Retard switch

randallb wrote:
It is obvious that a transmission retarder is seriously misunderstood. It is a primitive torque converter attached internally to the output shaft. When activated the transmission redirects oil into a series of enclosed vanes (thus the primitive converter analogy) and as these vanes shear the oil it acts like a brake and applies retardation to the output shaft. The ones equipped with a joy stick allow you to set the retarder's braking level, by increasing the amount of oil pumped in to the retarder, much like setting up a trailer brake controller. The ones equipped with an activation button or switch usually rely on the program in the ECM to control oil volume and pressure in to the retarder. They are a very effective way of controlling downhill speed but the "weakness" is the amount of heat absorbed by the trans fluid which is then dissipated through the engine cooling system. Why can this system be used on a diesel? A diesel cooling system is designed to move a large amount of heat which is generated when large amounts of fuel are dumped in to the engine under high load conditions. Conversely, when coasting downhill absolutely no heat is produced and dumping the retarder heat in to the cooling system works. A diesel with an open thermostat will bottom out any temperature gauge when going downhill. The downshift can be manual or controlled by the ECM but the retarder is the primary downhill speed controller. You can, with sufficient retarder size and oil volumes, control downhill speed while remaining in high gear. The only restriction to the size of the retarder is the case size of the transmission.
Randy

Of course your assuming your just coasting downhill. If your ascending hills and descending in high ambient temps one could possibly overheat the cooling system or push it close to the engines cooling limit. Adding the heat build up from the retarder is what is described to monitor as mentioned by the people who built the system.

Telling someone to just use high gear and the retarder in such a situation could get them into a whole heap of trouble real fast. IMHO using the correct gear ( usually the same as you went up in) will help keep the engine temps and trans temps in a safe zone. Shutting off the retarder in a high heat scenario and selecting proper gearing is what's suggested by the manufacturer