Kotouč gearboxes racing transmissions
Kotouc gearboxes
Racing transmissions

Understanding the Valve Body 1/2

The Valve Body of an automatic transmission contains a complex series of passages controlled by valves to direct fluid flows. We will look how this functions. Part 1 of 2

The valve body consists of an upper valve body, a lower valve body, a manual valve body and various covers. The body halves are separated by a separator plate which contains openings that control the flow of fluid between valve circuits. The valves control fluid pressure and switch fluid from one passage to another. Hydraulic circuits extend to the transmission housing and are connected either by direct mounting or through oil tube passages.

The valves are a precision fit to their bore in the body, and their position in the bore is determined by a balance between spring tension and hydraulic pressure. Hydraulic pressure within the valve body will vary based on the throttle position or pressure modulating valves. In the case of a non-ECT transmission, pressure also varies based on vehicle speed through the governor valve.

In order to understand what the many valves do in the valve body, they have been separated by function as listed below:

  • Pressure Control Valves
  • Shift Control Valves
  • Timing Valves
  • Pressure Modulating Valves

Pressure control valves

Pressure control valves regulate hydraulic pressure within the transmission. Hydraulic pressure is required to lubricate and remove heat from the fluid. Pressure is also necessary to apply the clutches, brakes, and bands that hold planetary gear components of the transmission. There are times when high pressure is necessary and other times when it is not. The primary concern with high pressure is that engine power is lost and excessive heat is generated. Heat brakes down the transmission fluid and robs it of its properties. Additional load on the engine affects fuel economy, so by regulating pressure, less load is placed on the engine.

The Primary regulator valve adjusts the pressure from the oil pump to all the hydraulic circuits in the transmission. The purpose of the valve is to reduce engine load and power loss. High pressure causes hard shifting and creates more heat, reducing fluid life. By reducing pressure, less power is required to rotate the pump and less heat is generated.

Pressure has a direct effect on the holding force of clutches and brakes. It should be higher when accelerating the vehicle and lower as the vehicle picks up speed.

The output of the valve is called “line pressure”, the highest oil pressure in the transmission. The position of the primary regulator valve is determined by throttle pressure, line pressure and spring tension. Spring tension pushes the value up for higher line pressure. Line pressure is routed to the top of the valve and counters spring tension to reduce line pressure. The overall effect is a balance between line pressure and spring tension.

At the base of the valve, throttle pressure is applied to push the valve upward, increasing line pressure. The greater the throttle opening, the greater line pressure becomes as the pressure regulator valve bleeds off line pressure from the oil pump. This is why adjustment of the throttle cable results in a change in shift feel sue to the change in line pressure.

Line pressure is also increased when reverse gear is selected. Line pressure from the manual valve is directed to the bottom of the valve pushing it upward, increasing line pressure by as much as 50%.

The Secondary Regulator valve regulates pressure to the torque converter and lubrication pressure. Spring tension pushes the valve upward to increase converter pressure. Converter pressure acts on the top of the valve to create a balance between it and spring tension. In some applications, throttle pressure is used to assist the spring in increasing converter pressure. Increased secondary regulator pressure provides for a firmer application of the lock up clutch under higher torque conditions.

The Oil Cooler Bypass valve prevents excessive pressure in the circuit to the oil cooler. The circuit is a low pressure system which routes oil through the cooler in the tank of the radiator and back to the sump of the transmission. The valve is spring loaded in the closed position and opens when pressure exceeds the spring rate.

The Pressure Relief valve regulates the oil pump pressure so that is does not rise above a predetermined maximum value. A calibrated spring is used to control the pressure by holding the valve against its seat.

Governor valve is found on all non-ECT transmissions. It is mounted on the output shaft of the rear wheel drive transmissions or is driven from the drive gear in the transaxle transmission. It balances the line pressure routed from the manual valve and the centrifugal force of the governor weights to produce hydraulic pressure in proportion to vehicle speed. The greater the speed of the output shaft, the greater the governor pressure.

Below 10mph, the centrifugal force is low and line pressure entering through the drilled passage in the valve to the base of the valve pushes the valve upward blocking the line pressure passage and opening the drain at the top land.

As vehicle speed increases, the weights move outward and the governor valve is pushed down by the lever of the inner weights. The governor valve position is balanced between centrifugal force acting on the lever at the top of the valve and governor pressure at the base of the valve.

As the governor rpm increases (middle and high speed) the outer weight movement is limited by the stopper of the governor body. Increased governor pressure acting on the base of the valve works against spring tension. With increased rpm’s, the centrifugal force of the inner weight and spring tension places additional force to push the valve down.

Throttle valve pressure is produced in response to the throttle opening angle. When the accelerator pedal is depressed, the downshift plug pushes the throttle valve upward by means of the spring, creating throttle pressure. The throttle valve supplies throttle pressure to each shift valve and acts in opposition to governor pressure. This is why throttle cable adjustment affects shift timing in non-ECT transmissions.

Throttle pressure also affects line pressure either directly or through throttle modulator pressure. Hydraulic pressure affected by throttle opening is directed to the base of the pressure regulator valve to increase line pressure when engine torque is increased. Additional line pressure serves to provide additional holding force at the holding devices to prevent slippage.