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Old 04-05-2014, 08:22 AM   #1
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Reprogramming the Jake Brake

Our coach is built on the Freightliner XC-R chassis. It has the 400 hp Cummins ISL and the Allison 3000 6-speed transmission. This is a very common chassis-engine-transmission combination.

The ISL has a true Jake Brake - a 2-stage compression brake - as opposed to an exhaust brake. However, what I am about to describe ought to work on ANY chassis with an engine brake and the Allison transmission.

When using the Jake Brake, I found the braking action to be very aggressive. The Allison transmission is programmed to shift down to the next lower gear as soon as it can and not over-rev the engine in the next lower gear. It targets 2nd gear and will shift down all the way to 2nd as the speed drops. Those downshifts were dramatic and always caused a heavy braking surge as the downshift took place. That surge is hard on the drive train...and the passengers!

To say it another way, the Jake Brake is programmed to deliver 100% braking effort every time you use it. But there are times when less than 100% braking effort is desirable. Slowing for an interstate off-ramp, driving in stop and go 35 mph in surface street traffic or descending a gradual grade are all situations where the Jake Brake is useful, but not at 100% braking effort.

I took my coach to the local Allison transmission shop and had them reprogram the TCM to stay in 6th gear when the Jake Brake is energized. Now, instead of the computer controlling when the downshifts take place, I control the downshifts via the Allison transmission controller. The job took only 30 minutes and cost about $60. They connect a computer to the diagnostics port and make ONE change in the programming, changing "second" to "sixth" in the Jake Brake program.

What a huge difference!
Now I can actually use the Jake in many situations where I previously could not. Depending on the gear I choose, the braking effort can be gentle or aggressive, as I need. Slowing for an interstate off-ramp is now a joy, using 5th gear in the slow-down. No more slamming into 4th gear at 45 mph! Driving in stop and go traffic at 35 mph in 4th gear provides just the right gentle braking effort needed to speed up and slow down with the traffic. I don't need to use the service brakes until it's time to actually stop.

If you do nothing with the shift controller, the transmission will still downshift to lower gears as you slow down, but it does so at much slower speeds than before. The end result is nice, gentle braking right down to 15 mph where the Jake disengages.

To those of you who claim that doing this will diminish the braking effort available in an emergency stop, you may have a small point. However, an emergency stop is an event lasting only a few seconds. From the time you hit the service brakes to the point where the Jake is providing meaningful braking effort is...a few seconds! By the time the Jake fully engages, the emergency is over.

If you are in the habit of placing your transmission in "D" and forgetting about it, then this mod may not be for you. I don't drive our coach that way. I take a more active role in driving over the road, and often use the transmission controller to choose the correct gear for the particular situation I am in at the moment. The computer is NOT always right in this application. It can't be...it cannot anticipate anything, it can only react to something that has already happened. You have eyes that can look ahead and see things that are about to happen. You CAN anticipate, whereas the computer cannot.

This was the best $60 I ever spent on our coach. In my opinion, they should come from the factory programmed this way.
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Old 04-05-2014, 08:49 AM   #2
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I'm not sure how the motorhome is wired but in trucks the engine brake has a selector switch that you can switch from low med and high 2cyl, 4cyl, or all 6cyl giving you the choice of how much brake you want without resetting transmission settings.
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Old 04-05-2014, 09:03 AM   #3
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Thanks Rich!! I have wanted to get mine reprogrammed so it's a little less aggressive and slamming down into 2nd. If they do it for $60 it's worth it!!
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Old 04-05-2014, 09:09 AM   #4
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Originally Posted by Fkrel View Post
I'm not sure how the motorhome is wired but in trucks the engine brake has a selector switch that you can switch from low med and high 2cyl, 4cyl, or all 6cyl giving you the choice of how much brake you want without resetting transmission settings.

The OP stated that the coach has a low and high setting. Until you drive the ISL you would not understand what he is saying.

I don't even use the high setting because of the stopping power on the low side.

I can tell you that the computer controlled downshifts can be very violent at times because I have the ISL too.

I have wanted to do the same as the OP for a years now. I have thought about going to 5 gear for the downshifts but 6 is most likely best?

You can always mainually downshift if you want more stoping power.

I am sure a few others out there with the ISL will chime in.
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Old 04-05-2014, 02:16 PM   #5
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Yes, I hate the standard 2nd and prefer not to use it most of the time. When I get around to it I will change it to 4th or 5th. In a true emergency the normal time delays aren't going to add a lot towards reducing stopping difference.

Another possibility is to get it programmed to "Latch" mode which doesn't act when you just ease off the throttle but comes in if you touch the brake pedal. That way you can just coast without having any auxiliary braking at all.
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Old 04-05-2014, 07:01 PM   #6
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I don't own an ISL. Mine's a CAT C7 with an Exhaust Brake. Similar situation in that I found with the Exhaust Brake activated it downshifted to '2'. I did not like it, as it was too aggressive.

I had mine reprogrammed to '4' when the Exhaust Brake is actuated, which is a 1:1 drive ratio. In this engine/Allison 3000 tranny situation, it works just right for me.

I agree with the above posts. Allison will reprogram your downshift selection for you when the engine/exhaust brake actuates. '2' is way too aggressive.

I love how mine drives and downshifts now with the Exhaust Brake activated.

I also had it reprogrammed to 'Latch' mode, and I find that much more drivable as well.

P.S. Allison did my reprogramming for free. Just have to talk nice to them.. If they do charge you $60 it's a bargain.
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Old 04-05-2014, 10:52 PM   #7
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Originally Posted by Fkrel View Post
I'm not sure how the motorhome is wired but in trucks the engine brake has a selector switch that you can switch from low med and high 2cyl, 4cyl, or all 6cyl giving you the choice of how much brake you want without resetting transmission settings.
Coaches with the ISL have a 2-stage Jake Brake, low and high. Low uses 3 cylinders, high uses all 6. There is a 3-position switch on the panel, Low, High and Off.

Between the transmission reprogramming and the choice of low or high, I now have enough choices that I can come up with just the right braking effort for any situation. I love it!
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Old 04-06-2014, 08:58 PM   #8
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Rich,

Most folks aren't aware that if you select "1" (First Gear), the exhaust brake will engage. I have used it on mine on an 8% grade in Colorado. I also used it for testing and setting back pressure on a PacBrake when working with PacBrake on an ISL issue.

I am assuming when manually selecting first gear with a Jake (engine brake,) the same thing will happen. Try it and let us know.
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Old 04-06-2014, 10:43 PM   #9
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Most folks aren't aware that if you select "1" (First Gear), the exhaust brake will engage.
Is that sitting parked with the engine running, parking brakes on and selecting 1st gear.

If so, that would be very useful when checking and lubricating the pacbrake.
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Old 04-07-2014, 06:56 PM   #10
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Rich, Most folks aren't aware that if you select "1" (First Gear), the exhaust brake will engage. I have used it on mine on an 8% grade in Colorado. I also used it for testing and setting back pressure on a PacBrake when working with PacBrake on an ISL issue. I am assuming when manually selecting first gear with a Jake (engine brake,) the same thing will happen. Try it and let us know.
Yes...I can engage 1st gear and still use the Jake Brake, at VERY slow speeds. You could come down the side of the Washington Monument in 1st gear with the Jake Brake on high and never touch the service brakes! ;-)
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Old 04-07-2014, 07:26 PM   #11
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I have a cummins ISB with the Allison transmission does anyone know what kind of brake I may have? I notice awkward shifting and delayed engine braking and find this topic interesting.
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Old 04-07-2014, 08:54 PM   #12
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I had the same issue - preselect gear was too low. It was 4th in my case. In addition, the Jake on my M11 was wired as a one speed (high). I had Cummins change the one speed to a two speed (high-6 and low-3) and had Allison reprogram the ECM to preselect 5th. Like Rich, I really like the versatility this setup gives me. 5th gear and low Jake are just right for those long easy downhills.
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Old 04-07-2014, 09:24 PM   #13
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I have a cummins ISB with the Allison transmission does anyone know what kind of brake I may have? I notice awkward shifting and delayed engine braking and find this topic interesting.
The only brake that can be fitted on the ISB is an exhaust brake, not a compression one.
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Old 04-07-2014, 09:28 PM   #14
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Thanks.
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