Hydraulic Components

Hydraulic Components


The transmission pump is driven by the torque converter shell at engine speed. The purpose of the pump is to create fluid flow and pressure in the system. Pump pressure is a variable pressure, depending on engine speed from idle to full throttle. At some speeds, pump pressure exceeds transmission requirements and must be controlled by the pressure regulator valve.

Three types of oil pumps are installed in automatic transmissions: the gear, rotor, and vane. The gear-type pump is the most commonly used.

Many people misunderstand the concept of how pumps move liquids. It is often thought that the action of a pump sucks liquid in, but this is not true. What actually happens during pump operation is that the liquid moves from an area of high pressure (atmospheric pressure) to an area of low pressure (below atmospheric pressure).

The spinning pump generates a low pressure area at its inlet chamber. A vent in the transmission admits atmospheric pressure to the transmission oil pan or sump. This higher atmospheric pressure acting on the transmission fluid in the oil pan forces the fluid into the low pressure area of the pump at its inlet chamber. The fluid is picked up by the pump gears, which move it to the pump's outlet chamber. Here the fluid is squeezed out of the pump.


The valve body can be best understood as the control center of an automatic transmission. The purpose of the valve body is to sense the load on the vehicle's engine and drivetrain and the operator's driving requirements.

The valve body is machined from aluminum or iron castings. Many precisely machined holes are located in the valve body to accommodate the various valves.

The valve body is made of two or three main parts. Valve bodies and related parts are bolted together and the bolts set to specifications with a torque wrench.

Some valve bodies are bolted directly to the transmission housing so the valve body becomes a part of the housing assembly. Internally, the valve body has many fluid passages called worm tracks.


The purpose of a valve is to start, stop, or direct and regulate fluid flow. Generally in most valve bodies, three types of valves are used: check ball, poppet, and, most commonly, the spool.

Check Ball Valve

The check ball valve is a ball that operates on a seat located on the valve body. The check ball operates by having a fluid pressure or manually operated linkage force it against the ball seat to block fluid flow. Pressure on the opposite side unseats the check ball. Check balls and poppet valves can be designed to be normally open, which allows free flow of fluid pressure, or normally closed, which blocks fluid pressure flow.

Check balls can be seated while directing fluid flow. Other applications of the check ball have two seats to check and direct fluid flow from two directions, being seated and unseated by pressures from either source.

Pressure Relief Valve Check ball valves can be used as pressure relief valves to relieve excessive fluid pressure. The check ball is held against its seat by spring tension that is stronger than the fluid pressure. When the fluid pressure overcomes the spring tension, the check ball is forced off its seat, which relieves excess pressure. As soon as the opposing fluid pressure is relieved, the spring tension forces the check ball back onto its seat.

Poppet Valve

A poppet valve can be a ball or a flat disc. In either case the poppet valve acts to achieve its purpose, which is to block fluid flow. Often the poppet valve has a stem to guide the valve's operation. The stem normally fits into a hole acting as a guide to the valve's opening and closing. Poppet valves tend to pop open and close, hence their name.

Poppet valves close against a valve seat. In some applications fluid pressure holds the valve closed or seated. When the stem of the poppet valve is pushed, the valve pops open, permitting fluid flow. When the opening force on the stem is released, the poppet valve closes, blocking fluid flow.

Spool Valve

The most frequently used valve in the valve body is the spool valve. A spool valve looks similar to a sewing thread spool. The large circular parts of the valve are called the spools. There are a minimum of two spools per valve. Each spool of the assembly is connected by a stem. The stem is not a precisely machined part of the valve. Adjoining spools and the valve stem form a space called the valley. Valleys form a fluid pressure chamber between the spools and valve body core. Fluid flow can be directed into other passages depending on the spool valve and valve body design.

Precisely machined around the periphery of each valve spool is the valve land. The land is the part of the spool valve assembly that rides on a very thin film of fluid in the valve body bore. The land must be treated very carefully because any damage, even a small core or scratch, can impair smooth valve operation. As the spool valve moves, the land covers (closes) or uncovers (opens) ports in the valve body.

The vertical part of each spool valve forms an area called the reaction area. Forces acting against the reaction area to move the spool valve include spring tension, fluid pressure, or mechanical linkage.

All valve bodies are designed with ports to support spool valve operation. Each spool valve has a fluid inlet port. Aligning with the spool valve's valley, but not necessarily in direct alignment with the inlet port, is the outlet port. If fluid pressure builds excessively, the spool valve moves to open the exhaust port that protects the system's parts. Each spool valve area has a vent to prevent buildup of fluid between the valve assembly and valve body. Since fluids are incompressible, fluid buildup would hinder valve travel.

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