2.0.0 TROUBLESHOOTING TRANSFER CASES
The transfer case is the mechanical unit that splits power between the front and rear
wheels (axles) (
Figure 8-19
). It is attached to the rear of the transmission or transaxle
and receives power from the transmission or transaxle output shaft. Power leaves the
transfer case through two output shafts. In addition to various drive gears, transfer
cases may contain drive chains, a differential assembly, or a viscous coupling. The term
all-wheel drive (AWD) in heavy-duty trucks usually refers to a chassis with a front drive
axle in addition to rear tandem drive axles.
Figure 8-20
shows the location of the
transfer case on a typical truck chassis.
Figure 8-19 - Three-shaft design transfer case.
Transfer cases can transfer drive torque directly using a 1:1 gear ratio, or can be used
to provide low gear reduction ratios of 2:1 additional to those in the transmission. The
drop box design of a transfer case housing permits its front driveshaft output to clear the
underside of the main transmission
Most transfer cases are available with power takeoff (PTO) capability and front axle
declutch. The front axle declutch is used to option-drive to the front axle when
negotiating steep grades or slippery or rough terrain. Both the PTO and front axle drive
declutch are driver engaged by dedicated shift levers.
NAVEDTRA 14050A
8-28