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Old 08-03-2021, 06:39 AM   #1
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Engine Braking Cummins/Allison

Our '19 RV has the 360hp cummins and a 3000 allison gearbox. If I use the exhaust brake the rev's can peak at up to 3000 RPM which I feel is a high number since data I have on the frame build has a 2600 RPM governed number.

The exhaust brake does indeed help slow the RV down, but when in use it sets the target range at 2. When driving in hills etc. at speeds of 65 MPH I am thinking this is too low a target as brakes will be applied to keep things in line when the exhaust brake meets its match.

Given my apprehension about the high RPMs I have been using a downshift approach letting the engine provide braking without the exhaust brake. If the revs get to 2500 or so I pulse the brakes to drag it back down.

I am wondering how others take advantage of the exhaust brake and any modes the gearbox provides. I have not used the modes much at all. The first selection is Mode 5, which I believe is a mode that delays upshifts.
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Old 08-03-2021, 07:40 AM   #2
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The shift selector displays 2 but the transmission will only go to the gear that is safe. I have a Silverleaf VMSpc that shows the actual gear it is in when it is working.


When I use the exhaust brake I push the button on the side console. If in cruise control I tap the brake to disengage. The display will go to 2 and the exhaust brake will engage. If I am gaining speed I will stab the brakes hard to slow the coach down, I do not continually hold the brake down, this will prevent the brakes from overheating.



One thing that helps is to anticipate the need of having to use the engine brake by slowing down as you are cresting a hill. This will reduce the overall RPM/gear that the exhaust brake will have to start out at.



I drove south on I75 in KY & TN last week and I seldom had to apply additional braking, I let the Pacbrake to it's thing. If I did use it I used hard shot braking applications to slow the rig down.
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Old 08-03-2021, 07:48 AM   #3
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My old isb/Alison 3000 did the same thing. I would not worry about it, as the transmission will not allow a downshift to revs that are not acceptable. I believe that (within bounds) it is ok to exceed the governed revs while slowing. I just did the same thing - used the brakes to slow things a bit.

If you are on a grade where the rpms keep going up and the engine brake is not enough, then you are just plain going too fast for the grade. Slow down 10, 15, 20 mph and see where the engine settles. You should find an equilibrium between the grade and your rig where it does not speed up. That speed is going to be defined by the physics of the thing - not the sign on the side of the road…
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Old 08-03-2021, 07:54 AM   #4
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WRT Physics, Indeed.

That is how I have managed things by down shifting (without the exhaust brake) at this point. I guess my point is the exhaust brake is a bit more aggressive than I am comfortable with. I have wondered about setting the target at 4 or 5 prior to engaging the exhaust brake but have not tried it yet.

I suppose I can try things on flat ground as well and will experiment a bit when we get rolling (again) tomorrow.
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Old 08-03-2021, 11:09 AM   #5
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My old exhaust brake on the Cummins 6BT and Allison isn't linked to anything. For me the sweet spot is fourth gear, although I have to manually select that. It may not drop into fourth immediately, depending on speed.

I've found that exhaust brake activiation in fifth or sixth does pretty much nothing while activation in fourth and and down lower gears slows things down right smartly. I wouldn't want it drop straight into second.
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Old 08-03-2021, 11:12 AM   #6
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Another option is to have any Allison dealer re-program the Allison ECU for a higher "exhaust brake pre-select gear".


You can choose any gear you like. I had mine re-programmed to 5th.


In terms of max permissible engine RPM, please contact Cummins with your engine serial number. Engines DO allow higher RPM under no load than governed RPM. You need to know what it is.
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Old 08-03-2021, 11:24 AM   #7
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My unit will keep downshifting as we go slower. RPM around 3k
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Old 08-03-2021, 11:29 AM   #8
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WRT Physics, Indeed.

That is how I have managed things by down shifting (without the exhaust brake) at this point. I guess my point is the exhaust brake is a bit more aggressive than I am comfortable with. I have wondered about setting the target at 4 or 5 prior to engaging the exhaust brake but have not tried it yet.

I suppose I can try things on flat ground as well and will experiment a bit when we get rolling (again) tomorrow.


Oh … if that is the case …. There were plenty of times where I would control my speed with gears, then when it snuck up, toggled on the engine brake …. Which would not increase revs, but slow the rig. When slowed enough, I would toggle off the engine brake, and repeat as necessary. Would that work for you?
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Old 08-03-2021, 11:31 AM   #9
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I would suggest KNOWING what RPM your engine can handle (from Cummins) rather than trying to find a "work around".
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Old 08-03-2021, 01:59 PM   #10
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Oh … if that is the case …. There were plenty of times where I would control my speed with gears, then when it snuck up, toggled on the engine brake …. Which would not increase revs, but slow the rig. When slowed enough, I would toggle off the engine brake, and repeat as necessary. Would that work for you?
It's a possibility. We are pretty much out of the mountains at this point but there is always room to try things out.

Getting in touch with Cummins is indeed appropriate.
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Old 08-03-2021, 01:59 PM   #11
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Another option is to have any Allison dealer re-program the Allison ECU for a higher "exhaust brake pre-select gear".

You can choose any gear you like. I had mine re-programmed to 5th.
Give that man a cigar! This is the answer.

I had our coach reprogrammed to stay in sixth gear with the Jake Brake engaged. It was the best change I ever made to the coach. I control what gear I want for braking. And if I do nothing, the transmission shifts down to the next lower gear at 1,000 RPM. Makes it really nice for using the Jake Brake in slowing for an off ramp, or on a gentle grade where max braking would be way too much.

I once asked a Freightliner guy why they programmed the exhaust/Jake brakes on their chassis to target second gear and shift down to a lower gear as soon as it was possible to do so. In other words, they are setting the engine brakes to deliver 100% of available braking 100% of the time. His answer was revealing. He said (paraphrasing now) that Freightliner did not believe that the typical motor home driver knew how to properly use an engine brake, so they just set it max braking and left it there. Gee, thanks.

I use my Jake brake 99% of the time I'm on the road. Without the aggressive downshift pattern in play, it provides gentle braking that I can use anywhere, any time. About the only time I turn it off is when I'm in a residential area where the slight additional noise could be a problem.

Reprogramming the transmission to stay in sixth gear took about 20 minutes at an Allison dealer, and cost $50.
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Old 08-03-2021, 02:23 PM   #12
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I chose 5th gear pre-select, as I can't imagine not wanting the single downshift anytime I want engine braking. 5th gear in our coach, still below even governed RPM is still over 70 MPH. Remember, 6th gear in the Allison is .65 and 5th is .75. Not that much difference.



Again, it is YOUR choice-- do what works best for how you want to be able to control YOUR drivetrain.
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Old 08-03-2021, 02:38 PM   #13
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Give that man a cigar! This is the answer.

I had our coach reprogrammed to stay in sixth gear with the Jake Brake engaged. It was the best change I ever made to the coach. I control what gear I want for braking. And if I do nothing, the transmission shifts down to the next lower gear at 1,000 RPM. Makes it really nice for using the Jake Brake in slowing for an off ramp, or on a gentle grade where max braking would be way too much.

I once asked a Freightliner guy why they programmed the exhaust/Jake brakes on their chassis to target second gear and shift down to a lower gear as soon as it was possible to do so. In other words, they are setting the engine brakes to deliver 100% of available braking 100% of the time. His answer was revealing. He said (paraphrasing now) that Freightliner did not believe that the typical motor home driver knew how to properly use an engine brake, so they just set it max braking and left it there. Gee, thanks.

I use my Jake brake 99% of the time I'm on the road. Without the aggressive downshift pattern in play, it provides gentle braking that I can use anywhere, any time. About the only time I turn it off is when I'm in a residential area where the slight additional noise could be a problem.

Reprogramming the transmission to stay in sixth gear took about 20 minutes at an Allison dealer, and cost $50.
Now that is interesting! Thanks for posting!

I was just on the phone with Cummins and we went over some details regarding the engine which is a B6.7 360 RV HP 2600 Governed.

The rep took the serial number and related that the Max Overspeed RPM is 4200 and if you are there 15 seconds, damage will occur. He was a bit fuzzy on wether or not this was cumulative (15 seconds total over several episodes or just one) but seemed sure it was 15 seconds at one time.

Max HP @ 2400 - 2600, Max Tq @ 1800 - 2300 and Max HP/Tq is at 2300.

He found another number RPM Governor Brake = 2630 but did not know what this was referring to.

He suggested talking to Allison, and I will be to see about having the target range set to 5 or 6 so I can select the target gear beyond which ever I eventually have programmed. 5 does seem reasonable to me.
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Old 09-11-2021, 07:54 AM   #14
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Ok I spoke with an Allison Rep and did some experimenting. The tech was unsure of what they could do, but he was local and pointed to by the corporate site. Since they were Detroit/Allison he was a bit put off but said sure stop in and we will see what we can do. I might. I probably won't.
That is because it seems that the exhaust brake forces a particular target range and once in it, it cannot be changed. On a few occasions now I have set the EB on and attempted to raise the target range without success. So it appears the target range programmed by the EB function is cast in bits somewhere that prevents modification.
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