Allison’s hydraulic retarder is basically a vaned flywheel
in the transmission housing. The transmission directs oil into
the retarder housing to absorb the vehicle’s energy through the drive
shaft. The absorbed energy is converted to heat and
dissipated through the vehicle’s cooling system. Resistance to
the flywheel, augmented by stators
on the inside of the housing, delivers braking power to the
driving wheels. More oil in the housing means stronger braking.
And since there’s no mechanical friction or wear to shock the
drive train, you’ll have better control of maintenance costs,
Allison models are available with output retarders
mounted on the output shaft, behind the gearing. They generate
the greatest braking at high drive shaft RPM and work
independent of engine speed or gear ratio.
MCI Service Information - April 2003
Allison™ Transmission Retarder Operation
This tip is being written as an informational tool for the
correct operation of Allison WTEC transmissions equipped with
hydraulic retarders. When used properly, this feature assists in
the deceleration of the coach which also reduces the wear of
The transmission retarder is enabled by a switch on the driver's
console. When enabled, the transmission retarder is activated by
two different methods; either with the application of the
service brake pedal or by releasing the throttle pedal. A
joystick on the driver's console selects the amount of
retardation. Typically, retardation increases as the lever is
moved to the rearmost position, but some coaches have been set
up with maximum retardation in the
foremost position, so refer to the decal next to the joystick
for the specific retarder operation.
Inside the air brake valve are two switches that activate the
retarder as the brake pedal is applied. Retardation increases
with brake pedal travel.
For smooth operation and minimal heat generation, the suggested
method of operation is initial application of the retarder in
the lowest joystick position, and slowly moving the joystick
towards the maximum retardation position as increased
deceleration is required.
An Allison transmission retarder provides secondary vehicle
braking via the transmission. Transmission retarders are
usually more effective than engine brakes at lower RPMs; at
higher RPMs they tend to even out. Transmission retarders
generate a lot of heat and their maximum braking horsepower is
usually limited by the RV’s cooling system. Transmission
retarders are priced at a premium to engine brakes so they tend
to be very costly. Allison
Imagine standing over a 55 gal drum with a boat oar, and you're stirring the oar around and around. When there's only air in the drum, it's easy to move the oar. Now, fill the drum with a viscous fluid like oil, and try stirring. Not so easy, now. The retarder works the same way. through a mechanical connection, the drive wheels are trying to turn a bunch of little paddles in a fluid. Since energy can be neither lost or gained, rolling motion is converted to heat through shear, and dissipated into the air. Steven Hill
The hydraulic retarder is a most simple device, and has only
one moving part -- a rotor or "paddle wheel." The truck wheels
turn the rotor through the transmission. The rotor throws the
oil in the rotor cavity against the fixed stator vanes, which
resist the oil flow. This makes it harder for the rotor to turn
and correspondingly makes it harder for the truck wheels to
turn, thus slowing the vehicle. The oil does all the "braking"
work, absorbing the heat generated by the braking action. The
oil is circulated to the heat exchanger. When retarder action is
not called for, the valve is closed, the oil is evacuated form
the cavity, and there is no power absorption in the retarder.
Bear in mind the hydraulic retarder is what the name implies --
a retarder, and will not bring the vehicle to a complete stop,
but only retard its forward motion. Since the capacity of the
retarder or its retarding action is directly related to its RPM,
it will be most effective at higher engine and vehicle speeds.
Retarder switch on, Joy stick in zero position (fully forward on
my coach), let off the power, no retard.
Retarder switch on, Joy stick in any position other than zero,
let off the power, the retard to match the Joy stick position comes on.
Retarder switch on, Joy stick in zero position, let off the
power and lightly press the brake, 30% retard engaged. Press
harder, 66% retard engaged; press even harder, 100% retard
I believe the light pressure on the brake that provides 2 psi in
the chamber calls for the 30%, pressing harder to achieve 7 psi in the chamber results in
66% and pressing to increase to 10+psi results in 100% retard.
Theory and Operation of The Jake Brake Engine Brake