Testing For Intermittent Conditions

If the Tech-1 displays a code intermittently, or if after a test drive a code does not reappear, the problem is most likely a faulty electrical connection or loose wiring. Terminals (including grounds) should always be the prime suspect. Intermittents rarely occur inside sophisticated electronic components such as the Transmission Control Module (TCM).

Intermittent trouble codes in the 4L30-E automatic transmission electrical control system may be stored by the TCM. These codes can be helpful in diagnosing intermittent conditions.

USING SELF-DIAGNOSTIC SYSTEM TO ISOLATE SUSPECT CIRCUITS

If an intermittent condition is being diagnosed, the TCM self-diagnostic system can be used to help isolate the suspect circuit:

1. First, display any TCM trouble codes which may be present in the TCM and follow the appropriate trouble code chart.
2. Then test drive the vehicle, attempting to duplicate the failure condition. A description of the driving circumstances under which the failure occurs, if available, can be helpful in duplicating the condition.
3. After duplicating the condition, check for any TCM fault codes that may have been set and follow the appropriate fault code chart.
4. If no fault codes were stored, follow step 5 of the TEST DRIVING section.

CHECKING SUSPECT CIRCUITS

If the system malfunction is not repeated, a good description of the vehicle behavior may be helpful in locating a "most likely" component or circuit. Most intermittent problems are caused by faulty electrical connections or wiring. When an intermittent failure is encountered, check suspect circuits for:

1. Poor mating of connector terminals or terminals not fully seated in the connector body (backed out).
2. Improperly formed or damaged terminals. All connector terminals in a problem circuit should be reformed to increase contact tension.
3. Poor terminal to wire connection. This requires removing the terminal from the connector body to inspect.
4. Dirty, corroded, or distorted terminals and/or connectors.
5. Loose, dirty, or corroded ground connections. HINT: Any time you have an intermittent in more than one circuit, check whether the circuits share a common ground connection.
6. Pinched or damaged wires.
7. Electro-Magnetic Interference (EMI). HINT: Check that all wires are properly routed away from spark plug wires, distributor wires, coil, and generator. Also check for improperly installed electrical options, such as lights, 2-way radios, etc.

USING TECH-1 F3: SNAPSHOT MODE TO HELP LOCATE INTERMITTENT FAULTS

On 1991 models, use the F3: SNAPSHOT mode of the Tech-1 to help isolate the cause of an intermittent fault. The snapshot mode will record information before and after the problem occurs. Set the snapshot to "trigger" on the suspect code (or codes) or, if you notice the reported symptom during test drive, trigger the snapshot manually.

After the snapshot has been triggered, command the Tech-1 to play back the flow of data recorded from each of the various sensors. Signs of an intermittent fault in a sensor circuit are a sudden unexplainable jump in data values out of the normal range.

When checking trouble codes that indicate "high voltage signal", the circuit should be tested for:

1. High sensor resistance.
2. Open in wire or poor wire connection to sensor.
3. Open or poor connection to ground.

When checking trouble codes that indicate "Low voltage signal", the circuit should be checked for:

1. Low sensor resistance.
2. Sensor feed wire short to ground.
3. Low system voltage.