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.
3.
To obtain maximum retarding, maintain the top
governed speed of the engine through appropri-
ate selection of gears when the engine brake is
in use.
4.
When either the clutch or throttle pedal is de-
pressed, the engine brake is deactivated.
CAUTION
If the engine brake fails to shut off when either
the throttle or clutch pedal is depressed, place
the dash switch in the OFF position and do not
use the engine brake until the throttle or clutch
switch system is repaired. If the engine brake
fails to shut off when the dash switch is turned
off, the engine should be shut down and the en-
gine brake repaired before continuing operation.
Failure to do so could result in damage to the
engine.
Turbo Brake (MBE4000 only)
For high braking output, the Mercedes-Benz
MBE4000 engine can be equipped with an optional
turbo brake. The turbo brake redirects the exhaust
gas flow through the turbocharger to provide 600
brake horsepower.
The turbo brake can be operated either manually or
automatically, through the cruise control function.
Because the charge air pressure is maintained at a
high level during braking, full throttle response is
available immediately, if the operator desires it, with-
out any turbo lag.
The turbo brake is maintenance-free, highly reliable,
and adds virtually no weight to the engine.
Constant-Throttle Valves (MBE4000
only)
To increase braking performance, the Mercedes-
Benz MBE4000 engine can be equipped with con-
stant throttles in each combustion chamber. Small
valves built into the cylinder head allow a small
amount of compressed air to escape through the ex-
haust port during the combustion stroke. The
constant-throttle valves are open during the entire
time that the engine brake is activated. Although
some braking ability is lost because the valves are
constantly open, constant-throttle braking is quieter in
operation than other types of engine brakes.
When both the exhaust brake and the constant
throttles are installed, a three-position switch on the
Engines and Clutches
7.21
