Use the following tables when diagnosing a symptom complaint.
The following Symptom tables are located in this section:
Important Preliminary Checks Before Starting
Perform
Powertrain On Board Diagnostic (OBD) System Check
before using the Symptom Tables, and
verify all of the following:
| • | The powertrain control module (PCM) and malfunction indicator
lamp (MIL) are operating properly. |
| • | There are no DTCs stored. |
| • | Ensure that the engine coolant temperature is not above 130°C
(266°F). This condition causes the PCM to operate in Engine Coolant Over
Temperature-Fuel Disabled Mode. While in Engine Coolant Over Temperature-Fuel
Disabled Mode, the PCM turns the fuel off to 3 cylinders at a time in order
to keep engine temperatures from reaching damaging levels. The system perceives
Engine Coolant Over Temperature as a lack of power, miss, or rough idle. |
| • | Verify the customer concern and locate the correct symptom in
the table of contents. Inspect the items indicated under that symptom. |
Visual/Physical Check
Several of the symptom procedures ask for a careful visual/physical
check. This step is extremely important. The check can lead to correcting
a problem without further checks and can save valuable time. This check
includes:
| • | Inspect the PCM grounds for being clean, tight, and in their proper
location. |
| • | Inspect the vacuum hoses for splits, kinks, and proper connections,
as shown on the Vehicle Emission Control Information label. Inspect thoroughly
for any type of leak or restriction. |
| • | Inspect the mass air flow (MAF) sensor installation. The arrows
on the plastic portion of the sensor must point toward the engine |
| • | Inspect the air intake ducts for being collapsed or for having
damaged areas. |
| • | Test for air leaks at the throttle body mounting area, the MAF
sensor, and the intake manifold sealing surfaces. |
| • | Inspect the ignition wires for cracking, hardness, proper routing,
and carbon tracking. |
| • | Inspect the engine harness wiring and terminals for proper connections,
pinches, or cuts. |
Intermittent
Important: Inspect for any improper installation of electrical components if an
intermittent condition exists. Inspect for aftermarket theft deterrent devices,
lights and cellular phones. Ensure that no aftermarket equipment is connected
to the Class II circuit. If you can not locate an intermittent condition,
a cellular phone signal communication may cause the condition.
Important : The problem may or may not turn on the malfunction indicator lamp (MIL)
or store a DTC. DO NOT use the Diagnostic Trouble Code (DTC) tables for intermittent
problems. The fault must be present in order to locate the problem.
Faulty electrical connections or wiring cause most intermittent problems.
Perform a careful visual/physical check for the following conditions:
| • | A faulty mating of the connector halves or a terminal not fully
seated in the connector body (backed out). |
| • | An improperly formed or damaged terminal. |
| • | Reform or replace the connector terminals in the problem circuit
in order to insure proper contact tension. |
| • | A faulty terminal to wire connection requires removing the terminal
from the connector body in order to repair the connection. |
Road test the vehicle with a J 39200
digital multimeter (DMM) connected to the suspected circuit.
An abnormal voltage when the malfunction occurs is a good indication that
there is a malfunction in the circuit being monitored.
Use a scan tool in order to help detect intermittent conditions. The
GM Techline equipment (scan tool) has several features that you can use to
locate an intermittent condition. Use the following features to find intermittent
faults:
| • | You can trigger the Snapshot feature in order to capture and
store engine parameters within the scan tool when the malfunction occurs.
You can then review this stored information in order to see what caused the
malfunction. |
| • | Using the scan tool Freeze Frame buffer or Failure Records buffer
can also aid in locating an intermittent condition. Review and capture the
information in the Freeze Frame and/or Failure record associated with the
intermittent DTC being diagnosed. Drive the vehicle in the conditions that
were present when the DTC originally set. |
Important : If the intermittent condition exists as a start and then stall, inspect
for DTCs relating to the theft deterrent system. Inspect for improper installation
of electrical options such as lights, cellular phones etc.
Any of the following may cause an intermittent MIL with no stored DTC:
| • | The ignition coils shorted to ground, or arcing at the ignition
wires or the spark plugs. |
| • | The ignition control (IC) wires routed too close to the secondary
ignition wires, coils, or the generator. Ensure that all of the circuits from
the PCM to the ignition coils have good connections. |
| • | An open diode across the A/C compressor clutch, or other open
diodes. |
