compartment. The manual subtransmission is used in
provides four forward gear ratios and sufficient strength
conjunction with a manual or an automatic transmission.
for heavy-duty use. In this case, the subtransmission
uses a planetary gearset with hydraulically actuated
b. Automatic. The automatic subtransmission is
holding elements whose operating principles are much
used by some manufacturers, not as an auxiliary
the same as the drive train of an automatic transmission
transmission, but as a unit that becomes an integral part
(para 23-5). The shifting of the subtransmlssion is
of the main transmission. In this configuration, a basic
integrated into the control system of the main
light-vehicle type automatic transmission design is
transmission and shifting is programmed so that both
integrated with a subtransmlsslon.
transmissions function as one.
Section III. OVERDRIVES
the rear of the transmission case.
26-5. Purpose. A transmission overdrive (fig. 26-4)
transmissions, an overdrive is obtained by the gear ratios
provides a gear ratio less than 1:1. It reduces engine
provided in the transmission.
wear by requiring less revolutions of the engine for a
given mileage than required if direct drive is used, and
26-6. Configurations. The overdrive unit sometimes Is
reduces gasoline consumption by providing a more
suitable gear ratio for high speeds on level roads. When
Installed in the vehicle as an integral part of the
In operation, the overdrive reduces the engine rear-axle
transmission, much like a subtransmission (para 26-4).
gear ratio by approximately 30 percent. A freewheeling
In other applications, the overdrive unit is located
device usually Is Incorporated in the overdrive, which
between the transmission and the driving axle, In series
also helps to save gasoline. The overdrive usually Is a
with the drive shaft. The configuration Is similar to an
separate unit bolted to
auxiliary transmission (para 26-1).
Figure 26-4. Overdrive Unit