Various combinations of the above factors are used to
26-7. Unit Construction (Fig. 26-5). The basic
produce the three modes of operation that will be
overdrive unit is a planetary gear-overrunning clutch
discussed in the next paragraph.
combination. This combination is necessary in an
overdrive unit because the unit must be able to shift while
26-8. Operation. The overdrive unit is semi-automatic
the vehicle is moving. This contrasts with the sliding
in operation. When a minimum speed is reached
subtransmissions that can be operated only while the
(usually about 25 miles per hour) the overdrive unit
vehicle is not moving. The two factors that control the
automatically will engage if the driver releases the
overdrive modes of operation are:
accelerator pedal momentarily. A control lever also is
provided that locks out the overdrive unit mechanically.
a. The sun gear is locked to or released from the
The modes of operation are:
pinion gear cage. This is accomplished by a mechanical
linkage from the operator's controls that, through a fork
a. Locked Out (Fig. 26-5). The locked-out mode of
and collar, slide the sun gear in or out of mesh with the
the overdrive is used whenever the use of overdrive is
gear teeth cut into the pinion carrier.
not desired. The locked-out mode is selected by the
operator through the movement of an instrument panel
b. The sun gear is held stationary or allowed to
mounted lever. The movement of this lever slides the
rotate freely. This Is accomplished by a solenoid-
sun gear into mesh with the internal gear teeth in the
operated pawl that engages a notched plate that is
pinion carrier, resulting in the two members locking
attached to the sun gear. The solenoid is actuated by an
together. The locking of the sun gear to the pinion carrier
electrical control system (para 26-9).
Figure 26-5. Overdrive in Locked-Out Position.