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Old 08-23-2015, 09:13 AM   #1
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Unhappy Cummins ISL Engine Brake

I have a 2008 Fleetwood AT with ISL. The engine brake does not work in both settings Do not know which setting is working ( low or high) Is this a problem in the program or wiring?

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Old 08-23-2015, 09:21 AM   #2
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If your switch is like mine, it flips forward and toward the rear. Either direction will bring the tranny down incrementally as the vehicle slows. I believe either direction brings the tranny to 2nd eventually.

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Old 08-23-2015, 10:08 AM   #3
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Maybe, but I suspect you are just expecting more than it will deliver in some circumstances. It's an engine brake (Jacobs type) rather than exhaust brake, and the two positions brake with either 3 cylinders or all 6. Neither position has a lot of effect at high speeds because the tranny can't downshift much without over-revving the engine. As you slow down (use the service brake to help if needed), it continues to downshift and you get more and braking effect.

You should see the indicator on the shifter go to "2" as soon as the engine brake engages, but the actual gear in use will depend on the mph speed. Above 60 mph the tranny stays in 6th gear and you don't get a lot of effect. The tach will show an RPM increase, though, and as the speed falls the tranny downshifts to 5, 4, etc. and you feel greater braking.
Gary Brinck
Former owner of 2004 American Tradition
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC
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Old 08-23-2015, 01:41 PM   #4
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Something isn't right. My Jacobs is programmed to target 4th gear. On a 6% downgrade I can maintain ~60mph with the low setting - rocker switch down on the bottom. If I need to slow down further for a curve or the grade steepens, I rock the switch to the forward position (high) and my coach slows down.

The low setting activates the exhaust valves on three cylinders, the high setting activates all six cylinders.
2003 Alpine Coach 40MDTS - 400HP Cummins ISL
Can Am Spyder Roadster
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brake, cummins, engine

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