THEORY OF OPERATION - CONTINUED
BRAKE AND HYDRAULIC FAN SYSTEM - CONTINUED
This allows pressurized oil from front and rear brake accumulator ports to flow through upper and lower
passages and upper and lower valve spool passages. Oil then flows through front and rear service brake
outlets to apply service brakes. Simultaneously, pressurized oil flows into upper and lower cavities and
through upper and lower valve spool passages. Oil pressure at service brakes is equal to oil pressure in
upper and lower cavities.
Oil pressure in lower cavity and force from lower spring act to balance lower valve spool against force of
pressure in upper cavity, just as pressure in upper cavity creates a force against bottom of upper valve
spool. Upper valve spool is balanced by upper spring force at top of spool.
Force from upper and middle springs is balanced by force applied to pedal. Upward movement of upper
and lower valve spools causes front and rear brake accumulator ports to be covered. Pressure in indepen-
dent valve sections creates feedback force, allowing operator to modulate pressure to service brakes.
Upper and lower valve spools balance between ports for hydraulic oil tank and ports for service brakes to
maintain brake pressure until pedal position changes. A change in pedal position means more or less brake
pressure is necessary.
If piston moves down to apply more compression on upper spring, upper and lower valve spools will move
to allow more pressure oil to flow to front and rear brake outlets to service brakes. This results in a higher
oil pressure in upper and lower cavities, which is necessary to maintain balance in upper and lower valve
Transmission Neutralizer Override Control Rocker Switch.
Transmission neutralizer override switch is located on switch panel on upper right side of cab (TM 5-3805-
291-10). Switch is a momentary rocker switch. Transmission neutralizer override switch allows an opera-
tor to disable functioning of transmission neutralizer.
N OT E
The transmission neutralizer's default status is active. Whenever the machine is turned on, the transmis-
sion neutralizer will be enabled. Indicator light on dash panel display will illuminate when transmission
neutralizer is disabled.
Press top of switch to disable transmission neutralizer. When transmission neutralizer is disabled, use of
left brake pedal will not disengage transmission. Left brake pedal will function as a conventional brake
pedal. Transmission neutralizer indicator light on dash panel display will illuminate (TM 5-3805-291-10).
If transmission neutralizer has been disabled, press top of switch to activate it. When neutralizer is active,
operator may depress left brake pedal to engage transmission neutralizer. Transmission neutralizer will
disengage transmission as service brakes are applied. This allows an operator to use accelerator pedal to
increase engine speed for better hydraulic response. There is also a gain in fuel efficiency and a reduction
in transmission oil temperatures.
Left Brake Pedal Position Sensor.
Left brake pedal position sensor sends an input to powertrain ECM, continuously updating ECM on left
brake pedal position. ECM records changes in position, then activates appropriate solenoid valve.
Left brake pedal position sensor is a PWM sensor, continuously generating a PWM signal. Duty cycle var-
ies in proportion to pedal position. ECM receives PWM signal from position sensor and measures duty
cycle to determine pedal position. Sensor signal frequency is constant at 5,000 Hz. Machine electrical sys-
tem provides +battery voltage to pedal position sensor for operating power.