GM/Detroit 6.5L – Diagnostics – DIAGNOSTIC APPROACHES

Three diagnostic approaches can be used to solve driveability problems: code-based diagnostics, symptom-based diagnostics, and system-based diagnostics. Each has a specific purpose and should only be undertaken as directed by the On-Board Diagnostic (OBD) check.

Code-Based Diagnostics
Code-based diagnostics centers around the Diagnostic Trouble Codes (DTCs) stored by the PCM. These codes are systematically numbered so that each code relates to a specific fault at a specific component. Using a “Scan” tool or the Malfunction Indicator Lamp (MIL), you can identify and correct the code condition(s) by following the appropriate trouble tree.

As much as possible, code numbering consistency is used in all GM vehicles. Depending on vehicle options and equipment, code numbers may or may not apply to specific vehicles. When using code-based diagnostics, always begin with the lowest numbered code first.

A complete list of 6.5L EFI diesel DTCs is contained in section 7 of this book. The list includes the code number, name, and the page number on which the circuit description for the code is located.

Symptom-Based Diagnostics
Symptom-based diagnostics are used for conditions that don’t set Diagnostic Trouble Codes but still result in driveability complaints such as sluggish performance, hard starting, or intermittents. These conditions generally produce normal “Scan” tool data values. Procedures for diagnosing driveability symptoms are found in section 2, “Driveability Symptoms,” of the Driveability and Emissions manual.

Covered conditions include:
• Intermittents
• Hard Starts
• Surges and/or Chuggles
• Lack of Power, Sluggish, Spongy
• Fuel Knock/Combustion Noise
• Poor Fuel Economy
• Excessive Smoke


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 Service” in the Service 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

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