transmission retarder

[Bill1170]

This may have been discussed before but if so I missed it. While driving on interstate hwys or good 4 lanes ,without much stopping , do most drivers keep the retarder on or off. Does it seem to effect mpg?
It should not have been called a transmission retarder but I guess a Jake brake, I have had retarders on previous coaches.

Bill C

[Jon Wehrenberg]

The two (Jake and retarder) are different. I can generalize by saying when I had a Jake on my previous coach I turned it on as necessary.

I have a retarder now and I turn the "master" retarder switch on every time I am driving, and use the steering column mounted control when needed. By doing that I have retarder assistance standing by and ready for whenever I apply the brakes, and I can assist the cruise control by manually controlling retarder operation on the steering column mounted control.

Neither the Jake or retarder add "drag" or affect your mileage when driving. The Jake turns your engine into an air compressor and then dumps the compressed air at the end of every compression stroke when it is called upon to work.

The retarder acts like a torque converter when working by resisting the flow of transmission fluid with the amount of retardation varied by the control. While both are slowing you down, and can be seen as preventing the bus from taking full advantage of the energy stored by a bus starting to descend a hill, both are actually just using some of the energy from free wheeling down a hill to regulate the speed of a descent.

Whatever you have feel free to make use of it. There is no down side.

I would recommend you monitor the transmission temps when using a retarder, and although Allison says the transmission computer limits the temps, and that temps up to 300 degrees are OK, they also prove by their fluid change schedule that the resultant high temps from a retarder cut fluid life cycles in half.

There are no restrictions or limits relating to the use of a jake brake.

[dreamchasers]

Bill,

I can share the way I use the Jake Brake. On my Country Coach, a Jake Brake enable button is selectable, on or off. I normally like the Jake brake helping slowing the coach, so I keep it on or enabled most of the time. When the accelerator pedal is depressed, sending a fuel demand signal to the ECU, the Jake is enabled, but not actually engaged, slowing the vehicle down. When the fuel demand is off and cruise control off, the jake will engage.

Perhaps others can offer their feedback.

Hector

[dale farley]

Because I like the assistance of the Jake brake, I usually set it on "Low" while in urban/suburban areas and set it to "High" when in hills/mountains. I don't usually use it on the interstate if I have my cruise engaged, but I make sure I can quickly switch it on if needed.

[Bill1170]

Thanks for the replies,I ask for the info and then have been out of town.
I do like the way the jake brake works and so I leave it on most of the time and am glad to hear it does not effect mpg.

[Larry W]

I do not know if leaving my Jake on affects my mileage, I never turn it off. In fact I leave the jake on high most all the time. At cruise rpm (1500 to 1600) in 6th gear the jake does not do a lot to slow the bus down. Here in the west we have numerous roads that are up and down grade. I set the cruise to about 1600 rpm and when starting down a grade that is steep enough to make the bus speed up I will shift into 5th gear and most times the cruise will maintain it's set speed down hill. When slowing to exit the freeway for a rest stop I keep down shifting to the next lower gear when the engine speed drops to 1400 rpm. This allows very little use of the brakes. In urban areas I use a gear that will maintain a rpm range of about 1600 to 1700 rpm. When some pulls out if front of me the jake will start to slow the bus before I can put my foot on the brake pedal.
Just how I do it, do what fits you.

[Jon Wehrenberg]

The jake will work best at high RPM. It turns your engine into a very poor efficiency air compressor, so the more RPMs the greater the force of holding you back. A jake has a theoretical retarding force equal to about 1/2 your rated horsepower. The only downside to the use of any retarding device is it is dangerous on slippery surfaces.

If we all wanted to get ridiculously detailed we can say any type of retarding device does cost us mileage, but its cost in terms of fuel is insignificant compared to the safety benefits we derive. By using a braking device, either the transmission retarder or the jake, we are using some of the energy ot took to crest a hill and throwing it away in the form of asking it to slow the coach on the downhill side. We are covering the ground going down the hill at a slower speed rather than allowing the bus to coast down at whatever speed it can obtain, thus reducing the engine running time, and having a much higher speed at the bottom delaying the need to step back on to the gas pedal due to the momentum. This of course ignores speed limits and safety and just theoretical.

In a plane where there are no speed limits applicable this concept works.

But forget about that very small cost of using the jake or retarder. Their use helps us have much greater control of the momentum our very heavy buses have and provide a very valuable braking assist when needed.

[dale farley]

Jon, Among other things you mentioned, the slippery surfaces is a good point. I know the manual says don't use the Jake on slippery surfaces (rain, ice, etc.).

[Jon Wehrenberg]

I've got a lot of experience driving all sizes of vehicles on slippery surfaces. I can say with absolute certainty that if a jake or retarder is used on a slippery surface that drive axle could lose traction if the retarding force exceeds the traction available. That means out of the 8 wheels, you are now left with 4 to provide braking force and directional control.

Our braking systems are precisely balanced by Prevost engineering varying the brake surface area, the brake chamber sizes, and adjustments to the slack adjuster arm lengths to evenly apply braking forces. When you add a retarder to just one of the axles you lose that balance and that one axle is locking up with brake application. A locked wheel has virtually zero braking or steering force.

This is most evident in the snow belt, but I have learned here in the south that long periods between rain showers produce the same results when it rains. The oils that have dripped onto road surfaces for weeks between rainfalls create ice like conditions when it rains. So my advice would be to be very cautious with the use of the jake or retarder when the potential for slippery road conditions exists.

There are other snow belt drivers here and maybe they can add to these comments.

[jack14r]

If the coach has ABS and the ABS activates my Prevost manual states it will disconnect the retarder,I don't remember about the jake.