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TM 55-1740-200-14
OPERATING INSTRUCTIONS
reverse  speed.  S e c o n d through fifth gears are
2-1. INTRODUCTION.
synchronized. Gear selection is made manually by the
gearshift lever located in the cab.
2-2. This chapter contains operating instructions for the
Type MB-4 towing tractor. The principles of operation
2-10. TRANSFER CASE. The transfer case transfers
precede the operating instructions to provide easier
transmission output to the front and rear axles with gear
understanding of the tractor operation.
reduction. The transfer case  i s equipped with a
differential which may be manually locked in or locked
2-3. PRINCIPLES OF OPERATION.
out, depending on road conditions. When one axle loses
traction the differential action may be locked out which
2-4. The operating principles and basic fundamentals
provides the effect of locking the output shafts together
with which the operator is familiar have been omitted.
to transmit identical rpm to each axle. Differential action
O p e r a t i n g procedures of the more complicated
is controlled manually by a lever in the cab and should be
components and systems are explained in the following
used only under slippery road conditions. Because of the
paragraphs.
differential, the vehicle is in 4-wheel drive at all times.
2-5. ENGINE. The engine operation is the same as any
2-11. AXLES. Propeller shafts connect the identical
standard, flat-head, six cylinder, internal combustion
front and rear axles to the transfer case. Each axle is
engine.
equipped with a standard differential and heavy duty
steerable type wheels. The wheels encase the wheel brake
2-6. FUEL SYSTEM. Engine fuel is provided by a
assemblies and provide a mounting surface for the tire
standard fuel system and regulated by a velocity type
assembly.
governor. The governor is an integral part of the
carburetor and regulates fuel supply by closing the
2-12. STEERING SYSTEM. The hydraulic steering
throttle valve when fuel-air mixture velocity exceeds the
system (figure 2-1) is powered by the engine driven
governor setting.
hydraulic pump (1) which pumps fluid from the reservoir
2-7. HYDRAULIC TRANSMISSION. The hydraulic
(2) directly to the front control valve (9). The front
t r a n s m i s s i o n  transfers torque  to the mechanical
control valve directs fluid to the front cylinder (8) when
transmission through an integral fluid coupling. During
actuated by the steering wheel (3) and bypasses all other
normal driving the hydraulic transmission input and
fluid to the flow divider (5). The flow divider sends a
output rpm are the same. During heavy pulling the output
maximum of 30 psi to the rear control valve (6), all over
rpm is much slower than the input rpm. Under these
30 psi is returned to the reservoir. The rear control valve
conditions the hydraulic transmission has multiplied
directs fluid to the rear cylinder (7) when actuated by the
torque to transfer full engine power to the transmission
rear wheel steering lever (4). When the steering lever is
and prev!ent engine stall. The hydraulic transmission fluid
released, the rear wheels automatically return to the
is cooled by circulation through a heat exchanger located
straight ahead position. Although the front and rear
in the radiator.
control valves are hydraulically connected, front and rear
control are completely independent.
2-8. CLUTCH. Fluid under pressure is available to the
clutch at all times. When the solenoid control switch,
2-13. BRAKE SYSTEM. The hydraulic brake system
located on the gearshift knob, is pressed the solenoid
(figure 2-2) is a standard hydraulic system equipped with
opens a spool valve. With the valve open, oil circulates
a vacuum booster unit (6). The booster unit is connected
through and disengages the clutch to allow manual
to the intake manifold to obtain the vacuum necessary
selection of transmission gears.
for proper operation. When the brake pedal (3) is
depressed the master cylinder (2) hydraulically opens the
2-9. TRANSMISSION. The transmission is a mechanical
air intake valve on the booster unit. When the valve
synchro-mesh unit with five forward speeds and one
opens, air enters the booster unit through the air cleaner

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