IK1300138 - TerraStar Transfer Case Diagnostic and Operational Information

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8/10/2013

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Title: TerraStar Transfer Case Diagnostic and Operational Information
Applies To: TerraStar 4x4 Models

Description
TerraStar 4x4 model vehicles are built with a FABCO TC-28 transfer case with 4 feedback switches and 2 electric actuators. The body
controller monitors the feedback switches to regulate when it grounds each one of the 4 relays that control the amperage and polarity to
each of the 2 electric actuators. The transfer case has 3 modes, 4x2 high, 4x4 high, and 4x4 low.

Interlocks To Change Ranges
To shift the transfer case from 4X2 high mode to 4x4 high mode, the following interlocks must be met:






Key is in the run position
Engine speed is above programmable parameter 3047, Engine_Operating_Threshold. Default setting is 300 RPM
Vehicle speed is below programmable parameter 2812, Axle_Engage_Threshold. Default setting is 24 MPH
Axle differential speed is below programmable parameter 3052, FR2RR_Vel_Diff_ABS_Threshold. Default setting is 0.93 MPH
Front axle switch is in the upper position and has a good status

To shift the transfer case from 4x4 high mode to 4x4 low mode, the following interlocks must be met:








Transfer case is in 4x4 high mode
Vehicle speed is below programmable parameter 3045, Range_Engage_Threshold. Default setting is 3 MPH
Transmission is in the neutral range
Service brake is depressed
Key is in the run position
Engine speed is above programmable parameter 3047, Engine_Operating_Threshold. Default setting is 300 RPM
Range switch is in the lower position and has a good status

When shifting from 4x4 high to 4x4 low mode, the front axle indicator lamp in the front axle switch will flash. This is normal operation.

DLB Template
Click HERE for the DLB template for diagnosing this.

Signal Descriptions
Signal

B/C Pin

Transfer case Mode

Description

Xfer_Case_4x4_Cmd

1601-E13

N/A

This is the ground command to the 4x4 relay

Xfer_Case_4x2_Cmd

1601-E9

N/A

This is the ground command to the 4x2 relay

Xfer_Case_High_Cmd

1601-F4

N/A

This is the ground command to the high range relay

Xfer_Case_Low_Cmd

1601-F5

N/A

This is the ground command to the low range relay

Transfer_Case_Front_Axle_Disengaged

1602-F11

Front axle is not engaged when checked
in DLB

This is the signal from switch SW1, indicating the
transfer case position.

Transfer_Case_Front_Axle_Engaged

1602_F13

Front axle is engaged when checked in
DLB

This is the signal from switch SW2, indicating the
transfer case position.

Transfer_Case_Driveline_Engaged_Low

1602-F8

Transfer case is in low range when
checked in DLB

This is the signal from switch SW3, indicating the
transfer case position.

Transfer_Case_Driveline_Engaged_High

1602-F9

Transfer case is in high range when
checked in DLB

This is the signal from switch SW4, indicating the
transfer case position.

Switch Locations

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IK1300138 - TerraStar Transfer Case Diagnostic and Operational Information

Page 2 of 3

Troubleshooting
When the transfer case is any range, the switch that corresponds with that location will be closed, completing the feedback circuit to the
body controller. The high and low range switches should never both be closed at the same time. The front axle engaged and front axle
disengaged switches should never both be closed at the same time either.
The first step to diagnosing the system is to understand which range the transfer case currently is in. There are two ways to figure this out.
The first way is to place the vehicle in neutral with the engine off and wheels chocked, and comparing the input and output rotations of each
shafts are they are being turned by hand in comparison to each other. The second is to drive the vehicle and see it the speedometer is
accurate, and if the vehicle is capable of reaching highway speeds. The transfer case low range is a 2.2:1 underdrive. This means that when
the transfer case is in low range, for every 2.2 turns of the output shaft from the transmission to the transfer case, the driveshafts that
connect to the axles turn one time. Once the transfer case range is understood, this should be compared to the reading of the feedback
switches in DLB with the key on. The table above gives signal descriptions for each range. If the feedback signals do not match the current
position of the transfer case, the body controller will not send a signal for the transfer case to shift.

Circuit Diagram

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IK1300138 - TerraStar Transfer Case Diagnostic and Operational Information

Page 3 of 3

Transfer Case Information
The parts and service manual for the FABCO TC-28 transfer case can be obtained by clicking HERE.
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Copyright © 2013 Navistar, Inc.

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