Intermittent conditions can be especially difficult to isolate. Use the following procedure for diagnosing intermittents.
1. Most intermittent problems are caused by faulty electrical connection or wiring. Perform careful visual/physical checks as described earlier in this section and in the Service Manual. Check for:
• Poor mating of the connector halves or 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 in order to check. Refer to “Wiring Harness Sen/ice” in the Sen/ice Manual.
2. If a visual/physical check does not find the cause of the problem, the vehicle can be driven with a voltmeter connected to a suspected circuit. A Tech 1 “Scan” tool can also be used to help detect intermittent conditions. An abnormal voltage or Tech 1 “Scan” tool reading when the problem occurs indicates the problem may be in that circuit. If the wiring and connectors check OK, and a diagnostic code was stored for a sensor circuit, substitute a good sensor and recheck.
3. An intermittent MIL/“Service Engine Soon” lamp with no stored diagnostic code may be caused by:
• MIL/“Service Engine Soon” lamp wire to PCM shorted to ground.
• “Diagnostic Request” terminal wire to PCM shorted to ground.
• PCM power supply problems.
• Internal PCM intermittent problems.
4. Check for electrical system interference caused by a defective relay, PCM driven solenoid, or switch. They can cause a shard electrical surge. Normally, the problem will occur when the faulty component is operated.
5. Check for improper installation of electrical options such as lights, 2 way radios, etc.
6. Check for open diode across A/C compressor clutch, and for other open diodes (refer to wiring diagrams).
7. If a problem has not been found, refer to the “PCM Connector Symptom” chart in the Driveability and Emissions Service Manual.
System-Based Diagnostics
System-based diagnostic routines are used, as directed by the OBD check, when data parameter(s) are abnormal but not so far out of range or in combination with other parameters to set a code. Covered systems include: Fuel System, Glow Plug System, Exhaust Gas Recirculation (EGR) System, Transmission Controls, Crankcase Ventilation System, and Air Intake System
