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Old 05-06-2012, 11:03 AM   #29
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Quote:
Originally Posted by Algoma View Post
2 points here. Firstly, most Allisons are programmed so that when you hit the exhaust brake the transmission shifts down to 4 as this is a direct drive gear and not an overdrive like 5 and 6 which provide little braking effort. Second, the engine is designed to exceed the normal governed speed with the exhaust brake on as braking force increases with engine speed. However there is still a limit (around 3000rpm) and the transmission will shift up to 5 if this is exceeded. I don't think the engine brake equipped engines allow this as you are physically changing the valve timing, not just slowing the exhaust gasses.
Good piece of info Algoma, I am impressed with the amount of info that a person can obtain on this forum I need to find out were you folks find and research some of this information. But as slow as I am on the computer I will keep this forum as my best asset for information regarding are Alpine.
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Old 05-06-2012, 11:07 AM   #30
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Thanks for all the research and info Mike. We just returned from a trip to Yuma and had numerous occasions to use the Pac Brake. Now, it may just be my imagination getting the better of logic, but I would swear that when the switch is activated in the upper position that the PB slows the coach faster and has a greater braking effect at lower speeds than when the switch is activated in the lower position. In the lower position, the braking effect seems to "run out" when you get down to around 20 MPH but when the upper position is switched on, it seems to stay fully engaged clear down to being stopped. I will try your method #3 the next time I have the old girl out on the road to see if it is imagination or fact! Thanks again Mike for all your help for all things Alpine.
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Old 05-06-2012, 12:08 PM   #31
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It doesn't make any diffrence whether you have a jake brake or Pac Brake, anybody that can connect to the Allison port can program to operate in any gears you want. I had my ISC pac brake and my ISL Jake brake both programed for 4-5-6 gears and it works good for me. I just manually shift gears down if I want to be in 5th or whatever. I figure if the brake is there , I might as well be able to use it. Even though it could be done for low gears, I don't think you need it. At this point, it would be hard to say what gears it works in all coaches, as some could have been reprogrammed over the years.
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Old 05-07-2012, 02:43 AM   #32
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John - First you have exhaust brake, different than engine brake, yours works off the exhaust gas passing out the pipe, ours starts long before exhaust is created, by actually shutting all the intake and exhaust valves in engine. No exhaust is created in our operation, since the fuel is shut off so combustion does not happen. It is basically just a large air compressor (really expensive one at that). I have hit 2400 RPM once when not paying attention, my transmission did not shift up to 5th gear.

Next, Cummins told me specifically, not to ever exceed 2600 RPM in engine brake mode no matter what. So I won't argue with the folks who fix them, you may do as you please.

With EB you have different parameters that may allow higher RPM's when in EB mode, ours is different, so our engines may not need that high of RPM's. Regardless, I have enough trouble with mine now, without screwing up the engine too.
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Old 05-07-2012, 07:24 AM   #33
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This is from the PacBrake datasheet :-

"Pacbrake exhaust brakes are designed and approved ENGINE SAFE at 300 RPM above the engines maximum governed RPM."
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Old 05-07-2012, 11:16 AM   #34
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John, that may be true, but the engine governor does not work when you are not pressing on the pedal. It is stated in the manual, it won't govern engine RPM while going down hill.
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Old 05-07-2012, 11:27 AM   #35
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I beleive the +300 rpm is saying the engine in freewheeling mode has a higher safe rpm than when driven by diesel combustion. In fact that's quite handy in either case of exhaust or Jake brake. If top speed is 2600, then you get 11.5% more braking than if same governed speed applied.
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Old 05-07-2012, 12:43 PM   #36
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Monty, I don't dispute that, otherwise how could you go 300 rpm over the governed speed. In my case the C8.3 is rated at 2800 and if I exceed that in 4 going downhill the transmission will shift up to 5 to prevent over-revving. I have to use the service brakes to drop my speed so that the transmission can shift back to 4 and I get exhaust braking back. This regulates my speed to around 55. The engine brake on the ISL is far more efficient so you probably don't have this problem.

Kraig, the pre-select can be programmed to any gear, I was just pointing out that the factory default is to select 4 when you apply the exhaust brake. I agree that programming it to 3 or even 2 is a bit extreme for a motorhome. I did change mine once to no pre-select so that I could use the exhaust brake in 5 and 6 but as I pointed out earlier there is very little braking effect in these gears. The engine brake may be different.
I had a problem with my throttle position sensor and as a result the exhaust brake would not activate so I went back to pre-select 4 so I could diagnose the problem more easily. Now that is fixed I would probably go back to no pre-select for the reasons you mentioned.
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