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Copyright © 2009 ATSG
AUTOMATIC TRANSMISSION SERVICE GROUP
Technical Service Information
Figure 9
OIL LEVEL CONTROL
EFFECTS OF INCORRECT FLUID LEVEL
The "Float" prevents
expanded or overfilled
ATF from reaching the
gearset chamber.
ATF above
the full level.
The oil level is controlled with a "float", that is an
integral part of the valve body assembly, as shown in
Figure 9. The "float" is positioned so that it can plug
the opening between the transmission sump and the
gearset chamber so that the rotating gearsets do not
create foaming, aerate the fluid, or force it out the
breather. With normal oil levels, the lubricating oil
which flows constantly out of the gearset, flows back
to the sump through the bottom case opening. When
the oil level rises (as fluid is heated), the oil presses
the "float" against the opening. The "float" therefore
seperates the transmission sump from the gearset
chamber. The lubricating oil which continues to flow
out of the gearsets is thrown against the case wall and
returns to the sump through the upper opening, as
shown in Figure 9.
A low fluid level allows the oil pump to take in air
along with the fluid. Air in the fluid will cause oil
pressures to be low and pressures will develop slower
than normal.
If the transmission is overfilled, the gearsets will
churn the fluid into foam. This aerates the fluid and
can cause the same conditions that occur with low
fluid levels. Foaming also causes fluid expansion
which can result in fluid overflow from the vent or fill
tube. Fluid overflow can easily be mistaken for a leak
if inspection is not done carefully.
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