#8158
October, 2018
Page 1 of 4
SUBJECT:

Input/Output Functions Available on Chevrolet® Silverado® 4500HD, 5500HD and 6500HD

MODELS AFFECTED:

1700, 1750, 2700, and 2750 Series Transmissions

Introduction
The purpose of this Service Tip is to provide a brief description of the input and output functions available to theuser on Allison Transmission equipped Chevrolet® Silverado® 4500HD, 5500HD, and 6500HD vehicles.
Definitions• Input Functions send discrete signals to the Transmission Control Module (TCM) indicating an operator’srequest(s) or the operating state of other vehicle systems. Specific transmission operation or function willthen be coordinated with the indicated state of the vehicle.
• Output Functions send discrete signals out of the Transmission Control Module (TCM) for use in controllingthe operation of other non-transmission components. The output functions may be turned on and off basedon the state of related input function signals or the operating state of the transmission.
Application and Usage
When the transmission is purchased from the vehicle manufacturer, the required input and output functions mustbe specified at that time and are usually in the form of standard “packages.” The dealer sales person can help
determine which packages are available. In many cases, multiple input/output packages have been created forvehicles used in specific vocations. If the function programming in a transmission control module must be alteredafter the vehicle is delivered, consult with the nearest authorized service location for reprogramming instructions.
There may be a charge for this service.
Depending on the selection, both input and output signals can be broadcasted by wire or through GMLAN datalinkmessages.
The TCM cannot contain all of the available input/output functions. A vehicle’s duty cycle usually determines which
combination of available inputs and outputs is chosen. Your vehicle dealer can advise you regarding which
input/output packages are released in their chassis.
Not all functions are compatible. Use of certain functions excludes the possibility of using others. Under nocircumstances should emergency vehicle calibrations and option packages be used in non-emergency vehicle
applications and vice versa.
Information contained in this publication is current at the time of publication and is subject to change without notice.

ST8158EN

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Copyright © 2018 Allison Transmission, Inc. All Rights Reserved.

#8158October, 2018
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Input Functions
Name

Description

Input via GMLAN or Input Wire

SECONDARY MODE INPUT A

When the function is enabled, the TCM willcommand the transmission to shift according
to the secondary shift calibration. Operationin primary mode resumes when the function
is disabled. Primary mode will also be
re-enabled each time the TCM is re-initialized.

GMLAN

The TCM activates the PTO Drive Interface
Output by switching power to one wire when
the following conditions have been met:
• The function has been requested
PTO DRIVE INTERFACE 1 INPUT C1• Throttle position is lowGMLAN• Engine speed is within Customer
Modifiable Constant limits• Output speed is within Customer
Modifiable Constant limits

AUXILIARY FUNCTION RANGEINHIBIT – SINGLE INPUT E

This function is enabled when a switch is
opened to break the circuit between an input
wire and ground. When an interfaced auxiliaryvehicle function is activated, as denoted by
the open state of the enable switch, the TCM
commands an inhibit on all transmission shifts
from Neutral-to-Drive, Neutral-to-Reverse,
and, if applicable, Park-to-Drive and Reverse.

Wire 101

ENGINE BRAKE INTERFACE(STANDARD) INPUT H

This function is enabled when one wireis switched to send power to the TCM.
When the throttle is closed, the transmission
lockup clutch is engaged and this function
is enabled. The TCM activates the function
by switching one wire from ground to open.
Simultaneously, the TCM will commandpreselect downshifts based on the specified
Preselect Strategy, Standard or Low, andthe Customer Modifiable Constants.

GMLAN

ANTI-LOCK BRAKE SYSTEM(ABS) INPUT Y

This function disengages the transmission
lockup clutch, modifies the downshiftschedule, and disables the retarder (ifequipped) during vehicle braking conditions
which activate the vehicle’s ABS system.

GMLAN

SERVICE BRAKE STATUS INPUT AA

This function is enabled when a switch isclosed to complete the circuit.

GMLAN

GRADE BRAKING/REGENERATIVEINPUT BSThis function is enabled when one wire isswitched to send power to the TCM.

GMLAN

Copyright © 2018 Allison Transmission, Inc. All Rights Reserved.

#8158October, 2018
Page 3 of 4

Name

Description

Input via GMLAN or Input Wire

HIGH N/v RATIO INPUT CC

This function is enabled when a switch is
closed to complete the circuit between an
input wire and ground, and a switch between
a wire and switched power is opened. Thiscondition alerts the TCM that the transfer
case shift mechanism has been shifted to theLow position which results in a high N/v ratio.

GMLAN

Output Functions
Name

Description

ENGINE BRAKE INTERFACEOUTPUT A

This function is enabled to signal that the TCM isin a state to allow engine brake activation. Whenthe throttle is closed, the transmission lockup clutch
is engaged and this function is enabled. The TCM
activates the function via GMLAN.

Output via GMLAN orOutput Wire

GMLAN

Simultaneously, the TCM will command preselectdownshifts based on the specified Preselect Strategy,Standard or Low, and the Customer Modifiable
Constants.

SUMP/RETARDER TEMPERATURE
INDICATOR OUTPUT B

This output function is activated by the TCM switching
one wire from open to ground when the TCM detects
one of the following over-temperature conditions in thesump:

Wire 164• 121º C (250º F) for 15 minutes• 128º C (262º F) for 1.5 minutes• 132º C (270º F) for 3 seconds

RANGE INDICATOR (5thGeneration DEFAULT IS ALWAYS
NEUTRAL/PARK) OUTPUT C

This output function is activated by the TCM switching
one wire from open to ground, which occurs when
the specified gear (or gears) is commanded by
the TCM. The range at which activation occurs
is adjustable with diagnostic tools.

Wire 145

OUTPUT SPEED INDICATORA OUTPUT DThis output function is activated by the TCM switchingone wire from ground to open, which occurs when
the TCM detects that a preset output speed has
been exceeded in either the forward or reverse
direction. The speed setting at which activation
occurs is adjustable with diagnostic tools.

Wire 105

Copyright © 2018 Allison Transmission, Inc. All Rights Reserved.

#8158October, 2018
Page 4 of 4Name

Description

Output via GMLAN orOutput Wire

The TCM activates the PTO Drive Inter-face Output
by switching power to one wire when the following
conditions have been met:
• The function has been requested
PTO DRIVE INTERFACE 1
OUTPUT G1• Throttle position is low

GMLAN• Engine speed is within Customer Modifiable
Constant limits• Output speed is within Customer Modifiable
Constant limits

TRANSMISSION SERVICEINDICATOR OUTPUT O

RANGE INHIBITED INDICATOROUTPUT AD

This output function is activated by the TCM
switching one wire from open to ground indicatingthat transmission service is required. The function
is deactivated when the service condition has beenaddressed and the indicator reset.

GMLAN

This output function is activated by the TCM switching
one wire from open to ground when the TCM istemporarily limiting transmission shifts. An activeRange Inhibited signal alerts the operator to thefollowing:• Shifts from Neutral-to-Drive or Neutral-to-Reverseare inhibited.
• Directional change shifts are inhibited.
• The transmission is in Neutral even though a
Drive range or Reverse is selected.

Copyright © 2018 Allison Transmission, Inc. All Rights Reserved.

Wire 124 and GMLAN