Fourth gear overdrive operation is controlled by the
powertrain control module (PCM). The module operates
an overdrive solenoid on the valve body to cause
a 3-4 upshift. An overdrive switch, in the instrument
panel, allows the driver to select, or turn off the overdrive
feature. The switch must be in the ON position
before a 3-4 upshift will take place.
The overdrive switch does not operate the overdrive
solenoid directly. Instead, the switch acts as a
signal device that tells the PCM when overdrive has
either been selected, or turned off. When the switch
is turned On, the PCM records the signal; it then energizes
the overdrive solenoid when vehicle speed
and sensor inputs are at required levels.
Speed, temperature, and throttle position are
among the electrical signals supplied to the PCM to
operate the overdrive solenoid on the valve body. The
solenoid contains a check ball that opens and closes a
vent port in the 3-4 shift valve feed passage.
The overdrive solenoid and check ball are not energized
in first, second, third or reverse gear. The vent
port stays open diverting 2-3 valve line pressure
away from the 3-4 valve.
The overdrive solenoid is energized by the PCM.
The solenoid check ball closes the vent port when the
solenoid is energized. This allows line pressure from
the 2-3 shift valve to act directly on the 3-4 valve.
Line pressure on the 3-4 valve moves the valve to
upshift position. This exposes feed passages to the
3-4 timing valve, quick fill valve, 3-4 accumulator
and ultimately to the overdrive piston.
Line pressure through the timing valve moves the
overdrive piston into contact with the overdrive
clutch. The boost valve provides increased fluid apply
pressure to the overdrive clutch during the 3-4 upshift
and during fourth gear operation.
The overdrive piston engages the overdrive clutch
by pressing directly against the clutch pressure plate.
The overdrive clutch also disengages the direct clutch
during 3-4 upshifts.
As fluid pressure extends the overdrive piston, the
piston contacts the direct clutch hub pressing it rearward.
This action compresses the direct clutch spring
relieving spring load on the clutch pack. The clutch is
disengaged once spring load is relieved.
The 3-4 accumulator cushions overdrive clutch engagement
to smooth the transition into fourth gear.
The accumulator is charged at the same time as apply