7 ELECTRONIC CONTROL SYSTEMS DIAGNOSTICS 367
Operation
The ECM contro ls the starting system. The clutch
switch or transmission neutral switch provide input to
the ECM. Both switches prevent the starter from being
engaged unless the transmission is in neutral or the
clutch is depressed.
Start Relay
The engine starter relay controls voltage to the starter
motor. Turning the ignition switch to start position
supplies current to energize the relay at Pin 86. If the
engine is not running and the driveline is not engaged,
the ECM Pin X3–23 will enable the relay by suppling
a ground circuit to Pin 85 of the relay. When the relay
is closed, current passes through the relay to the pins
on the starter solenoid.
Before troubleshooting, inspect circuit connectors
for loose or damaged pins or wires. Wires and
connections must be free of damage or corrosion.
When connectors corrode, a white residue will be
present and must be removed. Make sure the
batteries are fully charged. To ensure accurate
readings, check battery cables and grounds for clean
and tight connections.
Clutch Switch
Manual transmissions use the clutch switch to supply
a signal to the ECM indicating that the driveline
is disengaged. A 12 V signal on the Driveline
Disengagement Switch (DDS) circuit indicates that
the clutch is disengaged. A 0 V signal indicates that
the clutch is engaged.
Neutral Switch
Allison LCT t ransmissions use the neutral position
switch to supply power to the starter relay and a signal
to the ECM that the driveline is disengaged. Vehicles
programmed fo r Allison AT/MT transm iss ion s re ce ive
a 12 V signal on the DDS circuit indicating that the
transmission is out of gear. A 0 V signal indicates that
the transmission is in gear. When the transmission is
in gear no power is available to the ECI relay.
WTEC MD with Auto Neutral
Allison MD World Transmission Electronically
Controlled (WTEC) transmissions (with optional
Auto Neutral) have a crank inhibit system with an
additional relay. The relay inhibits cranking when
the transmission is in auto neutral. Pin 6 of the
transmission ECU controls 12 V to Pin 86 of the
starter relay. Pin X3–8 of the ECM receives 12 V from
the WTEC Auto Neutral re lay when the transmission
is shifted to neutral or auto neutral. Without the
additional relay, the DDS input (Pin X3–8) allows
cranking in auto neutral.
Electronic Control Module (ECM)
When the ECM recognizes that t he engine is not
running and the driveline is not engaged, the ECM
will ground Pin X3–23. This provides a current path
for the ECI relay to close when the Start switch is
engaged or the starter button is depressed.
When the ECM recognizes that the engine is running
or the driveline is engaged, the ECM will open Pin
X3–23. This prevents the ECI relay from closing and
the starter motor from engaging.
Fault Detection / Management
When the on demand Engine ON standard test is run,
an open or short to ground can be detected on the coil
side of the EC I relay.
Tools
• EST with MasterDiagnostics® software
• EZ-Tech® interface cable
• Digital Multimeter (DMM)
• Breakout Box
• 3-Banana Plug Harness
• 500 Ohm Resistor Harness
• Relay breakout harness
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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