THEORY OF OPERATION
This work package provides a functional description of the major systems and explains how the systems of the
120M Motor Grader work together. The 120M Motor Grader is an all-purpose, wheeled machine used for spreading
and evening various types of granular material (dirt, stone, sand, etc.). The 120M is powered by an in-line, six-
cylinder, direct-injected, turbocharged and intercooled diesel engine. Hydraulically operated implements allow
blade positioning for forward or backward grading, ditch or embankment grading, and snow removal. A scarifier
allows loosening of compacted material prior to grading. Front leaning wheel and frame articulation provide
The machine contains the following systems:
1. Engine. The engine is a liquid-cooled, in-line 6-cylinder, 4-cycle, turbocharged engine. The VHP (variable
horsepower) Plus engine develops 138 to 173 hp at 2,000 RPM and 693 lb-ft of torque at 1,400 RPM and runs
on JP8 or No. 1 or 2 diesel fuel. The engine delivers power to the transmission.
2. Powertrain. The powertrain consists of the transmission, differential, and bevel gears; final drive and tandems;
and the rear axle. The manually shifted transmission is an electronic clutch pressure control, or ECPC
transmission with eight forward speeds and six reverse speeds. The transmission is electronically controlled
by an electronic control module (ECM) and hydraulically actuated.
3. All-Wheel Drive (AWD). The AWD system utilizes charge and piston pumps along with piston motors to drive
each front wheel. The AWD can be used together with the rear drive wheels or by itself to move the machine.
4. Hydraulic and Steering System. The hydraulic system is a proportional priority pressure compensated
(PPPC) system. The PPPC system is a load-sensing system. The PPPC system uses a load-sensing pump for
the implement and steering in order to perform the pressure compensation function. The implement control
valves have an internal flow compensator. This flow compensator performs the proportional priority function.
The main implement ECM controls the steering.
5. Braking and Hydraulic Fan System. The brakes and hydraulic fan use a common pump to supply these
systems with hydraulic pressure. A pump control valve adjusts pressure according to the load of either the
brakes or the hydraulic fan.
6. Secondary Steering. The secondary steering system provides an emergency backup for the failure of the
steering control valve and a failure of the steering pump. The secondary steering system operates under three
modes: The secondary steering pilot valve is activated, the secondary steering pump is activated, or the
secondary steering pump is activated and the secondary steering pilot valve is activated.
7. Implement System. The machine is equipped with an implement system that is electronically controlled and
hydraulically actuated. The machine is equipped with fully electronic left and right joysticks. The joysticks
enable the operator to send commands to each implement ECM. In order to control the systems, the machine
is equipped with two implement ECMs.
8. Air Conditioning and Heating. The air conditioning system is a closed-circuit orifice tube that uses the
refrigerant, which is under pressure, to cool the operator compartment. The heating system uses the coolant
from the engine cooling system to heat the operator compartment.
9. Monitoring System. The monitoring system includes an indicator and gauge display and a messenger
display. Both are used to alert the driver of the machine to normal and abnormal operating conditions. The
Messenger monitoring system continuously monitors all machine systems. The Messenger consists of both
software and hardware components. The messenger communicates with the ECMs of systems via the Cat
Data Link. The messenger can indicate warnings that have occurred in the ECMs of other systems.