7 ELECTRONIC CONTROL SYSTEMS DIAGNOSTICS 545
VSS Circuit O peration (Manual and Allison
Transmissions)
Figure 508 VSS circuit diagram (manual and Allison transmissions)
Fault Detection / Management
The ECM performs diagnostic checks on the VSS
circuit when the engine is operating at 0 mph. The
ECM transmits a voltage signal on the VSS circuit and
determines if the return voltage is out of range high
or low. When a fault condition is detected, the ECM
disables the cruise control and power takeoff. If the
road speed limiting option is enabled, the ECM will
limit engine rpm for all gears.
The ECM will not set DTCs for VSS circuit failure for
vehicles equipped with Allison transmissions.
NOTE: See truck Chassis Electrical Circuit Diagram
Manual for circuit numbers, connector and fuse
locations. To diag n os e Allison transm issio n VSS
sensor problems, see truck Elec trical System
Troubleshooting Guide and Allison maintenance
and diagnostic manuals.
VSS Diagnostic Trouble Codes (DTCs)
DTCs are read using the EST or by counting the
flashes from the amber and red ENGINE lamp.
DTC 141
VSS signal ou t -o f -ra ng e low
• DTC 141 is set by the ECM when an out of range
low condition is detected in the VSS circuit.
• When DTC 141 is active the amber ENGINE lamp
is not illuminated.
DTC 142
VSS signal out-of-range high
• DTC 142 is set by the ECM when an out of range
high condition is detected in the VSS circuit.
EGES-270-1
Read all safety instructions in the "Safety Information" section of this manual before doing any procedures.
Follow all warnings, cautions, and notes.
© August 2008 Navistar, Inc.
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