Engines brake, depress the throttle or clutch pedal, or move the dash switch to OFF. 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 main- tain desired vehicle speed. IMPORTANT: When descending a grade, remem- ber 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 retain- ing their effectiveness. WARNING The engine brake must be disengaged when shift- ing gears.
The clutch must be used if the dash switch is in the LO or HI position. If the engine brake is engaged when the transmission is in neu- tral, the braking power of the engine brake can stall the engine, which could result in loss of vehicle control, possibly causing personal injury and prop- erty damage. A driver may descend slower than control speed by se- lecting a lower gear, one that will not overspeed the en- gine. The engine brake retarding force will then be suffi- cient to cause vehicle deceleration. Occasional deactivation of the engine brake may be necessary to maintain the designated road speed under these condi- tions.
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. Maximum brak- ing occurs with the use of the lowest gear that does not exceed the rated engine speed. A rule of thumb for gear choice is to select the gear that normally would be used to climb an approaching downhill grade. Generally, this same gear can be used with the engine brake for a controlled descent of the hill. The engine brake can be used to stop a vehicle if the service brakes quit working.
By energizing the engine brake as soon as a service brake problem is apparent, a retarding effect is applied to the vehicle. As grade conditions permit, the driver can progressively down- shift, using the engine brake in each gear. Eventually, the engine brake will stop the vehicle. "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. 4.28