AIR SYSTEM AND BRAKES CONTINUED
When the park brakes have been engaged automatically, the park brake switch must be
reset manually. When the air pressure in the system has returned to the normal operating
pressure, the knob must be pulled upward into the ENGAGED position. Next, push down
on the park brake control in order to disengage the park brakes.
If the air system fails or if the pressure drops below the required pressure in order to disengage the brakes, two
pressure switches for the park and secondary brakes will close. When the pressure switches are in the CLOSED
position, the pressure switches will send an electrical signal to the transmission/chassis ECM. The transmission/
chassis ECM will send an electrical signal to park brake switch. When the park brake switch receives the signal, it
will return to the ENGAGED position and the park brake solenoid valves will close. When the park brake solenoid
valves are in the CLOSED position, the air pressure in the park brake circuit will escape from the quick release and
holding valves. The park brakes are now engaged.
Accessory Air Circuit (Differential Lock)
Air pressure for the accessory air circuit flows from the service brake control valve through the pressure protection
valve to the accessory circuits.
When the differential lock switch is activated, a signal is sent from the switch to the differential lock solenoid valve.
The differential lock solenoid valve will open, allowing pressurized air to flow to the quick release valve. The pres-
surized air opens the quick release valve and air pressure then flows to the differential lock. The differential is now
When the differential lock circuit is deactivated, the differential lock solenoid valve will close. The air in the circuit
will be exhausted through the quick release valve. The differential is now unlocked.
END OF WORK PACKAGE