Pierce Enforcer, Operator'S Manual

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Driving the Vehicle / 5-55
OPERATION
5-29.4 Hydraulic Retarder
The hydraulic retarder (Allison
®
Transmission Retarder or equivalent) uses the viscous property of transmission fluid
to retard the driveline. When activated, transmission fluid is introduced into the retarder housing and energy is
absorbed into the fluid through the opposing action of spinning vanes. This retards the vehicle through the rear
wheels and adds heat to the transmission fluid. This heat is dissipated through the transmission cooler and radiator.
Retarder Brake Capacity
The maximum braking capacity of Allison retarders is determined by the transmission Electronic Control Unit (ECU)
calibration. This capacity is specified at the time of purchase to provide the low, medium, or high capacity setting.
This capacity setting indicates the maximum retardation force with the system fully applied. Low settings will provide
auxiliary braking force with moderate cooling system heat loads. High settings will provide aggressive auxiliary
braking, but may limit the amount of time that the retarder can be applied before exceeding the cooling capacity of
the vehicle.
Control Systems
The control system may be specified in many versions. The most popular control systems are described below.
Overheat Warnings
Because the retardation force is created by the churning action of transmission fluid within the retarder housing, a
significant amount of waste heat is generated. This heat is transferred to the radiator coolant via a shell and tube
heat exchanger. The radiator is thus ultimately required to dissipate heat generated both during vehicle acceleration
(engine and transmission heat load) and during braking (retarder heat load). During heavy use and high ambient
Observe the following cautions when driving a vehicle equipped with a hydraulic retarder.
Apply and operate the retarder only when the engine is at closed throttle.
Observe transmission and engine temperature limits at all times. Select the lowest possible
transmission range to increase the cooling system capacity and total retardation available.
In the event of overheating, decrease the use of the retarder; use service brakes.
Observe the retarder “over-temp” light to ensure that the vehicle control system is functioning
properly.
Refer to the Allison Transmission operator’s manual for additional cautions.
Throttle 100% The retarder applies fully whenever the throttle pedal is released.
Throttle 50% - Brake 50% The retarder is applied to half its capacity when the throttle pedal is released,
and applied to its full capacity when the brake pedal is depressed.
Throttle 33% - Brake 67% The retarder is applied to one third its capacity when the throttle pedal is
released, and applied to its full capacity when the brake pedal is depressed.
Hand Lever Control The retarder is applied when the throttle pedal is released. The percent of
application is controlled by a six-position hand lever.
Foot Pedal The retarder is applied when a dedicated foot pedal is depressed. Retardation
force increases proportionally as the pedal is depressed farther.