Allison Series 1000 Transmission Grade Braking Feature
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.
Some commonly asked questions on Grade-Braking.
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.
- 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.
- Does grade-braking work in Tow/Haul and normal mode? Yes, grade-braking is active in either mode.
- 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.
- 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.
- 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.
- How do I get out of a grade-braking shift? Grade-braking can be exited by depressing the throttle.
- 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.