engine is running, the pump is producing pressure. There
are several types of hydraulic pumps. These will be dis-
cussed in a later chapter.
The hydraulic pump draws transmission fluid from the
bottom or side oil pan, which is usually called the sump.
The pump pressurizes the fluid for use by the other
hydraulic system components, Figure 1-27. Modern
transaxles with two oil pans may have more than one
pump. In systems that contain three pumps, a three-gear
scavenger pump removes the fluid from the bottom pan
and pumps it into the side pan. The primary and secondary
pumps pick up the oil from the side pan and pump it to
the other hydraulic system components. On very old
vehicles, a pump was installed on the output shaft and it
produced pressure only when the vehicle was moving.
Output shaft pumps are no longer used.
The pressure regulator is installed in the outlet line
from the pump and controls overall transmission pressures,
usually called line pressures.
If the pump output becomes too great, the pressure
regulator valve opens, dumping oil back into the oil pan.
Once the pressure returns to normal, the valve closes. A
pressure regulator is shown in Figure 1-28. The pressure
regulator may be installed in the pump housing or in the
valve body.
24 Automatic Transmissions and Transaxles
MN 7
models
Roller bearings
Roller
bearing
Thrust
washers
Thrust
bearings
Thrust washers
Roller
bearings
Thrust
bearing
Thrust
bearing
Roller
bearing
Figure 1-26. Modern transmissions and transaxles contain many bearings. Some bearings are round types that serve the same pur-
pose as bushings. Others are flat rings that keep moving parts from rubbing against each other. (General Motors)