Today’s motor homes are coming with bigger engines and ever increasing horse power ratings as buyers demand more performance and the ability to tow heavier loads. Probably less advertised or thought of is controlling this mass while descending steep grades. During long downhill runs, the use of the normal service brakes only, can result in excessive heat build up and eventual brake fade, or failure. Fortunately, there are other methods or features that can be used to reduce the use of the service brakes. Let’s look at each.
Large class A gasoline powered coach. Operators gear their automatic transmissions to lower ranges and use the engine as a speed reducer. This works fairly well, as gas engines, with their throttle butterfly, have more resistance than the non restrictive intake of a diesel engine. Additionally, light duty transmission type retarders and after market exhaust brake kits are some of the other methods now being seen. Keep in mind, should you wish to add an exhaust type brake on any vehicle, be sure the transmission is capable of torque converter lock up or similar. There are transmission kits that may be available for your particular model to accomplish this, if needed.
Mid range diesel pusher type A coaches (26k lbs. to 34k lbs.) mostly are equipped with a Pac Brake, similar to the above photo. This was designed and made by Jacobson and is generically called an exhaust brake. The Pac brake consists of a butterfly like valve within the exhaust, that closes when braking is desired. This causes back pressure on the gases that are being forced through the exhaust system and create an engine drag. The engine’s resistance increases with R.P.M., therefore the transmission should be sequentially geared down to maintain the highest engine speed within a safe range. The downshifting is done automatically for you on most, if not all, coaches, and is usually programmed to seek as low as 2nd gear. To activate the Pac Brake, the following must be present. The Pac Brake activation switch must be turned on, the cruise control must be in the non-engaged mode and the throttle position must be at zero (0). Depressing the accelerator pedal while the exhaust brake is operating will cycle the auxiliary brake on and off, which may, in some driving circumstances, be beneficial. There is now an improved version of the popular Pac Brake, the PRXB. This new technology automatically increases the back pressure as the engine RPM’s falls giving it substantially more braking force throughout the RPM range. This is also an upgradable feature on existing Pac Brakes for around $800.
Upper mid to heavy motor coaches are usually equipped with full Jake Brakes. These may be one, two or even three speed brake models. The most common are the one and the two speed, or “Low” and “High”. Unlike an exhaust brake’s method of restricting the exhaust flow, the Jake actually alters the valve train’s internal sequence to emulate a compressor.
A single speed Jake uses all the combustion chambers or cylinders, which on most motor coaches is six. A two speed employs six for “High” and three for “Low”, while a three speed uses six, three and two for “High, Medium and Low” respectively. Jake Brakes work very well and are capable of producing higher stopping horse power than exhaust brake type.
Hydraulic, high horse power transmission retarders are not found on many coaches, but, never the less, are very effective. These would probably only be found on large heavy rigs and conversion type units. The Allison hydraulic retarder, for example, basically uses a vaned flywheel within the transmission housing. Oil is directed into the vanes which absorb the vehicles energy and converts it to heat. The engine’s cooling system then disperses the heat. This is generally the most expensive and effective method for supliment braking.
Compression type brakes are a very important feature, particularly on diesel pushers. The heavy vehicle weight causes the speed to increase at a rapid rate if not checked on steep down hill grades. The service, or foundation brakes, are not capable of being dragged or steadily used during long descents without creating excessive heat. This heat will eventually cause brake fade and may lead to total brake failure. Therefore it is important to know the braking features of your vehicle and how to best employ them. Always refer to the owner/operator manual for proper use and maintenance.
Next week, we will be looking at practical applications and using compression type braking. Also we will have some useful tips, and as usual, we hope to hear from you with your comments.
Until Then Lug_Nut