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Old 07-04-2016, 08:57 PM   #1
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When is the engine brake active

On a class A motor home with Freightliner chassis, 450 HP Cummins engine, and Allison 3000 transmission, is the engine brake active whenever the "Engine Braking" toggle switch is on to either Hi or Lo or just during that process when the Allison starts downshifting, usually toward 2nd gear?

Just wondering if I'm gaining anything until I press the brake peddle and the little green light on the dash shows up.

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Old 07-04-2016, 11:36 PM   #2
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normally the engine brake is engaged when the throttle is totally released. If you touch the throttle the brakes should disengage. Any downshifting would be irrelevant to the engine brakes. The hi and low is relative to how many cylinders will be active. Hi is all 6 cylinders and low is 3. If you have one three position switch center position is off and the other two positions are low and high. You will see more braking with higher rpms and most brakes are set to disengage below a certain rpm so as not to kill the engine. I have driven trucks where the jakes would stay engaged and kill the engine until the clutch switch engaged or the throttle was touched.
Hope this answers your questions.

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Old 07-05-2016, 12:07 AM   #3
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I have a 370 ISL with engine brake. I have the foot pedal on/off and a 2 position rocker on the left switch panel. The rocker is not labeled hi or low (blank). My old ears cannot hear a difference between the hi and low. I would like to label the switch as to hi and low. I would also like to change the foot pedal to a toggle, if possible. Any one have any input here...
Full time since Nov 2012 RVM#41 Gary and Marian Hill 2001 Monaco Dynasty Jack 38' 370 hp ....If it ain't broke I can still fix it.
Like putting shoes on an octopuss, so are the days of our lives....
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Old 07-05-2016, 12:07 AM   #4
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Allen.....On yours, once you turn the switch to "LOW" or "HIGH", the engine brake system is active, but not engaged. If you are driving down the road and take your foot off of the throttle, the engine brake will engage. It will stay engaged until you step on the throttle or drop down under 15mph (approximately).

Your engine brake will also engage while using cruise control. If you're descending a grade, the engine brake will allow you to gain about 6 mph above where the cruise speed was set and then engage. For me, this often has me going to fast. When I start down a grade, I decide if the engine brake should be on "LOW" or "HIGH". This comes from experience. I also thumb down my cruise speed, 5 times, so that my speed stays where it was set. Once the brake releases, I thumb the speed back up.

Every time your engine brake engages, here is a light on the dash that illuminates. You can also experiment a little with your engine brake. A few people recommended shifting to 5th or even 4th (depending on the grade) and then setting the cruise at a slower speed to maintain that speed on a steep grade.

Two weeks ago, I tried the downshifting technique on some really steep grades in Oregon. I shifted into 5th (engine brake on HIGH) and then allowed the speed to drop to 50 mph and then set the cruise control. The coach maintained the 50 mph all the way down the grade. If it started to increase a bit, I just thumbed the cruise speed down one or two times. This worked well and made for a comfortable drive.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab
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Old 07-05-2016, 09:47 PM   #5
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About a month ago while having the dash air conditioner fixed (the tech at a Freightliner shop determined Freightliner had originally only put about 1/3 the amount of freon required for the system to work so the compressor wouldn't even engage) and getting the dash "malfunction light" checked out I had the engine brake changed to come on when the service brake is touched instead of the throttle being released.
It was irratating to have the tranny start downshifting every time the throttle was released to coast short distances down a hill.

I'm going to try the cruise control method but maybe changing the activation from the throttle to the brake may also affect the cruise. At least now I know nothing is happening just because I move the Engine Brake toggle switch to Hi or Lo.
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Old 07-06-2016, 10:28 PM   #6
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Allison can program your tranny so it will not downshift so aggressively. Takes about 30 minutes and $50. Makes the Jake Brake MUCH more useful. I had them set mine so it stays in whatever gear it is in, even 6th. At 1,000 RPM it will down shift to the next lower gear.

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brake, engine

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