SIL 14­1K2K­17
September, 2017
Page 1 of 3
SUBJECT:

Servicing Low Rate Damper (LRD) Torque Converters, High Gain Neutral at Stop (NAS) Main Valve
Bodies, and Retracked NAS Components

MODELS AFFECTED:

1000 Series™ and 2000 Series™ Transmissions

Introduction:
Allison Transmission is offering three (3) new torque converter designs using a LRD. The LRD torque converter
uses a lower rate (less stiff) spring damper design.
Greater torsional activity with some of the newer 4 cylinder diesel engines can be attributed to engines with
smaller displacements and higher cylinder pressure. The LRD lower rate springs help reduce the affects of
these highly­torsionally active, 4 cylinder, diesel engines The LRD torque converter also uses a “learned’ lock
up apply command, found in software BCD161 and newer. The learned lock up command enables the torque
converter lock up clutch to engage at lower engine speeds.
LRD torque converter and High Gain NAS main valve bodies are standard components in FuelSense® 2.0 Max.
FuelSense® 2.0 Max allows for lower­range torque converter lock up, 1st range for close ratio transmissions and
2nd range for wide ratio transmissions. Using these components along with FuelSense® 2.0 Max calibration,
are designed to enhance fuel economy.
High Gain NAS valve body produces a higher Torque Converter Clutch (TCC) pressure. The higher TCC pressure
is achieved by using a larger TCC valve and spring in a larger TCC valve bore.
The new LRD design requires a higher Torque Converter Clutch (TCC) engagement pressure. To accomplish higher
TCC pressure, a main valve body modification incorporates a larger TCC valve bore inside diameter and a larger
TCC valve and spring combination. The new valve bodies will be referred to as the High Gain NAS main valve body.
NOTE: The LRD torque converters must be paired with High Gain NAS Main valve bodies. Failure to do
so will cause the torque converter to fail prematurely due to low lock up pressure.
Low Gain NAS main valve body parts will be serviced with either Low Gain NAS or High Gain NAS main valve
body components. Low gain NAS main valve body parts will be available until stock is exhausted. Former Low
Gain NAS main valve body TCC valve, P/N 29545994 and spring, P/N 29550070 will continue to be serviceable
with the Low Gain valve and spring. Refer to Table 1 for part numbers and part descriptions.

WBD / SL8021EN

3674885

Copyright © 2017 Allison Transmission, Inc. All Rights Reserved.

SIL 14­1K2K­17
September, 2017
Page 2 of 3
A TCM software update to BCD161 (released April, 2016) or later is required to apply logic that “learns” what
TCC pressure to use.
NOTE: Neutral at Stop–Standard is formerly known as Reduced Engine Load at Stop, or RELS. Neutral at
Stop–Premium is formerly known as Enhanced Converter Load Reduction, or ECLR.
Allison’s Service Channel shall continue servicing traditional and LRD torque converters with “like” parts.
Additionally, fielded Low Gain NAS main valve bodies can be serviced with either same replacement parts (as
long as they are available) or with High Gain NAS main valve body components (provided the TCM software
is upgraded to minimum BCD161 level).
NOTE: BCD161 software contains algorithms that learn to optimize lockup pressures.

Table 1. Torque Converters
Part Name

Engineering
Group

Original Part
Number

Low Rate Damper
Part Number

Description of Change

TC210, Torque Converter
Assembly

21­950

29546574

—

No change to original twin­plate
design

TC211, Torque Converter
Assembly

21­949

29546573

—

No change to original twin­plate
design

TC221, Torque Converter
Assembly

21­951

29546576

—

No change to original twin­plate
design

TC222, Torque Converter
Assembly

21­952

29546575

—

No change to original twin­plate
design

TC211D, Torque Converter
Assembly LRD

21­966

—

29556904

New Low Rate Damper,
single­plate design

TC221D, Torque Converter
Assembly LRD

21­967

—

29556905

New Low Rate Damper,
single­plate design

TC222D, Torque Converter
Assembly LRD

21­968

—

29556906

New Low Rate Damper,
single­plate design

Interchangeability:
The High Gain NAS main valve body design must be used with the Low Rate Damper torque converter design in
order to deliver the higher converter clutch apply pressure needed to appropriately apply the new lockup clutch
design. Failure to pair the LRD torque converter and High Gain NAS main valve body will cause the torque
converter to fail prematurely. Dowel pin location change on the NAS re­tracked valve body that feeds through the
NAS separator plate and into the High Gain NAS main valve body will physically prevent unintentional parts from
being assembled. Physically prevent mismatched parts from being assembled.
NOTE: Reference Service Information Letter (SIL) 13­1K2K­17 for NAS Valve Body Retrack changes
and interchangeability.

Copyright © 2017 Allison Transmission, Inc. All Rights Reserved.

SIL 14­1K2K­17
September, 2017
Page 3 of 3

Parts Modifications:
• Hardware Changes
– Low Rate Damper Torque Converter single plate design
– Main valve body gain change (larger TCC valve P/N 29557801 and TCC spring P/N 29557802) to
increase Torque Converter Clutch, TCC pressure. The High Gain NAS main valve body is found in
Engineering Group 16­1902

Copyright © 2017 Allison Transmission, Inc. All Rights Reserved.