Important Preliminary Checks Before Starting
Before using the Symptom Tables, you should have performed the
Powertrain On Board Diagnostic (OBD) System Check
and verify
all of the following are true:
| • | The PCM and MIL (Service Engine Soon) are operating correctly. |
| • | There are no DTC(s) stored. |
| • | Verify the customer complaint, and locate the correct symptom
in the table of contents. Check the items indicated under that symptom. |
Several of the symptom procedures call for a Careful Visual/Physical
Check. The importance of this step cannot be stressed too strongly -
it can lead to correcting a problem without further checks and can save valuable
time. This check should include:
| • | PCM grounds for being clean, tight, and in their proper location. |
| • | Vacuum hoses for splits, kinks, and proper connections, as shown
on the Vehicle Emission Control Information label. Check thoroughly for any
type of leak or restriction. |
| • | Mass Air Flow (MAF) sensor installation. The arrows on the plastic
portion of the sensor must point toward the engine. |
| • | Air intake ducts for collapsed or damaged areas. |
| • | Air leaks at throttle body mounting area, Mass Air Flow (MAF)
sensor and intake manifold sealing surfaces. |
| • | Ignition wires for cracking, hardness, proper routing and carbon
tracking. |
| • | Wiring for proper connections, pinches and cuts. |
Intermittents
Important: Problem may or may not turn ON the MIL (Malfunction Indicator
Lamp) or store a DTC. DO NOT use the Diagnostic Trouble Code (DTC) tables
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 body (backed out). |
| • | Improperly formed or damaged terminal. |
| • | All connector terminals in the problem circuit should be carefully
reformed or replaced to insure proper contact tension. |
| • | Poor terminal to wire connection. This requires removing the terminal
from the connector body to check. |
Road test the vehicle with a J 39200 Digital Multimeter connected
to a 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 to help detect intermittent conditions. The GM Techline
equipment (Scan Tool) have several features that can be used to locate an
intermittent condition. Use the following features to find intermittent faults:
| • | The Snapshot feature can be triggered to capture and store engine
parameters within the scan tool when the malfunction occurs. This stored information
then can be reviewed by the service technician to see what caused the malfunction. |
| • | Using a Scan Tools Freeze Frame buffer or Failure Records buffer
can also aid in locating an intermittent condition. Review and capture the
information in the freeze frame or failure record associated with the intermittent
DTC being diagnosed. The vehicle can be driven in the conditions that were
present when the DTC originally set. |
Loss of diagnostic code memory. To check, disconnect MAP sensor and
idle engine until the MIL (Malfunction Indicator Lamp) comes ON (The engine
speed may require to be increased to 2000 RPM). DTC P0107 should be stored
and kept in memory when ignition is turned OFF. If not the PCM is faulty.
When this test is completed, DTC P0107 should be cleared from memory.
An intermittent MIL (Malfunction Indicator Lamp) with no stored DTC
may be caused by the following:
| • | Ignition coil shorted to ground and arcing at ignition wires or
spark plugs. |
| • | PCM grounds, refer to
PCM wiring diagrams. |
If the intermittent condition exists as a start and then stall, check
for DTC(s) relating to the theft deterrent system. Check for improper installation
of electrical options such as lights, cellular phones etc.
All Ignition Control (IC) wires should be routed away from secondary
ignition wires, distributor, coil, and generator. All wires from the PCM to
distributor should have a good connection.
Check for open diode across A/C compressor clutch and for other open
diodes.
