DRIVEABILITY AND EMISSIONS 6E1-81
SYMPTOM DIAGNOSIS
PRELIMINARY CHECKS
Before using this section, perform the "On-Board
Diagnostic (OBD) System Check" and verify all of the
following items:
• The engine control module (ECM) and malfunction
indicator lamp (MIL) (Check Engine lamp) are
operating correctly.
• There are no Diagnostic Trouble Code(s) stored.
• Scan Tool data is within normal operating range.
Refer to Typical Scan Data Values.
• Verify the customer complaint and locate the
correct symptom in the table of contents. Perform
the procedure included in the symptom chart.
VISUAL/PHYSICAL CHECK
Several of the symptom procedures call for a careful
visual/physical check. This can lead to correcting a
problem without further checks and can save valuable
time. This check should include the following items:
• ECM grounds for cleanliness, tightness and proper
location.
• Vacuum hoses for splits, kinks, and proper
connections, shown on the " Emission Control
System Schematics". Check thoroughly for any type
of leak or restriction.
• Air intake ducts for collapsed or damaged areas.
• Air leaks at throttle body mounting area, manifold
absolute pressure (MAP) sensor and intake
manifold sealing surfaces.
• Ignition wires for cracking, hardness, and carbon
tracking.
• Wiring for proper connections, pinches and cuts.
INTERMITTENT
Important: An intermittent problem may or may not
turn on the malfunction indicator lamp (MIL) or store a
Diagnostic Trouble Code. DO NOT use the Diagnostic
Trouble Code (Diagnostic Trouble Code) charts for
intermittent problems. The fault must be present to
locate the problem.
Most intermittent problems are caused by faulty
electrical connections or wiring. Perform a careful
visual/physical check for the following conditions.
• Poor mating of the connector halves or a terminal
not fully seated in the connector (backed out).
• Improperly formed or damaged terminal.
• All connector terminals in the problem circuit should
be carefully checked for proper contact tension.
• Poor terminal-to-wire connection. This requires
removing the terminal from the connector body to
check.
• Ignition coil shorted to ground and arcing at ignition
wires or plugs.
• MIL (Check Engine lamp) wire to ECM shorted to
ground.
• Poor ECM grounds. Refer to the ECM wiring
diagrams.
Road test the vehicle with a 5-8840-2392-0 Digital
Multimeter connected to a suspected circuit. An
abnormal voltage when the malfunction occurs is a
good indication that there is a fault in the circuit being
monitored.
Use a Scan Tool to help detect intermittent conditions.
The Scan Tools have several features that can be
used to locate an intermittent condition. Use the
following features to find intermittent faults:
To check for loss of diagnostic code memory,
disconnect the MAP sensor and idle the engine until
the MIL (Check Engine lamp) comes on. Diagnostic
Trouble Code 34 should be stored and kept in memory
when the ignition is turned "OFF." If not, the ECM is
faulty. When this test is completed, make sure that
you clear the Diagnostic Trouble Code 34 from
memory.
Check for improper installation of electrical options
such as lights, cellular phones, etc. Check all wires
from the ECM to the ignition control module for poor
connections.
Check for an open diode across the A/C compressor
clutch and check for other open diodes (refer to wiring
diagrams in Electrical Diagnosis).
If problem has not been found, refer to ECM
Connector Symptom tables.
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