Engine Braking
Each engine manufacturer uses a specific engine brake for their engine. Regardless of the manufacturer, the engine brake is operated with the principles outlined in this chapter. Refer to the engine manufacturer's operation manual for details of their particular engine brake product.
Warning: To avoid injury from loss of vehicle control, do not activate the engine brake system under the following conditions:
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on wet or slippery pavement, unless the vehicle is equipped with antilock braking system (ABS) and you have had prior experience driving under these conditions;
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when driving without a trailer or pulling an empty trailer;
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if the tractor drive wheels begin to lock, or there is fishtail motion after the engine brake is activated.
Whenever vehicle braking is required on good road conditions, the engine brake may be used in conjunction with the service brakes. There is no time limit for operation of the engine brake. However, an engine brake does not provide the precise control of, and is not a substitute for, service brakes.
Warning: Usage of the engine brake as the primary braking system can cause unpredictable stopping distances, that could result in personal injury or property damage. Service brakes are the primary vehicle braking system.
Two dash-mounted switches control the engine brake:
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The On/Off switch (ENG BRK), is used to turn the engine brake on and off. See Fig.. When the engine brake is on, an amber light will illuminate behind the switch.
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The engine brake intensity switch (HI/MED/LO or, in some cases, HI/LO) controls the amount of engine braking. When the panel lights are on, the ENG BRK legend is backlit in green.
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Engine Brake On/Off
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Engine Brake Intensity Switch
Fig. 1, Engine Brake Switches
Since the engine brake is most effective at rated engine speed, gear selection is very important. Gearing down the vehicle within the limits of the rated engine speed makes the engine brake more effective. Recommended engine braking speed is above 1800 rpm and below the rated speed.
Warning: The engine brake must be disengaged when shifting gears using the clutch pedal. If the engine 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.
"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.
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.
For slower descent, select a lower gear, being careful not to overspeed the engine. Occasional deactivation of the engine brake may be necessary to maintain the designated road speed under these conditions.
If the engine is equipped with both cruise control and an engine brake, the engine brake can operate automatically while in cruise control mode. If the cruise control/engine brake function is turned on in the DDEC VI system programming, the engine brake will come on low when the set road speed increases a few mph (km/h) above the cruise set speed. The maximum amount of braking is selected with the dash switches. When the vehicle returns to the set cruise speed, the engine brake will turn off.
The engine brake will only operate when the accelerator pedal is fully released. Depressing the clutch will also prevent the engine brake from operating.
Vehicles equipped with ABS have the ability to turn the engine brake off if wheel slip is detected. The engine brake will automatically turn back on once wheel slip is no longer detected.
The DDEC VI system will deactivate the engine brake when the engine speed falls below 1000 rpm or when the vehicle slows down to a preset speed, depending on DDEC programming. This prevents stalling the engine.