Author Topic: Exhaust brake for steep, slow downhills?  (Read 279 times)

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Offline dbhutchins

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Exhaust brake for steep, slow downhills?
« on: September 05, 2018, 10:55:05 AM »
Hi Dave,
Appreciated your input on my exhaust brake the other day.  But I'm still wondering whether to have CAT reprogram my tranny.  As is, it won't allow exhaust braking below 3rd gear.  I live in a place with lots of steep roads (15% not uncommon) that I need to proceed down very slowly, where I think exhaust braking would be helpful.  But of course I go down in 1st or 2nd gear. I drove a big DP recently where the engine brake functioned down to 2nd gear and around 20-25 mph.  I have to say I liked that a lot more than having to use my wheel brakes below 30 mph on steep downgrades.  I think that a CAT dealer could change the tranny program to allow exhaust brake into 2nd and down to 20 mph.  Is that correct?  And are there any reasons I SHOULDN'T do that?  Very curious about this and will be happy for your advice.
Don Hutchins
'08 CC Allure 470 Cat C-9 w/ Cat tranny

Offline David.e.atherton

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Re: Exhaust brake for steep, slow downhills?
« Reply #1 on: September 11, 2018, 05:29:55 PM »
Hi Don, sorry for not getting back with you but sometimes Inget little confussed which
Post I have answered. Let go back to the exhaust brake problem, with an exhaust brake
there is a flapper that open and closes with a air cylinder and down shifts the transmission
to lower gear to better hold back motorhome on grades. The exhaust brake is only effective
at 45 mph or lower. ( note: higher mph exhaust pressure will over come the air cylinder
keeping the flapper open ). The question can the transmission be program to a lower speed
on the Allison transmission. The answer is yes and no, and basic it is best not to try changing
to lower gear. ( reason the lower gear in transmission holding engine back via transmission
gear train will only increase the engine rpm and now you would be required to use and
drag your foot brake to keep engine rpms below maxing out on engine tact.) use of the present
setup is correct for the type of brake you have. My suggestion and in my work had to
move very heavy earth moving machines and had what is called a engine compression brake
to help hold back heavy load but I used my transmission lower gears and dragged my
brakes lightly to hold my engine rpms down, it is my option only I would always go down
steep grades with the same gear that it used to climb the hill. What I would like to leave you with
Don, depend on your transmission down shift lower gears and hold back with foot brake
lightly and use exhaust brake along with this. The exhaust brake will not produce the  brake
force as you may think an To be on the safe side in middle of long steep grade the transmission
In lower gear will hold you back. Just remember exhaust brake will open flapper with exhaust
pressure higher than the little air cylinder. Ask Dave Dave Atherton Retired Cat Mechanic

Offline dbhutchins

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Re: Exhaust brake for steep, slow downhills?
« Reply #2 on: September 12, 2018, 06:02:14 AM »
Good information.  Thanks, Dave. 
I have a CAT tranny, but assume that functionally they work the same as the Allison.  Really appreciate that input!
Don
Don Hutchins
'08 CC Allure 470 Cat C-9 w/ Cat tranny

Offline David.e.atherton

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Re: Exhaust brake for steep, slow downhills?
« Reply #3 on: September 12, 2018, 06:16:15 AM »
Don, with the cat transmission it set up and tested with different values, than they use
the one that matches the engine setup together. Ask Dave, Dave Atherton Retired Cat Mechanic