Community Forums

koresample · 08-19-2013, 02:59 PM

we have a 1999 4080dp with 325hp cummins, allison 6 spd and exhaust brake. I am going to be doing some mountain traveling for the first time and want to know how others are selecting the appropriate gear for the grade. I am assuming going up I don't do anything just let the allison select the best gear or?
For going down what do you do and when? I normally travel with the exhaust brake on (except in town) but know i need to have it in a lower gear to minimize or eliminate the need to use the service brakes. When do you do this, at what rpm etc?

Mike

RickO · 08-19-2013, 03:03 PM

For descending, I never manually downshift but rather let the engine brake do it's own thing. It manages the downshifting and I just tap the service brakes from time to time to keep things in check.

For climbing, yes, you can just allow the Allison to do it's thing... so long as the temps aren't too hot. You may find that your engine begins to run too hot unless you downshift to keep the RPM around 2000. On steep grades, that might mean slowing down to whatever speed will allow you to hold that engine speed.

Good luck

Rick

wallynm · 08-19-2013, 03:05 PM

Going down we always try to go about the same speed as if we are going up the same grade. The trany may up shift but we just brake until it down shifts again with the EB on.

Romeo · 08-19-2013, 04:33 PM

Quote:

Originally Posted by koresample

... I normally travel with the exhaust brake on (except in town) but know i need to have it in a lower gear to minimize or eliminate the need to use the service brakes...
Mike

I only have experience with my exhaust brake which has a foot pedal instead of a toggle switch, I would think that travelling with the exhaust brake in the on position would preclude coasting, which saves fuel (???).

RickO · 08-19-2013, 04:45 PM

Quote:

Originally Posted by Romeo

I only have experience with my exhaust brake which has a foot pedal instead of a toggle switch, I would think that travelling with the exhaust brake in the on position would preclude coasting, which saves fuel (???).

I have a two stage dash switch engine brake and travel with it off so I can coast too. I like being able to drive that way and really believed it should improve my MPG, but I don't seem to get even the average of what folks with my similar set up get so... I don't get it.

Rick

"007" · 08-19-2013, 05:29 PM

This is in QT's 1 & 2 may explain what you need to know.
Wiring for brake is there if you have a problem also.

harleychief · 08-19-2013, 06:20 PM

Quote:

Originally Posted by koresample

we have a 1999 4080dp with 325hp cummins, allison 6 spd and exhaust brake. I am going to be doing some mountain traveling for the first time and want to know how others are selecting the appropriate gear for the grade. I am assuming going up I don't do anything just let the allison select the best gear or?
For going down what do you do and when? I normally travel with the exhaust brake on (except in town) but know i need to have it in a lower gear to minimize or eliminate the need to use the service brakes. When do you do this, at what rpm etc?

Mike

Mike,
The Allison transmission and the controls for it are bulletproof. When ascending turn off the exhaust brake, you don't want or need it going up hill. When descending try this: at the crest of the hill take your foot off the throttle so you don't build up speed, as you start your descent turn on the engine brake. Remember that as long as your foot is off the throttle the engine brake will be engaged. Apply the service brakes as required, remember that long easy braking is better then constant short hard applications. The transmission controls won't allow the engine to overspeed, just watch your gauges and keep everything as close to normal as you can.

Charlie

firedoc · 08-19-2013, 06:26 PM

Quote:

Originally Posted by harleychief

Apply the service brakes as required, remember that long easy braking is better then constant short hard applications.
Charlie

How about that folks?

Selah · 08-19-2013, 06:54 PM

Quote:

Originally Posted by firedoc

How about that folks?

That would be NO!
Do not ride your brakes going down grades. Long application of your brakes, even lightly, will cause them to over heat. First start down the grade at about the same speed you came up, or slower if the descent is much steeper. Use your e-brake or down shift or both, if your speed in creases too much use the service brakes the bring your speed down fairly quickly. Repeat as needed. The less time you are on the brakes the more they can cool down.
Disclaimer: I am not an expert, don't claim to be, don't want to be.

Mr_D · 08-19-2013, 07:02 PM

Quote:

Originally Posted by harleychief

Apply the service brakes as required, remember that long easy braking is better then constant short hard applications.

The short hard "stab" is what you're supposed to do. Dragging the brakes lets the brakes overheat since they don't have time to cool down. Hitting the brakes hard and then letting off lets them cool down.
This is per various manuals and papers I've read over the years.

RickO · 08-19-2013, 07:03 PM

Quote:

Originally Posted by harleychief

The transmission controls won't allow the engine to overspeed,
Charlie

Good point.

It's probably worth adding that we're not supposed to use the engine brake on wet or slick surfaces. That leave us with only downshifting and service brakes.

I once had to make a harrowing, long descent in the middle of a huge California rainstorm.

I was devoting my full attention to getting down that hill. There's no better time to get in a line of trucks and stay there.

Rick

harleychief · 08-19-2013, 07:07 PM

Folks I'm not going to get into an argument on braking techniques. If you pump the brakes hard and constantly you have a greater chance of overheating. I'm not saying to ride the brakes all the way down the mountain, but if you must apply the brakes it's better to do less often for longer periods. This is IMHO and 20+ years driving big rigs.

dennis45 · 08-19-2013, 07:11 PM

Quote:

Originally Posted by Mr_D

The short hard "stab" is what you're supposed to do. Dragging the brakes lets the brakes overheat since they don't have time to cool down. Hitting the brakes hard and then letting off lets them cool down.
This is per various manuals and papers I've read over the years.

This IS the accepted method. Do Not pump the brakes with short stabs.

buck454 · 08-19-2013, 07:14 PM

I, as another poster mentioned, watch the temp when traveling in some steep grades, when temp starts climbing I know to manually down shift. Interesting how fast the temp cools back down. Then, when I break to top of grade I shift back to norm.

My engine break is also a toggle and I happened to notice while traveling at night(which is not often) a small light above switch showing LO and HIGH. I'm sure this is a good thing, but what is the rule of thumb for when high, when low? Would this mostly be dependant to grade? Would high be the best for city limits (if legal) when approaching an abundance of intersection lights.

bill