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Old 11-14-2019, 04:49 PM   #1
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Engine Brake

I have seen posts on here before about having the Allison transmission calibrated to allow driver select of the desired gear for use with the Jacobs Brake. Looking for information on this and where did it get done. I prefer to have my transmission stay in a higher gear rather than continuing to downshift. This would be much better when in fluctuating speeds on highway traffic. Any help would be appreciated. Thanks in advance.
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Old 11-14-2019, 08:08 PM   #2
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Quote:
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I have seen posts on here before about having the Allison transmission calibrated to allow driver select of the desired gear for use with the Jacobs Brake. Looking for information on this and where did it get done. I prefer to have my transmission stay in a higher gear rather than continuing to downshift. This would be much better when in fluctuating speeds on highway traffic. Any help would be appreciated. Thanks in advance.
Well Sir,
What you might "prefer", might not be the best mechanical application for any and all given sets of circumstances that that which your engine and trans operate best at, for slowing or descending grades. I'm by far, no engine/trans engineer but, have driven fire trucks for a large percentage of my life and, I kind-a figure the folks that DO engineer the engine braking systems, engines themselves, and with the cooperation of the transmission, (pretty much always automatics in RVs), pretty much have figured out what works best.

Now, on some coaches, primarily ones with exhaust brakes, some folks have had the trigger for the application of the exhaust brake changed from instant on when the circumstances permit, to, only triggering when the service brake is touched, when descending grades or down off ramps etc. I'm not sure that can be done with a Jake (compression) brake or not. But, just some thoughts on this.
Scott
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Old 11-14-2019, 10:36 PM   #3
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The changes you want to make are done in the ECM. It requires a Cummins Insite program to access the computer and make changes. You can go to any Cummins shop or shop that works on Cummins engines and have them make the changes. It's usually charged at an hour labor.
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Old 11-15-2019, 03:52 AM   #4
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Thanks! For some reason I have had in my mind that it was an Allison Dealer I had to go to. Needless to say those are few and far between. Now that you have cleared my thinking I am sure I can acquire what I want along with the proper advice to make it easy on the engine and transmission. I never could understand why they want these things dropping to lower gears so fast at 65mph. All the manual transmissions I have driven worked well one gear down for just slowing down and not stopping. Thanks again.
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Old 11-15-2019, 05:28 AM   #5
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The changes you want to make are done in the ECM. It requires a Cummins Insite program to access the computer and make changes. You can go to any Cummins shop or shop that works on Cummins engines and have them make the changes. It's usually charged at an hour labor.

Wrong. You need an Allison transmission shop. Cummins has nothing to do with the trans target gear selection....its all in tbe TCM.
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Old 11-15-2019, 06:32 AM   #6
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Something to consider before you jump into this: Your Jake brake becomes less effective as RPMs drop, so the transmission shifts down to maintain braking. If you alter the shift points so that the trans stays in higher gear, you will need to manually shift when you need additional braking.
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Old 11-15-2019, 12:36 PM   #7
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Oh boy here we go again....OP, (Joe O) is this an Engine brake or an Exhaust brake? Jacobs makes both and both operate differently.
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Old 11-15-2019, 10:32 PM   #8
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Oh boy here we go again....OP, (Joe O) is this an Engine brake or an Exhaust brake? Jacobs makes both and both operate differently.
Gotta know that before any cogent information may be offered.
An exhaust brake works best at higher RPM's (2,000-2,300), an engine brake works at all RPM's.
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Old 11-15-2019, 10:45 PM   #9
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An exhaust brake works best at higher RPM's (2,000-2,300), an engine brake works at all RPM's.
To be specific, the effectiveness of an engine brake increases significantly as the engine speed approaches the engine's red line. For example, at peak the engine brake on my C-12 can develop over 300HP of braking out of the engine's peak output of 425HP.
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Old 11-16-2019, 03:12 AM   #10
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Oh boy here we go again....OP, (Joe O) is this an Engine brake or an Exhaust brake? Jacobs makes both and both operate differently.
It is a Jake hooked to a 400hp Cummins. I have no problem needing to manually shift it down. The brake is very effective right till it kicks out at about 5mph.
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Old 11-16-2019, 06:25 AM   #11
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Just to be clear, your transmission does not downshift right away to the gear that shows up on your shifter. The transmission is looking at input rpm and chooses the correct gear. My 97 Imperial with exhaust brake showed 2 on these shift pad but it took time and gradual loss of speed and engine rom before it downshifted all the way to 2. My current Motorhome has a Jake Brake and does the same thing. My advice is to leave it the way it is. The transmission logic is very intelligent.
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Old 11-16-2019, 07:59 AM   #12
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Jeez, he understands how it works, he just wants control of it. I wish i could do the same with my GMC pickup. The Ram pickup has it figured out the right way. I hated the aggressive downshifting on the 2 DP’s we had as well.
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Old 11-16-2019, 08:37 AM   #13
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The manufacturers have designed the system to be automatic and for maximum efficiency in engine braking. Which imo isn't necessary all the time. If you're lightly loaded and not towing the program could be much less aggressive. So I agree with the OP. A less aggressive downshift mode would be better for everyday driving in traffic, especially when lightly loaded and not towing. It would be nice to have user selected modes, but I would have to spend another couple hundred thousand dollars to get into that level of coach.

I've developed a way of just barely riding the gas pedal to keep the exhaust brake from activating. But that doesn't help fuel efficiency because the throttle is never really fully closed. Most of the time I leave the EB switch off and just coast, or manually downshift the tranny, or flip the EB switch on only when I really want it...which isnt very often because of its aggressive nature.

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Old 11-16-2019, 09:27 AM   #14
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You can have the "target gear" programming changed, whether it is a engine brake or exhaust brake. As already mentioned, the Allison doesn't go directly to that gear anyway - it chooses intervening gears as appropriate.

You might prefer having the target gear programmed for 4th rather than 2nd. Many motorhomes with engine brakes come programmed that way, so it may already be so. 4th is the top non-overdrive gear on an Allison 6 speed and most likely where you are running in highway traffic anyway. You can always manually downshift further on steeper grades.

Personally, I just flip the Engine brake switch off if traffic is causing too frequent downshifts.
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