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Old 04-13-2015, 09:31 PM   #43
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Thanks Marty for the info. I have to go to Vegas to a Detriot allison dealer. two and a half hour drive for me. I'm headig that way in June so will get it checked.
You have a CAT engine, why would you go to a DD shop to get it reprogrammed? Go to a CAT/Allison shop and get it done right.
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Old 04-14-2015, 08:21 AM   #44
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Mr D. Empire tractor in Phoenix AZ told me I have to go to a Detriot/Allison shop to check the software on the trans. Apparently they don't mess with Allison trans.
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Old 04-14-2015, 12:28 PM   #45
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Jeazzes fire up. I told U that anything after 55 mph in 5th 6th or any other gear it will down shift to forth . What more do U want. this getting ridiculous. I would remove my original post if I knew how! I'm going off into the sunset talking to my self. Ha Ha
Jeazzzzzzzzzzzzzzes SYL! You can get as sarcastic as you please and walk in to whatever sunset you like but, I and many on here have been trying to help you with your question of operation of your Jake and related RPMs. But, like myself, many here have repeatedly asked you to answer a very simple question pertaining to the EXACT SPEED at which yours downshifts to fourth. You keep re-iterating the fact that if you're in "5th or 6th" it automatically downshifts to forth when the Jake is applied. That is NOT SPEED, it's simply telling me what gear you may or may not be in.

Well, of course it will, if the engine AND SPEED OF THE COACH are at such lower limits of those upper gears "5th and or 6th" and you allow the Jake to be in operation. But, as many have already TRIED to tell you, the deceleration RPMs can, and most often DO, go considerably higher for JAKE and EXHAUST BRAKE mode than they do for a governed ACCELERATION mode.

And, you've eluded to the fact that you have off-road experience with CAT engine RPMs and high idle operations. Well, that's great but, in reality, it has nothing to do with the way these coaches, and Allison transmissions are programed for operation in these coaches. A diesel is a diesel but, can have varying limits of operation, based on the vehicle/piece of equipment it's mounted in.

Now, for some odd reason, you refuse to answer the simple question of what ACTUAL SPEED yours downshifts to fourth, when using the JAKE brake. Now, one, you DON'T KNOW what speed it happens or, are simply refusing to believe that, it's operating normally and that, it instantaneously increases the RPMs to the upper ALLOWABLE limits of the engine, as per both the engine ECM and, Allison TCM.

If your coach/trans is programmed to use 4th as a target gear then, that's up to you, to keep it that way. Mine, as well as many on here, is programmed to use 2nd as a target gear. Both target gears can be reprogrammed or, changed for the owners individual preferences. I'd just about bet my house that, if and when, you get done with your sunset walk, and you either call CAT/Allison or, just an Allison service center, that they'll ask you: "At what speed does it down shift to fourth"??? Have a nice day.
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Old 04-14-2015, 06:04 PM   #46
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I'm just wondering if the OP means the display immediately goes to 4th when the Jake comes on. My indicator does the same--when I turn the Jake on and let up on the accelerator, the display goes to 2nd, but the tranny does not downshift until the RPM is in a range where it is safe to downshift through the gears to the target gear--in my case, second..
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Old 04-14-2015, 07:55 PM   #47
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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 04-14-2015, 08:13 PM   #48
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The engine brake on the ISX preselects 3rd gear and I have three positions as well as off. I found that I very seldom needed the high position.
If I find I'm slowing down too much I can always feed in a little throttle.
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Old 04-14-2015, 09:37 PM   #49
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The statement that the Jake brake delivers 100% braking every time you use it not correct. If you have a two-stage Jake, you have either 50% or 100% braking. If you have a three-stage Jake, you have the choice of 33%, 67% or 100%. If the Jake on high is too aggressive, you can use a lower setting.
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Old 04-14-2015, 10:51 PM   #50
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The statement that the Jake brake delivers 100% braking every time you use it not correct. If you have a two-stage Jake, you have either 50% or 100% braking. If you have a three-stage Jake, you have the choice of 33%, 67% or 100%. If the Jake on high is too aggressive, you can use a lower setting.

Technically you may be correct, but in a real world application, you miss my point.

Whether you are in the LOW or HIGH position, if the transmission is programmed to downshift all the way to second gear, the downshifts are very aggressive and in most cases, much more than is necessary. By reprogramming the Allison to stay in 6th gear, I control when the downshifts take place. If I want heavy braking, I click the down arrow a couple of times and the tranny downshifts as soon as it can without over-revving the engine in the next lower gear. If I only need gentle braking, such as slowing for an interstate off ramp, I just knock off the cruise or take my foot off the accelerator and I get nice, gentle braking effort without all that aggressive downshifting, which is almost always too much braking, even in the LOW position.

I will say it again...this was the single best mod I have done to our motor home. It was also the cheapest. The improvement in drivability was phenominal!
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Old 04-14-2015, 11:54 PM   #51
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I'm just wondering if the OP means the display immediately goes to 4th when the Jake comes on. My indicator does the same--when I turn the Jake on and let up on the accelerator, the display goes to 2nd, but the tranny does not downshift until the RPM is in a range where it is safe to downshift through the gears to the target gear--in my case, second..
Peralko,
Yes Sir, we all know that the key pad goes to a preselected gear. Some, "2nd", some 3rd" and some, will say 4th. And you are correct that the trans will not downshift 'till a preprogrammed SPEED and RPM are achieved. The OP is not revealing what his speed is when this infamous "fourth gear" down shift happens. Nor is he revealing just what the engine RPM goes to, when all this happens. This is why I've repeated myself and, others have too in trying to find out the EXACT SPEED at which this forth gear drop happens.

As other posts have indicated, there are some slight engine/trans ECM and TCM differences and, not all rigs are the same. And, you CAN have things changed, if you desire.

If the OP is used to farm implement and or, off road grader/loader/tractor etc. engine operations and, the RPMs that those run at, then I can understand why he'd react because the trans and engine are working together to operate correctly which, in many cases, will present a much higher than normal engine RPMs, on the deceleration mode. As I've stated, my C-7 with the Allison MH3000 will hit 2500-2600 while jumping into fourth during a deceleration mode. I'm certainly not fond of that kind of RPM either so, I stab at the brakes a time or two and, that will drop it to a more "normal" diesel RPM.

Oh well, it's no biggie.
Scott
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Old 04-15-2015, 08:01 AM   #52
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The ISL has a true Jake Brake - a 2-stage compression brake - as opposed to an exhaust brake.
Yours does, my ISL has an exhaust brake. And I wonder if that's a big part of the hot debate over pre-select gears? There are those who like the low pre-select, and those who like it higher. Fortunately, it's easy to change so everybody should be happy.

But I wonder if my gut feeling is true: perhaps those who prefer a high pre-select gear are those with an engine compression brake, while those who prefer a low pre-select gear are those with an exhaust brake? It would be an interesting survey to ask people which type of braking system they have (exhaust, 2-stage engine, 3-stage engine) and what pre-select gear they prefer. I'll bet there's a strong correlation with engine brake owners preferring a high pre-select and exhaust brake owners preferring a low pre-select.

You mention that with the transmission in 5th you get gentle deceleration, and very aggressive braking when aggressively downshifting. With the exhaust brake, I get gentle deceleration when aggressively downshifting, and barely discernible braking when staying in 5th gear. I think that is the crux of the divide. If you have a 2nd gear pre-select, you would spend most of your time adjusting the transmission up because your engine brake is very effective and you get too much braking. If I were to have a 4th or 6th gear pre-select, I would spend most of my time adjusting the transmission down because the exhaust brake is not as effective and I would not get enough braking.

Could that be a big part of the difference of opinion?
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Old 04-15-2015, 08:09 AM   #53
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Jeazzes fire up. I told U that anything after 55 mph in 5th 6th or any other gear it will down shift to forth . What more do U want.
He wants to know at what speed you consider this to be a problem.

Unless you have unusually low gearing in your coach downshifting to 4th gear at 55 MPH is not a problem, as your engine RPMs should still be within a safe non-powered range.

Now, if it downshifts straight to 4th gear while you are doing 75 MPH, that is likely a problem.

But if it doesn't downshift right away at 75 MPH, but does it at a lower speed, then it's probably working properly. But that determination can't be made without more detailed information.

So, the real question is:

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This is why I've repeated myself and, others have too in trying to find out the EXACT SPEED at which this forth gear drop happens.
You say at 55 and any speed above. But dropping into 4th gear at 55 MPH is not a problem.

Let's put it another way: at what RPM is the engine turning when you think it's going too fast?
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Old 04-15-2015, 09:51 AM   #54
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Shapeshifter,


Have to burst your bubble. We have a three-stage Jake, and have it programmed with a target gear of second, and that's the way I like it. It works great that way, and why in the world would I want to manually downshift an automatic transmission when it works great the way it is set up? The only time I did that was when I was going so slow I had to downshift to first with the Jake on.
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Old 04-15-2015, 10:37 AM   #55
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Have to burst your bubble.
I'm talking statistically, not absolutely, hence the mention of a survey. One data point does not prove or disprove a theory. I'm more interested in the general trend.
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