2005 Commercial Driver’s License Manual beyond the selected gear (unless the governor rpm is exceeded. It is very important to use this braking effect when going downgrades. 2.3.4 – Retarders Some vehicles have "retarders" Retarders help slow a vehicle, reducing the need for using your brakes. They reduce brake wear and give you another way to slowdown. There are four basic types of retarders (exhaust, engine, hydraulic, and electric. All retarders can be turned on or off by the driver. On some vehicles the retarding power can be adjusted. When turned "on" retarders apply their braking power (to the drive wheels only) whenever you letup on the accelerator pedal all the way. Because these devices can be noisy, be sure you know where their use is permitted. Caution. When your drive wheels have poor traction, the retarder may cause them to skid. Therefore, you should turn the retarder off whenever the road is wet, icy, or snow covered.