Transmission Pump / Torque Converter Diagnosis

By: Dan Wilson

Before any transmission noise/vibration diagnosis is done the engine and drive train
should be ruled out. Check for worn u-joints that can cause a noise at all vehicle
speeds. Check for worn mounts that can cause a vibration or cause the transmission to
touch the frame or body of the vehicle.

Transmission oil pumps provide fluid pressure for the transmission and the torque
converter while the valve body regulates and controls the flow to provide shifting. Most
pumps today are variable displacement. Gear and rotor type are fixed displacement
while the vane type are variable displacement.

The difference is in a fixed displacement pump the pressure increases with rotational
speed sometimes providing unneeded pressure while in a variable displacement
pump the pressure is allowed to decrease with lesser demand. This saves energy and
decreases output without affecting speed.

Transmission pumps are driven directly from the engine through the torque converter.
The torque converter housing is attached to the engines crankshaft/flex plate and the
torque converters pump drive hub or internal drive attaches to the oil pump. This
causes the pump to operate when ever the engine is running. This is why the pump will
make a noise in all gears including park and neutral. A restricted or clogged
transmission filter will also make the same whining noise in a transmission pump.

This is important when diagnosing between a transmission pump and a torque
converter. A transmission pump makes noise in all gears and is directly related to
engine speed. A torque converter will make the most noise when the vehicle is placed
in gear and the wheels held stationary. This is because the turbine is held stationary
while the housing is spinning from engine rotation (working the bearing); while in park
and neutral the noise is gone because the whole unit is spinning. This noise usually
decreases as the vehicle begins rolling forward.

Diagnosing a transmission is the same as anything else and often ends up being a
process of elimination. Begin with the basics and remember if something isn’t moving
it’s not making a noise. This is important to remember when diagnosing planetary gear
sets and the drive train. When a vehicle is placed in drive with the wheels held
stationary the entire drive train is also held stationary. This can help in eliminating these
items as a potential cause of transmission noise.

Transmission Pump/Torque Converter Diagnosis

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