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Old 08-22-2011, 10:29 PM   #1
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Engine/exhaust brake

Hi, 2003 Jayco, Freightliner Chassis with 330 Cat and Allison Trans. Going down Cajon Pass yesterday, there was 2 "down shifts" if you will, the first was slight and the 2nd was much stronger. The 2nd one felt more like the exhaust brake on my old HR Endeavor. Exhaust brake switch is either off or on, no 1 or 2 step selection. I dont have all the manuals so I am a bit lost. I thought it has an exhaust brake, not an engine brake, but it acts like a 2 stage brake. Any ideas?

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Old 08-22-2011, 10:52 PM   #2
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First "downshift" may have been exhaust brake coming on and the second "downshift" may have been transmission actually downshifting into lower gear?

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Old 08-23-2011, 06:43 PM   #3
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On the Freightliner chassis w/ CAT engine you have an exhaust brake which works in conjunction with the Allison transmission. As soon as the exhaust brake engages, it tells the transmission to downshift to a pre-programmed selection, usually to 2nd gear as programmed from the factory.

The exhaust brake needs hi RPM's to work, thus the connection with downshifting the tranny.

The downshift gear selection is programmable by Allison. I reprogrammed mine to downshift to 4th gear when the exhaust brake is engaged. I find this selection 'just right'.
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Old 08-23-2011, 10:24 PM   #4
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JP, I'll just add this to Pusherman's post, anytime you allow the coach to coast down and the engine RPM drops below 2,000 the trans will shift down another gear, to keep the exaust brake at it's most efficent operation.(max hold back). Factory programing is to achieve second gear, and re-programing will stop downshifting in forth.
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Old 08-24-2011, 08:27 AM   #5
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On ours (ISB not CAT), turning on the exhaust brake at 55pmh or greater has only a little impact. Below that, it seems to be a combination of current speed and increase or decrease from it.

If you are on a hill, have the exhaust brake on and your RV is decelerating, you will get a downshift as soon as it is safe for the engine. Depending on the gear you started in, that may or may not be a big change. That process continues down to around 10mph for us. You can almost tell when the engine RPMs have dropped enough for a downshift after you go through it a couple of times.

I struggle with our setup on some Interstates, which are limited to around a 6% grade. 6% and the exhaust brake puts us into deceleration. I find myself playing with turning the exhaust brake off manually or pressing on the fuel pedal (which also turns it off) to try not to get to that next downshift point because it slows us down too much. I had the same problem on a 9% grade and a lower gear. We were either way too slow or running away.

As others have suggested, a lot has to do with how your RV is programmed. I tried to take advantage of downhills, even small ones, to figure that out when we first got ours. We've had enough situations that I pretty much can anticipate its quirks and work around them. I suspect that you'll end up doing the same.
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Old 08-24-2011, 12:03 PM   #6
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OK, thanks for the replies. I guess I did not feel the exhaust brake on my older coach, just the trans downshift. I have another question, if I reprogram the trans, can I manually downshift to 3rd and then 2nd with the exhaust brake on?

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