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File In Section: 07 - Transmission/Transaxle

Bulletin No.: 02-07-30-004A

Date: September, 2002

INFORMATION
Subject:
New Product Information - Grade Brake Feature of Allison(R) 1000 Series Transmission

Models:
2001-03 Chevrolet Silverado Pickup Models
2001-03 GMC Sierra Pickup Models
with Allison(R) LCT1000 Series Automatic Transmission (RPO M74)

This bulletin is being revised to add the 2003 model year and additional information for clarification. Please discard Corporate Bulletin Number 02-07-30-004 (Section 07 - Transmission/Transaxle).

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 preselecting 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.

Edited by: Mackin
 

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<H1>New Product Information - Grade Brake Feature of Allison 1000 Series Transmission #02-07-30-004A - (09/11/2002)</H1>
<TABLE>
<T>
<TR>
<TD>Table 1:</TD>
<TD>8.1L Gas Engine (RPO L18) with both 4.10 and 3.73 axle ratio</TD></TR>
<TR>
<TD>Table 2:</TD>
<TD>6.6L Diesel Engine (RPO LB7) with 3.73 axle ratio</TD></TR></T></TABLE>New Product Information -- Grade Brake Feature of Allison® 1000 Series Transmission


2001-03 Chevrolet Silverado Pickup Models


2001-03 GMC Sierra Pickup Models


with Allison® LCT1000 Series Automatic Transmission (RPO M74)


This bulletin is being revised to add the 2003 model year and additional information for clarification. Please discard Corporate Bulletin Number 02-07-30-004 (Section 07 -- Transmission/Transaxle).


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.



<H4>Important</H4>


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:
<TABLE width="100%" border=1><A name=tab1-944861><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=944861&pubid=297&evc=sm#tab1-944861" target="_blank">
<H4>8.1L Gas Engine (RPO L18) with both 4.10 and 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>


4624</TD></TR>
<TR>
<TD vAlign=center align=middle>


4-3</TD>
<TD vAlign=center align=middle>


3266</TD></TR>
<TR>
<TD vAlign=center align=middle>


3-2</TD>
<TD vAlign=center align=middle>


2543</TD></TR>
<TR>
<TD vAlign=center align=middle>


2-1</TD>
<TD vAlign=center align=middle>


1474</TD></TR></T></TABLE>


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.
<TABLE width="100%" border=1><A name=tab2-944861><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=944861&pubid=297&evc=sm#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 a
 

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.6 LB7 Diesel Engine Allison LCT1000 (M74) Perceived Engine Overspeed During Decelerate Grade Brake #PI00151 - (07/09/2002)6.6 LB7 Diesel Engine Allison LCT1000 (M74) Perceived Engine Overspeed During Decelerate Grade Brake


.


When servicing the vehicle with the VIN you entered, the following diagnosis might be helpful if the vehicle exhibits the described symptoms. <A name=ss1-898558><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=898558#ss1-898558" target="_blank">
<H5>Condition/Concern:</A></H5>


There may be a perception that the Duramax 6.6 (LB7) diesel engine overspeeds when the Allison LCT1000 (M74) automatic transmission TCM activates the grade braking function.


Grade braking is a feature incorporated into the Allison LCT1000 (M74) transmission that utilizes engine braking to slow a heavy vehicle on steep grades. Service bulletin 02-07-30-004 provides additional detail on how grade braking functions.


The TCM calibration will allow a transmission downshift to occur with the Duramax 6.6 (LB7) engine as long as engine RPM does not exceed 4,000 RPM. After a downshift, if vehicle speed continues to increase, an upshift will occur at 4,800 RPM engine speed. This is normal operation and no attempts should be made to correct or change this operation.
 
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