1-13. ENGINE THROTTLE CONTROL. The fuel metering in the injectors is controlled electronically. When
engine is shut off the fuel injectors are automatically positioned to the advance fuel position for starting. To
stop the engine, the fuel supply is shut off. The stop control lever is engaged by the electrically-operated shut-
off solenoid which is controlled via the ignition switch. The solenoid is always engaged when the engine is
a. Throttle Control - Driving Mode. The engine speed is controlled electronically from the electronic foot
pedal assembly to the DDEC which electronically controls the fuel injectors.
b. Throttle Control - Firefighting Mode. Variable, stabilized engine speed is required to provide infinite
control of the water pump pressure. To achieve this, an electronic vernier throttle control (located on the pump
control panel) is connected to the DDEC to regulate engine speed during firefighting operations.
1-14. TRANSMISSION AND DRIVE TRAIN
a. Transmission. The transmission is a four-speed automatic unit providing four forward gear ranges, neutral
b. Controlled Traction Differential. The tandem rear axles are both provided with Controlled Traction
Differentials (differential locks). The differential locks incorporate a friction plate assembly (clutch) which is
under constant spring pressure. When engaged, the differential lock prevents one wheel from slipping in
relation to the other wheel on the same axle. When disengaged, conventional differential action for normal road
conditions is restored. When engaged, the differential lock will slip sufficiently for the vehicle to negotiate turns
in a normal manner.
c. Differential Lock Control. The differential locks are engaged and disengaged by air-operated shift units. Air
flow to these units is controlled by a selector valve in the cab. The differential locks can be actuated at any time
except when one or more wheels are spinning.
1-15. AIR SYSTEM. In this paragraph, only that part of the air system comprising air supply and storage, and
driving accessory operation is covered. The air brake system is described in paragraph 1-16.
a. Air Supply and Storage System.
(1) Refer to Air System Schematics in Appendix G. Filtered air is supplied from the engine air box to the
compressor. Here the air is compressed and discharged via the drier into the supply reservoir.
(2) The air flow from the supply reservoir to the three rear service, front service, and isolated emergency
spring brake release reservoirs is restricted by pressure protection valves. These valves isolate the
supply reservoir until sufficient pressure is available to permit vehicle operation. When sufficient
pressure is obtained, pressurization of the remaining reservoirs takes place. The check valves prevent
backflow from the reservoirs once these are charged.
(3) The supply reservoir is open to the rear service reservoir whenever the ignition switch is set to ON.
(4) To provide quick pressurization of the rear service and front service reservoirs immediately after start-
up, the supply reservoir is maintained fully charged and isolated when the ignition is OFF.
(5) While the system is charging, the air flowing through the air drier is dehumidified. T he moisture is
precipitated and collected in the bottom of the drier. When the compressor unloads, the discharge air
opens a valve in the bottom of the drier and the accumulated moisture is blown out by a backflow of
pressurized air. This air, still hot from compression, also removes residual moisture from the drying
(6) Pressure gauges and low pressure indicator lights for the front and rear reservoirs are provided in the