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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.

  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.
  4. 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.
  5. 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.
  6. 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.
  7. Does towing a trailer have any affect? Yes, the TCM calculates the effect of the added mass of the trailer/load.
  8. How do I get out of a grade-braking shift? Grade-braking can be exited by depressing the throttle.
  9. 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.