All,
Driving to Scottsdale for Thanksgiving and I'm trying to figure the best technique for employing the Pac Brake in downhills. I can't find any data if I should be downshifting the Allison 4 speed from D to 3 or even lower gears on steep down grades.
My current use of the Pac Brake doesn't seem to offer too much deceleration. The Pac Brake is working, I've had my mechanic verify this. Leaving it in D on small downhills here in Central Texas offers nearly no deceleration. Not sure how it could be different on a longer downhill.
Any help appreciated.

All,
I found a link to help me out here:
http://www.rversonline.org/02CCConfPacBrake.html

Apparently a downshift is mandatory on downhills as high RPMs are required for the PacBrake to work properly, the link states says Cummins says 2800 RPM on a DECELERATION is no issue and is what should be done for the brake to work.

Interesting

My Pac Brake has been taken out and loosened up twice and every time has "frozen" within a few hours of use, so I have just about given up on the thing.

I think that before you allow your engine to rev to 2800rpm (even in deceleration) I would check with Cummins directly and refer specifically to a 6CTA8.3L. My specs indicate that it reaches 240HP @ 2300rpm. I am probably wrong but I always assumed that meant that redline was 2300rpm and if that's true then I would think that you would be increasing the rpm's way beyond it's capacity to even stay bolted down on the frame, even if they claim it can live in the "redzone".

I don't think my tach reading has ever gone much higher than 2300rpm, although in deceleration once I see it going over 2300rpm I apply the brake momentarily to slow down the engine speed.

As I read it the key is 2800 RPM at DECELERATION (foot off the accelerator). At deceleration the engine is producing virtually no power and the issue is can it spin -safely- that fast under no load, and the article I referenced says that Cummins says it's fine.
Now 2800 RPM with your foot on the accelerator is where the Redline is applicable as the engine is producing power.
I will discuss this with my mechanic today but from a purely fundamental mechanical perspective I believe that this procedure seems sound.

I just talked to cummins and I was told that with foot off the accelerator my engine could go up to 3700 rpm . you really know have to know what engine number is to get an answer.I am running a 300 hp 8.3 other sizes and age make a difference.
so there is no problem with my engine at 2800 rpm others might be different.
thanks for that hint.
lenny
ps I will still keep mine at about 2500 just afraid I guess

quote:

I will still keep mine at about 2500 just afraid I guess

I believe the issue is this, essentially if your not forcing (via downshift) the engine to reach approx 2800 RPMs on a downhill (foot off accelerator pedal=deceleration), then the Pac Brake can't produce enough back pressure to slow you down.
There is a video on YouTube where a truck driver is demonstrating how to properly use a Pac Brake in an extended downhill. He is downshifted showing his tach and rpms and speed, mentioning he is using no main brakes. Just type in Pac Brake and I'm sure you'll find it.
This brake is not as effective as a Jake Brake, and if not utilized exactly as prescribed I think the Pac Brake is really useless.
This is why I started this thread, there are technical issues in properly utilizing this engine brake, and if not properly done, I don't think they really do anything to help us on a downhill. And my coach likes to take off like a rocket!

I removed the Pacbrake on my 3116 cat and allison 5 speed. It stuck on one too many times... the last time causing exhaust manifold gasket to blow before I could pull over and manually release it. Now I just downshift on grades until I'm accerating too much and apply braking to slow right down. Seems to work for me and I think easier on the mechanicals.

Strato -and- Steve WI...

We all have coaches and motor / tranny combinations that are nearly the same.

I've never seen my RPMs any greater than 2300 either. I would probably freak-out a bit if I did.

I was also told that the Pac-brake was not very effective with the CTA8.3 and the Allison 4-speed combination. I do not remember where I had heard this, so there is no way I am claiming this to be fact.

I never knew my Pac-brake to work, but I never followed Strato's Procedure ...the world will never know. Two seasons ago my diesel mechanic said the Pac-brake manifold had a huge crack-in-it, so he removed-it and installed a custom fabricated adaptor connecting to the remainder of the exhaust system.

Gotta love the air-brakes though!

Here is the video I was referencing, it's in the first post on this forum:

http://www.duramaxforum.com/fo...-air-bag-review.html

You'll notice he is running his truck at redline with the PacBrake on. They are still selling these units so I'm not giving up on mine.
I'll report after my Brady Texas to Scottsdale AZ run on how it performed.

Hi folks.
About rpm limits. Ours revs to about 2600 rpms when upshifting from first to second, then the next shift points are at about 2300rpms. Of course it hits that rpm for only a second and then you are in a higher gear. Don't think I'd be down shifting to third to get it up to 2800 and hold it there, thats too scary.
We have an "Extarder" by Jacobs. Is that what we are talking about here? Don't think ours even works, ... or if it does I'm not doing it right.
We will be hitting some good hills in a few weeks when we head out to Arizona. Last time it didn't seem to do much, I just slowed down and geared down and used the brakes off and on to control speed on those big hills out west.
Can't wait to call the Barth home for the next 4 months!!
Don