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.
types of oil pumps are installed in automatic transmissions: the gear, rotor,
and vane. The gear-type pump is the most commonly used.
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
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.
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.
valve body is machined from aluminum or iron castings. Many precisely machined
holes are located in the valve body to accommodate the various valves.
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.
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.
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
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.
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 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.
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
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.
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.