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I was coming down a hill the other day towing some snowmobiles. I lefted off the throttle and touched the brakes and the truck downshifted and revved to something like 3500 RPM. I touched the gas pedal and it settled back to about 2200.


I thought the redline on these motors was about 3200 RPM. I knew there was a towing downshift thing that could happen but that seems a little weird to go into redline.


Am I just missing something (I hope).


Don Miller


2004 CC SB with 2100 miles
 

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That is the grade braking feature...Apparently it is normal for the engine to rev well past redline. IIRC, the computer will upshift again if the engine's RPM's reach a certain point.


This is a layman's description
...Some of the more knowledgeable folks here may want to elaborate a litttle.








Kevin
 

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turbospl311 said:
I was coming down a hill the other day towing some snowmobiles. I lefted off the throttle and touched the brakes and the truck downshifted and revved to something like 3500 RPM. I touched the gas pedal and it settled back to about 2200.


I thought the redline on these motors was about 3200 RPM. I knew there was a towing downshift thing that could happen but that seems a little weird to go into redline.


Am I just missing something (I hope).


Don Miller


2004 CC SB with 2100 miles

<H1>New Product Information - Grade Brake Feature of Allison 1000 Series Transmission #02-07-30-004A - (09/11/2002)</H1>
<TABLE><T>
<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></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>
<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></T></TABLE>


For the L18 gas engine, max
 

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Trippin said:
Cool, I feel better now about accidently reving mine to 4,000 on the dyno Saturday.
big diff. between no tps and wot under load
 

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Seen one rev past 5k before! Off the acellerator not down hill braking! Don't worry D-Max's are TUFF.Edited by: JRmac
 

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IF you'll carefully inspect your tachometer you'll find that it always has had a 4800 RPM redline, the red area that starts at 3200 is hatched, solid red begins at 4800 RPM.
 
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