DESCRIPTION AND OPERATION
Page 45
If the engine brake is active, it will automatically
be deactivated by depressing either the
accelerator or the clutch, or if the engine speed
drops below 900 rpm. As soon as all of the
engine brake “active” conditions are again true
(foot off the accelerator, for example), the engine
brake will again be activated. The engine brake
can be deactivated simply by returning the
dash-mounted control switch to the OFF position.
The exception to this is if the vehicle has an
automatic shifting manual transmission. In this
application, the engine brake is automatically
activated by the transmission even if the dash
switch is turned OFF. This feature is needed to
slow the engine RPM down rapidly, which allows
this type of transmission to shift properly.
The engine brake may be used simultaneously
with the vehicle service brakes to achieve the
greatest vehicle slowing power. Releasing the
vehicle service brake pedal does not deactivate
the engine brake.
When the engine brake is enabled at the same
time the V-MAC cruise control is in use, the
engine brake automatically activates when
necessary to slow the vehicle to the cruise set
point speed. One of two possible options can be
selected:
앫 The engine brake is activated 2 mph above
the cruise set point (default mode).
앫 The engine brake is activated as soon as
cruise control commands fueling to zero
(vehicle has just reached cruise set point
and fuel has been turned off).
The engine brake’s ability to control maximum
vehicle speed under cruise control is limited to
the selected retarding power of the engine brake.
If the engine brake dash-mounted control switch
is set to the LOW position, only half of the
available braking power is used. If the
dash-mounted control switch is set to the HIGH
position, the cruise control invokes full engine
brake power.
Deactivating the cruise control function does not
disable the engine brake.
The MACK PowerLeash™ engine brake may be
activated or deactivated by other vehicle systems
such as ABS and Headway control systems.
Refer to the literature concerning these systems
for additional information.
PowerLeash™ Mechanical System Operation
During an engine braking event, the inboard
exhaust valve opens at the top of the
compression stroke to provide engine braking. To
accomplish this, hydraulic actuators are
incorporated into the exhaust rocker arms, and a
special “engine brake” profile is incorporated into
the exhaust lobes of the camshaft.
The engine brake hydraulic actuator is controlled
by engine oil pressure. Oil under pressure is
supplied to the actuator through the “control”
gallery, in the rocker shaft, and the flow of oil is
controlled by the engine brake solenoid located
on top of the shaft. A second gallery in the rocker
shaft, the “constant” oil supply gallery, supplies a
constant flow of oil to the inlet and exhaust rocker
arms for lubrication.
When the engine brake is activated (dashboard
control switch in either “low” or “high” positions, a
no fuel [0% throttle] condition and the clutch
pedal released), the engine brake solenoid
energizes and directs the flow of “control” oil to
the hydraulic actuators. When oil enters the
actuator upper cavity, the actuator hydraulically
locks with the plunger extended to take up the
lash from the valve train. As cam rotation
continues and the lifter roller reaches the
transition point from sub-base to base circle, the
upward movement of the push rod opens the
inboard exhaust valve due to the hydraulically
locked actuator plunger.
5-111.bk Page 45 Monday, July 10, 2006 2:26 PM
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