Jacobs Engine Brake Operation
WARNING
Do not use the engine brake if road surfaces are
slippery. Using the engine brake on wet, icy, or
snow-covered roads could result in loss of ve-
hicle control, possibly causing personal injury
and property damage.
If the engine brake is engaged when the trans-
mission is in neutral, the braking power of the
engine brake can stall the engine and result in
loss of vehicle control, possibly causing personal
injury and property damage.
To engage the engine brake, the dash switch must
be in the LO or HI position and both the clutch and
throttle pedals must be fully released. To disengage
the engine brake, depress the throttle or clutch
pedal, or move the dash switch to OFF. Use the LO
position when driving on flat, open stretches of road.
If you need to use the service brakes to slow down,
switch to a higher position until there is no need for
the service brakes.
WARNING
The engine brake must be disengaged when
shifting gears. The clutch must be used if the
dash switch is in the LO or HI position. If the en-
gine brake is engaged when the transmission is
in neutral, the braking power of the engine brake
can stall the engine, which could result in loss of
vehicle control, possibly causing personal injury
and property damage.
Since the engine brake is most effective at rated en-
gine speed, gear selection is very important. Gearing
down the vehicle within the limits of the rated engine
speed makes the engine brake more effective. Rec-
ommended engine braking speed is above 1800 rpm
and below the rated speed.
IMPORTANT: Maximum retarder performance is
obtained when you use the lowest possible gear
without exceeding the recommended engine
braking speed.
"Control speed" is the speed at which the engine
brake performs 100 percent of the required downhill
braking, resulting in a constant speed of descent.
The control speed varies, depending on vehicle
weight and the downhill grade.
Below a set engine speed (set at the factory to 700
rpm), the engine control unit (ECU) will not engage
the engine brake. The engine brake will not work
below the set engine speed.
For faster descent, select a higher gear than that
used for control speed. Service brakes must then be
used intermittently to prevent engine overspeed and
to maintain desired vehicle speed.
IMPORTANT: When descending a grade, re-
member that frequent use of service brakes
causes them to become hot, which results in a
reduction of their stopping ability. Grade descent
speed should be such that the service brakes
are used infrequently and that they remain cool,
thus retaining their effectiveness.
A driver may descend slower than control speed by
selecting a lower gear, one that will not overspeed
the engine. The engine brake retarding force will
then be sufficient to cause vehicle deceleration. Oc-
casional deactivation of the engine brake may be
necessary to maintain the designated road speed
under these conditions.
WARNING
Using the engine brake as a primary braking sys-
tem when the service brakes are operable is dan-
gerous. This can cause long, unpredictable stop-
ping distances, possibly resulting in personal
injury or property damage.
Whenever vehicle braking is required, the engine
brake may be used with the service brakes. There is
no time limit for operation of the engine brake.
1.
After the engine is warmed up and the vehicle is
in motion, move the paddle switch to the desired
position, LO or HI. Depending on the engine
model, LO will provide 1/3 or 1/2 of the full brak-
ing capacity of the engine. HI will provide maxi-
mum engine braking.
2.
The engine brake activates when the dash
switch is in LO or HI position and the driver’s
feet are removed from both the clutch and
throttle pedals. If it fails to activate, take the ve-
hicle to an authorized Freightliner dealer for ser-
vice.
Engines and Clutches
7.17
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