Most practising diagnosticians will tell you that before you get down to the nitty-gritty of probing and measuring, diagnosing faults in complex control systems requires a combination of assessments performed through a structured, logical approach.

Vehicles since the 1980s have in increasing numbers been fitted with some form of electronic control system. An integral part of the control system is self diagnosis. This was initially designed to assist technicians in quickly and accurately diagnosing these new generation ‘black box’ electronic control systems. Prior to the proliferation of electronic control systems, technicians were primarily focussed on mechanical and basic electrical repairs, they were not experienced in diagnosing failures in these electronic systems, their circuits or components. Therefore, the self diagnosis system was a useful ‘crutch’ to aide technicians in their diagnosis.

Before the testing is performed obtaining an accurate description of the symptoms is imperative in order to make an accurate diagnosis. It is important not to make the mistake of effecting a repair based on a customer's description of the symptoms, and never let them troubleshoot the problem for you. Customers fault or symptom descriptions can be vague and typically non-technical. Equally important it not to let your superior knowledge or expertise pre-diagnose the fault without test, measurement and analysis.

The following procedure is suggested as a general diagnostic process:
 

• PERFORM A CUSTOMER INTERVIEW (DISCOVER EXACTLY WHAT IS HAPPENING AND WHEN)
• PERFORM A VISUAL INSPECTION (CHECK FOR BROKEN WIRES, SPLIT PIPES ETC.)
• INITIAL CUSTOMER ASSISTED TEST DRIVE (CUSTOMER DRIVES YOU WHILST TRYING TO REPLICATE THE PROBLEM)
• VALIDATE, ACKNOWLEDGE AND RECORD THE COMPLAINT
• PRE TOOL ANALYSIS PREPARATION (CHECK BASIC CONDITION OF PLUGS, LEADS, COILS, THROTTLE BUTTERFLY SERVICE AS NECESSARY)
• PERFORM SCAN TOOL ANALYSIS, (TEST DRIVE THE VEHICLE UNDER FAULT GENERATING OPERATING CONDITIONS)
• SCOPE/METER DATA ANALYSIS (TEST THE SYSTEM UNDER FAULT GENERATING OPERATING CONDITIONS)

Common Component Testing:

Non start then check:

Crankshaft Position Sensor (Inductive) - The Crankshaft Position Sensor (CKP) is a inductive sensor responsible for reporting engine speed and position to the Powertrain Control Module (PCM). The signal output is an Alternating Current (AC), its amplitude and frequency will be seen to increase with engine speed.  The inductive sensor is normally a two wire device, though some manufacturers use three wires, the third being linked to a coaxial braid in order to limit Radio Frequency Interference (RFI) that may corrupt the signal.

The gap in the waveform is due to the 'missing tooth' in the flywheel or reluctor and is used as a reference for the PCM  to determine the engine's position. This gap is normally located anywhere between 10° and 100° BTDC.  Some systems use multiple reference points on each revolution.  

Crankshaft position sensors tend to fail as they become hot and the windings become open circuit. In this instance the engine may stall, but will restart if it is left to cool down.  

The output voltage of this sensor will be affected by several factors.

1) The sensor air gap will in some cases be fixed and is non adjustable, while on other vehicles the air gap can be adjusted and measured using feeler blades. A larger air gap will reduce the voltage output from the sensor.

2) A failing sensor with shorted windings will also reduce the voltage output, while a sensor with an open circuit will have no output at all. The condition of the winding inside the crank angle sensor can be determined by conducting a resistance test with a multimeter.

3) A slower than normal cranking speed may also cause the output to be low.

Poor performance then check:

Mass Air Flow Meter -  The 'hash' on the waveform is quite normal and is due to induction pulses affecting the airflow as the engine is running.  The air is not drawn in continuously, but in time with the opening of the inlet valves. 

The voltage output from the Mass Air Flow Meter (MAF) will relate directly to the air flowing into the engine.  The sensor output should look similar to the example shown.  The waveform shows that the sensor output is approximately 1 volt when the engine is at idling.  This voltage will rise as the engine speed is increased, producing an initial peak.  This peak is due to the initial influx of air, and drops momentarily before the voltage is seen to rise again to another peak of approximately 4.5 volts. Be sure to rev the engine to its maximum. The minimum acceptable voltage the MAF should produce is approximately 4.5 volts. The maximum output voltage will depend on how rapidly the engine speed is increased, a lower voltage does not necessarily indicate a fault within the MAF sensor.

A MAF sensor which produces less than this during a max RPM free rev should be considered for replacement.

It is imperative not to assume that a low output MAF is at fault, as any other factors which affect airflow (poor mechanical performance, lack of fuel, ignition errors) will lead to a poor peak MAF output. These items must be checked and verified prior to final diagnosis.