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:
<UL =BULLET>
<LI>Throttle position
<LI>Brake state (The brake must be applied to activate grade braking)
<LI>Vehicle acceleration/deceleration
<LI>Grade/Load
<LI>Vehicle speed </LI>[/list]
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.
The following charts provide the maximum Pre-Select (manual) shift points:
<TABLE width="100%" border=1><A name=tab2-944861><A href="file:///C:/DATA/archieves/2001%20Silverado%20info/New%20Product%20Information%20-%20Grade%20Brake%20Feature%20of%20Allison%201000%20Series%20Transmission%20%2302-07-30-004A.htm#tab2-944861" target="_blank">
<H4>6.6L Diesel Engine (RPO LB7) with 3.73 axle ratio</H4></A>
<T>
<TR>
<TH vAlign=bottom align=middle>
Shift</TH>
<TH vAlign=bottom align=middle>
Transmission OSS RPM</TH></TR>
<TR>
<TD vAlign=center align=middle>
5-4</TD>
<TD vAlign=center align=middle>
4017</TD></TR>
<TR>
<TD vAlign=center align=middle>
4-3</TD>
<TD vAlign=center align=middle>
2832</TD></TR>
<TR>
<TD vAlign=center align=middle>
3-2</TD>
<TD vAlign=center align=middle>
2196</TD></TR>
<TR>
<TD vAlign=center align=middle>
2-1</TD>
<TD vAlign=center align=middle>
1214</TD></TR></T></TABLE>
<H4>Important</H4>
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.
<LI =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.
<LI =1>Does grade-braking work in Tow/Haul and normal mode? Yes, grade-braking is active in either mode.
<LI =1>Can I turn grade-braking off? No, automatic grade-braking is always an active part of the transmission control system.
<LI =1>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.
<LI =1>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.
<LI =1>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" ra