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Exhaust brake vs. Retarder

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I have been getting information about the main differences between the exhaust brake and engine brake. Now I am confronted by the term "retarder."

Once again, I wish to remind everyone that I am not American and I have known about the use of this term as to describe brake performance of Foretravel coaches.

Where I live, a retarder is considered a device totally different from an exhaust brake. It is a mechanical supplemental device that can be used only on dry roads other than rainy, just humid or snowy or else the truck/RV would lose control and would turn its back around. The exhaust brake is a device that, generally speaking, makes it possible to use the compressed gasses of combustion as a power brake as to avoid having to use the service brake or at least to help and support the service brake. I am not a technician, but this is more and less the difference.

In other words, in the U.S., is a retarder the same as an exhaust brake or what? Thanks in advance for any comments or technical support.

Steve

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Hi Steve,

In addition to the retarder, the engine/exhaust brake should not be used on wet/slippery pavement. The purpose of a retarder, engine or exhaust brake is the same: It is to help slow the RV without use of the service brake. They may accomplish this in technically different ways, but the objective is the same.

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In the US, the term retarder refers to an electromagnetic device within the transmission which magnetically puts drag on the transmission. This device is used on trucks, busses and could be used on RV's but is seldom found on any but the most expensive RV's.

An engine brake is accomplished by changing the timing of the valves so that the exhaust valve is closed when it would normally open. This causes the piston to compress the air within the engine cylinder. Since the cylinder is tightly sealed, this is a very effective way to slow the vehicle. Some engine brakes have several levels applying the breaking to three or six cylinders to accomplish different braking power as needed.

An exhaust brake impedes the flow of the exhaust from the engine in the exhaust pipe. Since the exhaust pipe seal isn't as positive as the seal in the cylinders in the engine, this brake is somewhat less effective.

I have driven large RV's with the engine brake and with the exhaust brake. I find the engine brake to be a much more effective device than the exhaust brake. Both will do the job, one just does it better than the other.

The reason that the retarder, engine brake or exhaust brake should not be used with wet/slippery surfaces is that these apply breaking forces to the roadway in a relatively uncontrolled way. When the retarder/engine/exhaust brake kicks in, it can happen suddenly and you have no control over how hard it slows the vehicle except to turn off the device.

With the service brakes you apply the force you want, gently or strongly. The service brakes also are moderated by the anti-lock braking (ABS) devices on most newer motor homes. The ABS prevents wheel lock up. You can push the brakes to the floor and the ABS system will keep the wheels turning so you still have steering control and avoid a skid. With the ABS keeping the wheels turning you will stop faster than if the wheels lock up. So the service brakes have means to prevent skidding where the retarder, engine or exhaust brake do not.

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There are three different "auxiliary" braking systems used on different diesel engines.: Exhaust Brakes, Engine Compression Brakes and Variable Geometry Turbo Brakes.

Exhaust brake: literally a "flap" which closes off exhaust flow just downstream of the turbo in the exhaust system. This causes back pressure (55 PSI on ours) which generates braking power. With the Allison transmission, it is usually tied with downshifting of the transmission to the "pre-selected" gear (usually 2nd or 4th). Think of it as a potato stuffed in the tail pipe.

Engine Compression Brake (aka: Jake brake) The exhaust valves are opened as the pistons reaches TDC (Top Dead Center) on the compression stroke after the engine has done the "work" of compressing about 18 volumes of intake air to 1 volume . If the exhaust were not let escape by the compression brake's opening the exhaust valves (i.e. coasting with brake off), the "compressed air" would mostly be returned as power to the engine forcing the piston back down. With the Jake brake on, the engine works to compress air in the cylinder, then the air is let out. This generates quite a lot more braking force than an exhaust brake. The smallest engines to offer an engine compression brake are the Caterpillar C9 and Cummins ISL.

Variable Geometry Turbo: The vanes reverse or aperture closes (depends on engine manufacturer) to create back pressure with much the same effect as an exhaust brake.

Another alternative used by Foretravel and some over the road busses us TRANSMISSION RETARDERS (see Wayne's link) on their Allison transmissions. Transmissions with retarders will have an "R" suffix such as 4060R. They generate even more braking HP than exhaust or compression brakes, but are more expensive.

Brett Wolfe

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So,

does anyone know how this is activated on my specific coach? And what does it sound like/feel like?

I just bought a 93 Monaco Dynasty with a PAC brake. Has a button for left foot.

the private couple said they never used it and don't know how. My neighbor who drives a fire truck has a much more sophisticated set up.

I tried every combination of pressing before I brake, while I'm braking, with no brake applied. I hear a sound of air pressure releasing (?) And I know the unit is there mounted on the exhaust. But I can not discern any slowing.

Isn't it fun to school up the newbie nimrod :-)?

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So,

does anyone know how this is activated on my specific coach? And what does it sound like/feel like?

I just bought a 93 Monaco Dynasty with a PAC brake. Has a button for left foot.

the private couple said they never used it and don't know how. My neighbor who drives a fire truck has a much more sophisticated set up.

I tried every combination of pressing before I brake, while I'm braking, with no brake applied. I hear a sound of air pressure releasing (?) And I know the unit is there mounted on the exhaust. But I can not discern any slowing.

Isn't it fun to school up the newbie nimrod :-)?

Welcome to the FMCA Forum.

With a 1993 coach, it is likely that you have the Allison MD3060 6 speed transmission (unless very early 1993 model) with TWO display shift pad.

If this is your set up, the left display indicates the "pre-select" gear. So when you hit "D", it shows "6". If wired properly, the exhaust brake, when you hit the switch AND the throttle is closed the left display will go from "6" to the "pre-select" gear for exhaust brake activation which is generally "2" or "4". Yes, it can be any gear, but those are the two most commonly chosen gears by chassis makers. AND you should feel the transmission start downshifting as soon as you turn on the switch and close the throttle. At the same time, the right display will begin showing lower gears with downshifting occurring as soon as it will not over-speed the engine in the next lower gear.

If closing the throttle and hitting the exhaust brake switch does not change the transmission display/cause transmission downshifting, suspect an electrical issue.

If the display changes and transmission does start downshifting, but you can't really tell a difference in braking, here is an easy test: Use the down arrow to select a gear that activating the exhaust brake switch would choose (lock it in the gear that the right display indicates). NOW, turn on then off and back on the exhaust brake. Feel for a difference in braking HP. You have eliminated the transmission as a variable. If no difference in braking, the exhaust brake may not be receiving the electrical signal to close at the solenoid controlling air to the exhaust brake, OR the brake may be physically seized from disuse/lack of lubrication.

If you do not have this transmission, please let us know what drivetrain you have and we can go over it's operation.

Brett Wolfe

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If the transmission does downshift automatically when the pacbrake is activated, check to insure the valve is activated. It should be moved a full 90 degrees to block exhaust gas.

On my rig everything seemed to work but be pacbrake didn't slow the vehicle any more than manual downshifting. The culprit was an exhaust leak through the exhaust manufold gasket. Replacing the gasket fixed the problem.

But is just learned TODAY, that the shop that did the work re-used the manifold bolts and also didn't lock the lock washers properly.

I learned my lesson. Have the specialist at the Cat engine people do the job, not an independent.

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I doubt that using the exhaust brake on wet roads is nearly the concern for a motor home that it is for a truck trailer combination (18 wheeler). The exhaust brake provides braking effort through the drive axle which in a class A motor home is in the rear of the vehicle. With a Jake Brake I doubt that there would be sufficient braking to lock up the drive axle except maybe on ice. With a truck/trailer combination the braking effort is only on the drive axles of the truck and none on the trailer. With a heavily loaded truck/trailer the trailer may be more than 3 times the weight of the truck and it indeed will push the truck down the road on a wet or slippery surface very well leading to loss of control. The Jacobs Brake website does caution against use of their exhaust brake on wet or slippery surfaces, however there is no differentiation between types of vehicles. In fact the type of vehicle is not mentioned. Any type of braking, service brake or exhaust brake needs to be done very carefully if the road is slippery. In my case if the roads are very slippery I prefer to park my motor home and wait for conditions to improve.

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