View Full Version : Tow / Haul - Grade Breaking
Last weekend my wife and I took our 11,000 lb fifth wheel over Cameron and Rabbit Ears passes here in Colorado - the bigger of the two being 10,200 feet and 7% grade. Everything was great uphill - on the straight aways I could maintain 55 mph - however heading down the passes I ran into a situation that seemed dangerous to me. After holding on the brake pedal for 3-4 seconds the truck would downshift (bringing RPM upto 3200) now with foot off of brake pedal the truck was not maintaining 50MPH - slowly creeping up. Again I pressed the brake pedal for 3-4 seconds - AGAIN the truck downshifted - now RPMs way up there (approx 3800 RPM and climbing) - this situation happened to me twice. Finally I took it out of Tow / Haul and manually downshifted 1 gear and then was able to maintain speed with occasional application of the brake pedal.
Is this normal??? am I doing something wrong??- one of my co-workers has had a similar experience on a steep grade with his 2001 GMC D/A and fifthweel.
Thanks in advance for your replies.
Paintdude
10-01-2002, 20:46
It is normal , if my truck is normal..Mine does the same thing.. :confused:
Tudge I experienced the same problem on a recent 5000MI trip. On one occasion I'm screaming past a big 18 wheeler and the light came on I don't think we're meant to be hauling &ss down those steep grades and expect the transmission only to hold us back those big trucks have Jake brakes as well and they descend those grades at a moderate speed. As soon as I started driving moderately down the long grades and started to pay attention and obeying those yellow signs that say 30 MPH through the curve,things went much better. It just took me a little longer but the drive was much more enjoyable. Also I used the tow/haul the entire trip towing my 26'TT and it worked flawlessly.I decided to slow at the summit let it get into the lower gears without the high RPMs and then stay there Stu
[ 10-01-2002: Message edited by: dmaxstu ]</p>
Pizza Man
10-01-2002, 21:12
I donno if its normal but my D/a does the same thing when towing the 12k trailer. In my 2500 I have to down shift manually.
Everything you wanted to know about grade braking and more:
The grade-braking algorithm's primary purpose is to utilize engine braking to slow a heavy vehicle on steep grades in order to reduce wear on the traditional braking system. The method used to slow the vehicle is by overriding the PRNDL position, effectively pre-selecting the next lower gear range automatically. Because the transmission is electronically controlled and there is no mechanical linkage that needs to be moved for a pre-select downshift, implementation of this algorithm can be done completely in software with no hardware modifications.
This algorithm takes into consideration several factors before commanding a pre-select downshift. These are the primary inputs:
Throttle position
Brake state (The brake must be applied to activate grade braking)
Vehicle acceleration/deceleration
Grade/Load
Vehicle speed
The determination of the PRNDL position is made by a logic subsystem. Several factors are calculated and weighted and then a sum is calculated. When this calculated sum surpasses a threshold, a PRNDL override is commanded.
Important
The shift speeds for grade-braking are controlled/determined as a result of "Calculated Sum" logic. However, the grade-braking shift point will never be greater than the Pre-Select Shift Point.
The following charts provide the maximum Pre-Select (manual) shift points:
8.1L Gas Engine (RPO L18) with both 4.10 and 3.73 axle ratio Shift
Transmission OSS RPM
5-4
4624
4-3
3266
3-2
2543
2-1
1474
For the L18 gas engine, maximum engine speed immediately following a preselect or grade braking downshift can be as high as 4,650 RPM. After a grade braking downshift, if vehicle speed continues to increase, an upshift will occur at 5,000 RPM engine speed.
6.6L Diesel Engine (RPO LB7) with 3.73 axle ratio Shift
Transmission OSS RPM
5-4
4017
4-3
2832
3-2
2196
2-1
1214
For the LB7 diesel engine, maximum engine speed immediately following a preselect or grade braking downshift can be as high as 4,000 RPM. After a grade braking downshift, if vehicle speed continues to increase, an upshift will occur at 4,800 RPM engine speed.
Important:
Grade Braking is not intended to reduce the need for great care by the driver when driving a heavily loaded vehicle down a grade. Drivers should continue to take all normal and appropriate actions to keep the vehicle under control at all times.
1)Does grade-braking feature work on level roadways while trailing or hauling heavy loads and applying brakes? The feature is actively calculating whether or not to command a downshift while on level roadways, but it is much less likely that it will command a downshift under these conditions. The reason is that the Transmission Control Module (TCM) is using vehicle acceleration/deceleration as a determining factor. Since the vehicle is more likely to respond to vehicle brakes on level surfaces vs on a grade, grade braking is less likely to command a downshift.
2)Does grade-braking work in Tow/Haul and normal mode? Yes, grade-braking is active in either mode.
3)Can I turn grade-braking off? No, automatic grade-braking is always an active part of the transmission control system.
Does Tow/Haul have to be selected? No. In normal mode, the grade-braking feature will pre-select only the 5-4 downshift, whereas in Tow/Haul mode, grade-braking has ability to select the 5-4, 4-3, 3-2 downshifts.
4)When will I get a grade-braking downshift? There is no "fixed" shift point for a grade-braking downshift, however, the grade-braking downshift will never occur such that the engine speed following the shift exceeds GM Truck guidelines. Also, the grade-braking downshift will never occur without depressing the brake pedal.
5)To which gear will grade-braking downshift in each mode? The downshift will always be to the next lower range, i.e.; it will not "skip" ranges. In the Tow/Haul mode, the grade-braking feature has ability to select the 5-4, 4-3, 3-2 downshifts. Normal mode grade-braking will not command downshifts below 4th range.
Does towing a trailer have any affect? Yes, the TCM calculates the effect of the added mass of the trailer/load.
6)How do I get out of a grade-braking shift? Grade-braking can be exited by depressing the throttle.
7)What happens if the vehicle is on ice and a grade-braking event occurs, and the vehicle begins to slip? Grade-braking monitors the front and rear wheel speeds and can determine if the vehicle is slipping. If a slip occurs, grade-braking will be exited and the transmission will up-shift to the normal gear range depending on throttle position and transmission output shaft speed.
Kennedy,
Great explanation! That should answer most, if not all questions on this feature. Many thanks.
Gee after reading John's post I forgot what I was going to say... THAT was a lot of info! Oh yeah... the BD exhaust brake will greatly improve the grade braking. It will reduce the needed shifts and RPM's to slow down your load. It is not a cure for carelessnes, but will make any long downgrade much easier to take.
Ahh, that was one thing that I was wondering about. I know a few "mentally-challenged" (to be politically correct...) people who always use cruise control during the winter time here in northern Idaho and was wondering what the Grade-braking might do in a similar situation.
Thanks Kennedy.
Conrad
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7)What happens if the vehicle is on ice and a grade-braking event occurs, and the vehicle begins to slip? Grade-braking monitors the front and rear wheel speeds and can determine if the vehicle is slipping. If a slip occurs, grade-braking will be exited and the transmission will up-shift to the normal gear range depending on throttle position and transmission output shaft speed.
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[ 10-01-2002: Message edited by: conradv ]</p>
Just FYI, I did NOT draft that...
I hope my exhaust brake gets here one of these days! Almost as slow as Amsoil :rolleyes:
[ 10-02-2002: Message edited by: kennedy ]</p>
Thank you John for your very informative post I appreciate, it makes a lot more sense now but I'm from the old truck driving school that taught that you got fired when you scored liners in cummins diesels if you revved them over a 1000RPM when cold. I'm having a very hard time getting used to these high RPMs in the Dmax.Stu
JK,
Should have ordered your brake from a "panhandler"
I could of had it to you by now. :D :D :D Just kidding! Terry
CanadaKev
10-02-2002, 23:57
JK, Excellent post!
Tudge,
Remember, transmission braking is only a brake assist. It is not, in and of itself, a BRAKE!
If the grade is steep enough, it also requires some brake pressure along with the "tranny downshift" to maintain a desired speed. So when you apply the brake for 3-4 sec then release and apply again for 3-4 sec, you are telling the tranny that you would like it to do more. That's why it grabs 3rd gear and revs 3800rpm(which is OK) but if you are more comfortable with it grabbing 4th and revving 2800, then maintain a nice gentle brake pressure and it will stay in 4th until you slow your speed enough that it will downshift to 3rd at a lesser rpm.
This is probably as clear as mud :confused:
but if you get out on the road and try a few different scenarios, you'll see what I mean.
It is a phenominal transmission once you fully understand it's capabilities.(Not that I do, but I am learning) ;)
Kev
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7)What happens if the vehicle is on ice and a grade-braking event occurs, and the vehicle begins to slip? Grade-braking monitors the front and rear wheel speeds and can determine if the vehicle is slipping. If a slip occurs, grade-braking will be exited and the transmission will up-shift to the normal gear range depending on throttle position and transmission output shaft speed.
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John- now I would NEVER say you didn't know about our trucks- NEVER...but since you typed later that you in deed did NOT draft the above- I will call you on it.
First off, we live in Vermont- we have snow- the grade braking DOES NOT stop, when a slip occurs. I have HATED this feature in my truck since DAY 1, from the first snow fly to the last of each season. I am dreading the start of winter here because of it.
When the truck downshifts- the rear end kicks around, due to loosing traction- you let off on the brake- it does NOT upshift...you give it gas- it has to increase in RPM's prior to it actually kicking up a gear. Personally I think the grade braking is GREAT...until it snows. then it SUCKS! Without the auto 4wheel drive- you have no control...... if our '01 had the AUTOTRAC 4 wheel drive system- you could push the auto button- and when the rear slipped- it would engage 4HI and away you would go. The way it is now, it takes about 5-6 seconds to get out of the grade braking situation- 5-6 seconds too long. What they need to have is a on/off switch- very similar to the tow/haul mode- if I want to grade brake I will- if I want to keep all 4 tires heading foward- please let me...type of thing.
again- Mr. Kennedy- I would NEVER second guess ANYTHING you have to say about our trucks... please do not think that I am.
Brandy tongue.gif
Drive a mile, walk the rest
11-19-2002, 16:14
MudNurI,
I agree with you. Let the driver drive the truck! Give us control over the grade brake. I don't want a "logic subsystem" making the decisions for me.
The operation of the grade brake is not correct. Use the jake brake in a semi as an example:
1) The jakes have an on/off switch.
2) The jakes and the brakes are separate brake systems. They can be used independently or simultaneously.
3) You apply brake pressure to bring the engine RPM back down to operating range. You don't downshift and bring the RPM's higher like the allison does when you touch the brakes.
In my experience, the grade braking is NEVER assertive enough in normal mode to amount to a hill of beans. In other words, it barely exists, if at all. One of the keys is IF you are touching the brakes and the vehicle is still gaining speed.
I found the grade braking to be quite a treat while towing in the mountains. As soon as you touch the accelerator pedal it will shift right back up...
After reading some of these posts, I checked mine on a drive to town. I found that in normal mode, for all intents there was no grade braking (perhaps because the Torque Converter unlocked at about 50 mph in OD, and was unlocked in 2,3, and 4). In T/H, it is locked in all gears except 1, and will unlock in 2 at 20 mph. I am not sure what happens in 1, but in either normal or T/H, there is quite a bit of drag, and my exhaust brake is effective above its cut out point.
For those of you towing in the mountains, an exhaust brake is worth the money. I shaved 30 minutes between Flagstaff and Phoenox (135 miles) with mine.
Drive a mile, walk the rest
11-20-2002, 09:55
TLA,
Thank you for your post! It made me realized that something is NOT right. I should not have a downshift into first gear in NORMAL mode. This is also documented in kennedy's tech tips. It says I should only have a 5 to 4 downshift.
Drive a mile, walk the rest
11-20-2002, 10:54
kennedy,
I know where you live. Loyal, WI is close to the longest, flatest, straightest road that I have every drove in my entire life. I am talking about HWY 13 going into Wisconsin Rapids. It is much steeper here.
I learned to drive in the snow in Wisconsin. If you screw up, you end up in a cornfield. Usually, nobody gets hurt and nothing gets broke. Anybody with 10 feet of binder twine can pull you out. I live in Washington state and frequently cross the Cascade range. The snow conditions here are more difficult and the damage is more severe. The pucker factor is much higher!
I don't agree with you when you say that grade braking is not assertive. Mine is very assertive! It will shift down at least 4 times going down White Pass running empty. Maybe something is not right. Mine will not upshift right away. It will stay in 4th until I hit 70 MPH.
I will agree that grade braking while towing is a treat. Especially if it is a dry day and you have an exhaust brake.
I understand that the vehicle must be gaining speed and the brakes must be on. I remember that GM transmissions used the brake switch (under the dash on the brake pedal) to control transmissions functions. Is this also true with the allison transmission? I would like to try adjusting the brake switch so that it requires more brake travel.
Are the transmission OSS RPM numbers adjustable? Could a good technician tune those limits so that the transmission would never downshift?
This truck is an early 2002. Do I need a TCM reprogram?
I have a question, mainly out of curiosity? With the grade braking feature on, does this enable the Allison to act like a standard transmission??? Or does the grade braking initiate some sort of internal transmission brake in addition to engine drag?
I have the 6 speed and am not overly impressed with the amount of drag the engine places on the truck. I know the reason for the lack of drag on a diesel, but my point is if the grade braking allows the Allison to act like a standard, it won't do a whole lot.
Just my $.02!
MaxRock
D-max Man
11-20-2002, 11:15
These engines were tested to over 5000 rpm in over run conditions with no problems so those RPM's should not be a concern at all.
Actually, most everybody comments about the rolling little "quickie" hills that get you! If you want flat, 80 out of Pittsville is flat and straight. If you go in the ditch around here, be prepared to roll over. Ditches are deep and roads are crowned.
I've towed across the "divide" so I do know what real hills are. In NORMAL mode the grade braking is non existent in day to day driving pure and simple....
I will reiterate: In NORMAL mode, the grade braking is almost non existent.
MaxRock asked
"With the grade braking feature on, does this enable the Allison to act like a standard transmission??? Or does the grade braking initiate some sort of internal transmission brake in addition to engine drag?"
What I think you are asking about is the Allison "retarder" that is used in some Allison transmissions. The transmission that is in our trucks does not have a retarder built into it. I used to own a vehicle with an Allison transmission and there was an additional pedal located where the clutch pedal would normally be found. When it was depresses it operated the retarder feature that was built into the Allison transmission. There was no doubt when you turned it on but you had to watch the transmission temperature gauge!
George
Drive a mile, walk the rest
11-20-2002, 12:33
A big thank you to everyone that has shared their grade braking experiences. I am quite certain that my NORMAL mode is not functioning NORMALLY!
Does anyone know a good mechanic in the NW Oregon or SW Washington area?
What works for me when towing in tow haule mode.
When I know there is an extreamly steep down hill run I slow down to 55mph foot on the brake the tranny will shift into 4th, in order to stay in 4th, 55mph approx.is maintained by using the brake lightly every 15 or 20 seconds.
To make up for lost time you can blast up the next hill or mountain.
good luck
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