Principles of fault analysis by diagnostic techniques

The serial code reader displays information that has been prepared by the vehicle electronics (ECM); the information is an interpretation of the vehicle operating environment and condition. The area of fault is indicated but confirmation is required for a correct diagnosis of the actual fault and its cause.

I always recommend complimentary products for the completion of the task; these are

• A multi-meter for direct measurement of the electrical signals, temperature, current or time base.
• A simulator to generate the correct signal or value which can be substituted for the suspected faulty component.
• The information or data which allows comparison with the measured value.
• An oscilloscope for visual representation of the signals

Diagnosis is a six-step process

• The vehicle presents a faulty operation or behavior to the mechanic and he uses the code reader to read faults or component values

• The mechanic uses a multi-meter or scope to measure the actual signal and he should compare the value to a source of data.

• If a difference exists between code reader and multi-meter values then a fault could exist in the ECM. If no significant difference exists then he will have identified the probable source of the fault.

• Final confirmation of the fault can be achieved by simulating a correct signal at the source of the input, i.e. the sensor.

• If the correct signal causes the symptoms to disappear then the fault exists in the sensor; if the fault does NOT disappear then the problem can be in the harness, connectors or ECM.

• Simulation is used as the final step in this process, but as vehicle electronics has advanced the ECM software can become counter-productive for this last step and may suppress faulty signal input, which will lead to miss-interpretation.

Steve Davies

March 2000