Hello Guys - After looking over this thread on exhaust brakes, I wanted to jump in and say a few words in hopes for others to better understand how exhaust brakes work. I’ve been RV’ing since the early 80's and I have millions of miles with Seminole Trucking running semis loads from Miami to La in my younger days.
After looking over this thread - It’s clear to see most owners are using their exhaust brake incorrectly, which is why most feel it’s not working properly. If you’re one of those owners waiting to get into Cummins to get the brake checked out, there’s a good chance you’re wasting your time. Exhaust brakes don’t just quit or wear down like service brakes. In fact, your exhaust brake is one of the most reliable components on your engine. Therefore, if you tell the service advisor your exhaust brakes are not holding on inclines, after a costly inspection, he’s just going to just tell you not using the exhaust brake properly. Keep in mind, if it takes 300-400 hp to climb a mountain, a 100-200 hp exhaust brake is not going to maintain a safe speed going back down. Most exhaust brakes fall far below engine HP when it comes to braking power and HP.
Therefore, exhaust brakes are not designed to be used by themselves, they require the drag from a fast spinning engine in a low gear to assist the exhaust brake. In addition, besides the engine spinning and the exhaust brake, you’ll also need occasional use of the service brake to maintain safe rpm and speed going down a steep grade.
Getting back to the braking problem, it’s important for RV owners to note, an exhaust brake is not like an hydraulic or air brake where you can apply more braking, your exhaust brake works at one set level, which is dependent on RPM and speed. Therefore, it’s up to the driver to assist the exhaust brake by first engaging a lower gear to help the exhaust brake maintain speed down a grade.
When you’re about to go down a steep grade, the first thing that must happen is the driver must first shift to a gear that is low enough to hold the coach from over-speeding and reaching redline. If you’re not certain of which gear then start off in 2nd or 3rd gear. Once you find the right gear for the grade and your coasting down the hill with safe RPM, you then apply the exhaust brake to “Help” or maintain that speed and rpm all the way down the hill.
The steeper the hill, the slower your speed will be, forget anyone behind you, put on your flashers and find a speed that can get you down the hill and back home safely. In most cases, your top speed going down a 6-8% grade, is going to be anywhere from 25 to 35 mph. This is why there are 35 mph speed limits for trucks going down the hill. As you go down the hill, keep your foot over the service brake. If your RPM starts to climb towards red line, apply your service brake lightly, to slow the coach back to a safe rpm. If you find yourself having to add service brakes more than 2 times, that means your transmission needs to be downshifted to a “Lower Gear”. If this is the case, firmly apply the service brake to slow the coach so you can safely shift to a lower gear. This lower gear and the exhaust brake should now easily maintain a safe speed (under 35 mph).
Save your Service Brake - Whatever you do, never ride your service brakes down a grade, the brakes either drum or disk will heat up fade, shortly you’ll have no brakes and no way to down shift and you’re in for the ride of your life. If you find yourself using the service brake going down a grade you'll need a lower gear. Unless needed, avoid using your service brake on grades.
Keep in mind, the Allison transmissions processor will always protect the engine and not the driver. By this I mean if you’re going down a hill and the engine and exhaust brakes are not sufficient to maintain a speed under red line, the transmissions processor is going to shift to a higher gear to prevent the engine from over-speeding. When this happens, you will lose both the engine braking and the exhaust brakes. If this happens “Don’t Wait” immediately apply the service brake to slow the coach back down and shift back to a lower gear that will prevent the engine from red lining.
Automatic Down Shifting - Note as you switch on the exhaust brake switch and release the throttle, the Allison’s processor will automatically begin downshifting the engine to create additional drag from the engine. Regretfully, the Allison processor does not always downshift to the right gear. At times it may only downshift to 4th gear when you really need 3rd gear to maintain speed. For this reason, I never use my transmission in Drive when in the mountains, I always climb and descend shifting the transmission manually for best climbing and decent.
Generally speaking including a Jacobs brake, there are three different exhaust braking systems for most RV's. There’s an engine brake, where braking is achieved internally in the engine using a protocol of cylinder and valves timing to create braking. There is a standard exhaust brake, which operates by partially blocking the exhaust downstream of the engine and turbo and finally a turbo brake, using variable geometry vanes in the turbo to create back pressure within the engine to slow the coach. Regardless of which brake system you have, they all work fairly well to “assist” in braking however none of these brakes can provide enough HP to hold the coach when coming down a steep grade, they all need the transmission in a low gear for additional drag from the engine plus the exhaust brake to work together. In addition, all exhaust brakes work best (highest hp) within a certain range of engine RPM.
In the old days in there was no exhaust brakes on RV’s. Back then when going down a steep grade, only the engine and the service brake where used to maintain a safe speed. Not until around 2000 did exhaust brakes begin to surface in some RV’s. With that said, a modern RV with disk brakes and in the correct low gear can still safely go down steep grades without the use of an exhaust brake. A good tip is always maintain your speed with your transmission in a lower gear, then use your exhaust brake, to further assist in controlling the speed of your coach down a grade.
Ride Safe - Mike
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