GM Service Manual Online
For 1990-2009 cars only

Inspection/Test

Action

DEFINITION: The problem is not currently present but is indicated in DTC History.

OR

There is a customer complaint, but the symptom can not currently be duplicated (if the problem is not DTC related).

Preliminary

    • Refer to Symptoms - Engine Controls for the Important Preliminary Checks Before Starting.
    • The fault must be present to locate a problem using the DTC table. If a fault is intermittent, the use of DTC tables may result in the replacement of good parts.

Harness/Connector

Many intermittent open or shorted circuits come and go with harness/connector movement caused by vibration, engine torque, bumps/rough pavement, etc. Check for this type of condition by performing the applicable procedure from the following list:

    • Move related connectors and wiring while monitoring the appropriate scan tool data.
    • Move related connectors and wiring with the component commanded ON (and OFF) with the scan tool. Observe the components operation.
    • With the engine running, move related connectors and wiring while monitoring engine operation.

If harness or connector movement affects the data displayed, component/system operation, or engine operation inspect and repair the harness/connections as necessary. Refer to Wiring Repairs and Connector Repairs in Wiring Systems.

Electrical Connections or Wiring

Poor electrical connections/terminal tension or wiring problems cause most intermittents. Perform a careful inspection of the suspected circuit for the following:

    • Inspect for poor mating of the connector halves, or terminals not fully seated in the connector body (backed out).
    • Inspect for improperly formed or damaged terminals. Test for poor terminal tension.
    • Inspect for poor terminal to wire connections including terminals crimped over insulation. This requires removing the terminal from the connector body.
    • Inspect for corrosion/water intrusion.
    • Inspect the harness for pinched, cut or rubbed through wiring.
    • Make sure wiring does not come in contact with hot exhaust components.

Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

PCM Power and Grounds

Poor power or ground connections can cause widely varying symptoms.

    • Test all powertrain control module (PCM) power circuits. Many vehicles have multiple power circuits feeding the PCM. Inspect connections at the PCM connectors, fuses, and any intermediate connections between the power source and the PCM. A test lamp or a DMM may indicate that voltage is present, but neither tests a circuits ability to carry sufficient current. Ensure that the circuit can carry the current necessary to operate the component. Refer to Power Distribution Schematics and Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    • Test all PCM ground and system ground circuits. The PCM may have multiple ground circuits. Other components in the system may have separate grounds that may also need to be tested. Inspect grounds for clean and tight connections at the grounding point. Inspect the connections at the component and in splice packs (where applicable). Ensure that the circuit can carry the current necessary to operate the component. Refer to Ground Distribution Schematics and Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

Temperature Sensitivity

    • An intermittent condition may occur when a component or connection reaches normal operating temperature. The condition may occur only when the component or connection is cold, or only when the connection is hot.
    • The following data may help to diagnose this type of intermittent condition:
       - Freeze Frame and Failure Records
       - Scan tool snapshot
       - Vehicle data recorder
    • If the intermittent is related to heat, review the following data:
       - High ambient temperatures
       - Underhood or engine generated heat
       - Circuit generated heat due to a poor connection, or high electrical load
       - Higher than normal load conditions, such as towing
    • If the intermittent is related to cold, review the following data:
       - Low ambient temperatures -- In extremely low temperatures, ice may form in a connection or component. test for water intrusion.
       - The condition only occurs on a cold start.
       - The condition is not present after the vehicle warms up.
    • Information from the customer may help to determine if the trouble follows a pattern that is temperature related.

Electromagnetic Interference (EMI) and Electrical Noise

Some electrical components/circuits are sensitive to electromagnetic interference (EMI) or other types of electrical noise. Inspect for the following conditions:

    • A misrouted harness that is too close to high voltage/high current devices such as secondary ignition components, motors, generator, etc. These components may induce electrical noise on a circuit that could interfere with normal circuit operation.
    • Electrical system interference caused by a malfunctioning relay, PCM driven solenoid, or switch. These conditions cause a sharp electrical surge. Normally, the problem will occur when the malfunctioning component is operating.
    • Improper installation of non-factory (aftermarket) add-on accessories such as lights, 2-way radios, amplifiers, electric motors, etc.
    • Test for an open diode across the A/C compressor clutch, and for other open diodes.
    • Test the generator for a bad rectifier bridge that may be allowing AC noise into the electrical system. Refer to Diagnostic System Check - Engine Electrical in Engine Electrical.

Incorrect PCM Programming

There are only a few situations where reprogramming a PCM is appropriate:

    • A new service PCM is installed
    • A PCM from another vehicle is installed
    • Revised software/calibration files have been released for this vehicle.

Important: DO NOT re-program the PCM with the SAME software/calibration files that are already present in the PCM. This is not an effective repair for any type of driveability problem.

Verify that the PCM contains the correct software/calibration information. If incorrect programming is found, re-program the PCM with the most current software/calibration information. Refer to Service Programming System (SPS) in Programming.

Duplicating Failure Conditions

If none of the previous checks are successful, attempt to duplicate and/or capture the failure conditions.

Freeze Frame/Failure Records data (where applicable) contains the conditions that were present when the DTC set.

  1. Review and record Freeze Frame/Failure Records data
  2. Clear DTCs using the scan tool.
  3. Turn the key to OFF and wait 15 seconds.
  4. Operate the vehicle under the same conditions that were noted in Freeze Frame/Failure Records data (as closely as possible). The vehicle must also be operating within the Conditions for Running the DTC. Refer to Conditions for Running the DTC in the supporting text of the DTC being diagnosed.
  5. Monitor DTC Status for the DTC being tested. The scan tool will indicate Ran, when the enabling conditions have been satisfied long enough for the DTC to run. The scan tool will also indicate whether the DTC passed or failed.

An alternate method is to drive the vehicle with DMM connected to a suspected circuit. An abnormal reading on the DMM when the problem occurs, may help you locate the problem.

Scan Tool Snapshot

The scan tool can be set up to take a "snapshot" of the parameters available via serial data. The Snapshot function records live data over a period of time. The recorded data can be played back and analyzed. The scan tool can also graph parameters singly or in combinations of parameters for comparison. The snapshot can be triggered manually at the time the symptom is noticed, or set up in advance to trigger when a DTC sets.

An abnormal value captured in the recorded data may point to a system or component that needs to be investigated further.

Refer to the scan tool user instructions for more information on the Snapshot function.