An electric retarder is spliced into the drive shaft and hangs down below the body of the vehicle. Looks like a big electric motor and works on the same general principals as an electric motor, only functioning backwards to resist the turning of the drive shaft, not adding to it (in electrical terms I barely understand, this is called "counter EMF"). Usually this device has eight poles, or electromagnets, surrounding the spinning armature. The poles can be activated two at a time (across from each other) in a crescendo until all 8 poles are energized, providing ever increasing braking effort. A four-position control wand on the steering column activates the retarder through a series of relays in a control box.
A transmission retarder is an add-on component of a heavy duty truck/bus automatic transmission. Works just like the torque converter of your car but produces braking, rather than acceleration. In a a torque converter, two "fans" or vanes face each other, closely spaced, in a chamber filled with transmission fluid. When the engine accelerates, it spins the input fan, which drags the fluid along with it, which in turns drags the second fan into motion, providing a fluid coupling between the engine and the automatic gearbox. In the transmission retarder, however, one of the fans is permanently fixed in place. The rotating fan, connected to the output shaft of the transmission, tries to turn the fixed fan via the transmission fluid, but it resists, causing a braking action. A hand controller introduces increasing amounts of transmission fluid into the retarder, providing increasing levels of braking action.
Neither form of retarder has any effect on the operation of the service brakes of the vehicle, although supplemental controls operating off the service brake system can be used to control a retarder. Neither form of retarder can bring a vehicle to a complete halt, but they can slow you down to a crawl. The action of both retarders generates a lot of heat; the electric retarder is cooled by fans built into the armature, while the transmission retarder is cooled by the transmission cooler built into the engine radiator. That is heat
not going into the service brakes, meaning that the service brakes remain cool and ready for emergencies or for the final stop. The normal sequence for using the retarder on a mountain grade is to select the appropriate gear, use the retarder as needed to maintain speed, and use the service brakes only for an emergency or the final stop. They work; I've used both kinds for 20 years on 6-mile-long, 10% grades.
I know, guys, TMI!!! Sorry--old school bus driver trainer. Back to narrow-gauge steam!
Mike
