Engines Road speed usually decreases when the exhaust brake is applied during a descent. When the ve- hicle is carrying a heavy load or the grade is ex- tremely steep, you may need to apply the service brakes occasionally. independent of, the service brakes. The amount of re- tarding or braking force is controlled by the driver. Pacbrake Operation Do not expect a retarding effect similar to sudden hard application of the service brakes. The ex- haust brake retards the vehicle with a smooth braking effect. The Pacbrake is controlled by the exhaust brake switch and the throttle pedal.
All applications are additionally affected, controlled, or governed by engine speed through transmission gear selection. The Allison MD3060 transmission has automatic downshifting when the exhaust brake is requested. See the information on the "Allison World Transmission" later in this chapter. During a descent, the tachometer usually shows a drop in rpm depending on the grade and the vehicle load. On some applications, when the Pacbrake is in use, it may be necessary to check that the cruise control is not set and that the throttle is in the idle position. Depending on the grade and vehicle load, you may or may not feel the retarding force acting against your body when the brake is applied.
The retarding force of the brake may not always be noticed, but it is actually preventing the vehicle from going much faster. On some applications the engine, transmission, cruise control, and the Pacbrake exhaust brake may electroni- cally interact with each other, which automatically oper- ates their functions. Following are some examples of the programmed options that may be available with the cruise control in the on position: Shutting Off the Engine Make sure the exhaust brake is turned off before shut- ting off the engine. The coast mode engages the Pacbrake when the service brake is applied and disengages when the service brake is released.
Pacbrake ® Exhaust Brake The Pacbrake exhaust brake is intended as a supple- ment to the service brakes and will not bring the vehicle to a complete stop. The Pacbrake will assist in the con- trol or reduction of road speed in conjunction with, or The latch mode engages the Pacbrake when the service brake is applied and the Pacbrake re- mains on after the service brake is released. The Pacbrake is disengaged when another input (de- pressing the throttle or clutch, engine speed 4.31