buildup at the apply piston by diverting a portion of the pressure to a second piston in the same hydraulic circuit. This delays and smoothens the application of a clutch or band.