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 (http://service.gm.com/servlets/BlobShtml?ShtmlFile=944861&pubid=297&evc=sm#tab1-944861)</TD></TR>
<TR>
<TD>Table 2:</TD>
<TD>6.6L Diesel Engine (RPO LB7) with 3.73 axle ratio (http://service.gm.com/servlets/BlobShtml?ShtmlFile=944861&pubid=297&evc=sm#tab2-944861)</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