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On-Board Diagnostic (OBD) System Check
Circuit Description
The on-board diagnostic (OBD) system check is a
starting point for any driveability complaint diagnosis.
Before using this procedure, perform a visual/physical
check of the ECM (engine control module), VIM
(vehicle interface module), and engine grounds for
cleanliness and tightness.
The OBD system check is an organized approach to
identifying a problem created by an electronic engine
control system malfunction.
Diagnostic Aids
An intermittent may be caused by a poor connection, a
rubbed-through wire insulation, or a wire broken inside
the insulator. Check for the following conditions and, if
a fault is found, perform repair or replacement.
• Poor
connection
connectors for backed-out pins, improper mating,
broken locks, improperly formed or damaged pins,
and poor pin-to-wire connection.
• Damaged harness: Inspect the wiring harness for
Battery
0.5B
Main
DLC
Relay
112
88
118 117 116 115 114
at
ECM:
Inspect
SBF
SBF
100A
50A
3W/L
SBF
50A
SBF
SBF
30A
60A
Glow
Relay
Glow
Relay
121 120 119
rubbed-through wire insulation or wire broken
inside the insulator.
Test Description
Number(s) below refer to the step number(s) on the
Diagnostic Chart.
3: The CHECK ENGINE lamp (MIL) should be "ON"
steady with the starter switch "ON"/engine "OFF". If
not, "Inactive CHECK ENGINE Lamp (MIL) Check"
should be used to isolate the malfunction.
4: Check the data communication circuit and ensures
that both ECM and VIM are able to transmit serial data.
5: When pins are jumpered and the voltage reads 0 V,
the cause is open wiring. When pins are not jumpered
and the voltage reads 24 V, the cause is short.
10: This test ensures that the ECM is capable of
harness
controlling the MIL and the MIL driver circuit is not
shorted to ground. If a problem presents, the vehicle
should be repaired using "Unblinking MIL Check".
Engine Control System 1A-47
IG
IG
(ST)
(ON)
#22
#19
10A
15A
CHECK
ENGINE
Lamp
113
109
99
ECM
(Engine
Control
Module)
95 87
LNW21ALF006001-X
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