9.2 Electronic Ignition, Timing & Misfire Diagnosis

Four Generations of Ignition Switching

The Skill Verification Program competency standard lists Different Types of Ignition System as required underpinning knowledge. You are expected to recognise which architecture is in front of you before choosing a test method.

Waste spark explained

A waste-spark coil has two HT towers and fires both simultaneously. On a four-cylinder engine, cylinders 1 and 4 are paired and cylinders 2 and 3 are paired. One cylinder is on its compression stroke and receives the useful spark; its partner is on its exhaust stroke, where cylinder pressure is low, so very little voltage is required and that spark is wasted harmlessly.

A practical consequence follows. Because the two plugs sit in series across one secondary winding, one plug fires centre electrode to earth electrode and the other fires earth electrode to centre electrode. The reversed-polarity plug wears its earth electrode faster, which is why waste-spark engines often specify a shorter plug replacement interval than a coil-on-plug engine of the same size.

Coil-on-plug

Coil-on-plug mounts an individual coil directly on each spark plug and removes HT leads entirely. The advantages are a longer coil charging period available per cylinder, no lead resistance losses and no cross-firing. The trade-offs are heat exposure in the plug well and boot tracking. A cracked or oil-soaked coil boot is one of the most common misfire causes in the field. Look for a white or grey carbon track burned into the rubber boot and a matching sooted ring on the plug insulator.

Ignition Trigger Sensors

The competency standard requires you to check ignition system sensors and replace faulty ones. There are two families.

Variable reluctance, also called inductive or magnetic pickup

• Two wires, no external power supply. The sensor generates its own alternating voltage.
• Output amplitude rises with speed, so the sensor produces very little at low cranking speed. A weak battery causing slow cranking can therefore create an intermittent no-start on a perfectly good sensor.
• Test with a digital multimeter on AC volts while cranking, typically 0.3-2.0 V AC, or better with an oscilloscope to see the waveform and the missing-tooth synchronisation gap.
• Coil resistance is typically 200-2,000 ohms; compare against the manual.

Hall-effect and magneto-resistive active sensors

• Three wires: supply at 5 V or 12 V, earth and signal.
• Produce a constant-amplitude square wave whose height does not fall with speed, so they read reliably down to zero engine speed.
• Test supply and earth first, then confirm signal switching with an oscilloscope or a meter capable of reading duty cycle.

The crankshaft position sensor tells the ECU crankshaft position and speed. The camshaft position sensor tells it which stroke a given cylinder is on so that sequential injection and coil-on-plug firing can be phased. Lose the crankshaft signal and the engine will not run at all on most vehicles. Lose the camshaft signal and many engines still start, but only after extended cranking while the ECU works out phasing by trial firing.

Checking and Setting Ignition Timing

The competency standard lists the ignition timing light as required equipment.

1. Warm the engine to normal operating temperature and confirm idle speed is within specification. A wrong idle speed gives a wrong reading because centrifugal advance may already be operating.
2. Disable computer timing control using the manufacturer's method. On older systems this means unplugging a timing-set connector or grounding a specific diagnostic terminal. On many modern engines base timing is not adjustable at all; it is fixed by the crankshaft reluctor and any deviation is a mechanical fault such as a jumped timing belt.
3. Clean the crankshaft pulley marks and the fixed pointer, then highlight the specified degree line with chalk or paint so it is readable under the strobe.
4. Clamp the inductive pickup around the number one cylinder HT lead, observing lead direction where the tool is polarity sensitive.
5. Strobe the marks and compare against specification, for example 10 degrees before top dead centre at idle.
6. Reconnect the timing control connector and confirm timing now advances as engine speed rises, which proves the advance function works.

Advance mechanisms to verify

• Centrifugal advance: timing should advance smoothly as speed increases. A stuck weight or broken spring gives flat or erratic advance.
• Vacuum advance: disconnecting and plugging the vacuum hose should retard timing at part throttle. A split diaphragm gives no change and usually a vacuum leak as well.
• Electronic advance: verify with scan-tool live data showing commanded spark advance against engine speed and load.

The Knock Sensor

A knock sensor is a piezoelectric device bolted to the engine block that converts the specific vibration frequency of detonation, typically in the 5 kHz to 15 kHz band, into a small alternating voltage. The ECU listens only in a narrow window around each combustion event. When it hears knock it retards ignition timing in small steps until knocking stops, then creeps advance back in.

Diagnostic points:

• Torque matters. A knock sensor tightened loosely or over-torqued will not couple vibration correctly and sets a circuit or performance code in the P0325 to P0332 range.
• A knock sensor fault commonly appears as reduced power and higher fuel consumption because the ECU applies a default safety retard.
• Never fit a knock sensor with a washer or thread sealant unless the manufacturer specifies one, because both change the mechanical coupling.

Misfire Detection and Diagnosis

Modern ECUs detect misfire by watching the crankshaft accelerate and decelerate between firing events through the crankshaft position signal. A cylinder that does not contribute torque produces a measurable deceleration, and the ECU counts those events per 1,000 engine revolutions.

Malfunction indicator lamp behaviour is a safety instruction, not a suggestion.

• Steady lamp - an emissions fault is stored and the vehicle can normally be driven to the workshop.
• Flashing lamp - a misfire severe enough to damage the catalytic converter is happening right now. Raw fuel entering the catalyst can drive it past 900 degrees Celsius. Reduce load and speed and get the vehicle in for repair immediately.

A disciplined misfire routine

1. Read the codes and the freeze-frame data before touching anything. Freeze frame records engine speed, load, coolant temperature and fuel trims at the moment the misfire was logged, which tells you whether this is a cold-start, idle or load problem.
2. Check misfire counters per cylinder in live data. A single cylinder points to ignition, injector or compression on that cylinder. A group of cylinders sharing one coil pack points to that coil. A random pattern points to a shared cause such as fuel pressure, a vacuum leak or a degrading crankshaft signal.
3. Swap components between cylinders. Move the coil from the misfiring cylinder to a healthy one, clear codes and re-run. If the misfire follows the coil, the coil is faulty. Repeat with plugs and injectors. This one technique resolves a large share of misfires with no parts purchase.
4. Only then reach for the oscilloscope to examine secondary ignition patterns, injector waveforms or relative compression.

Safety rules the practical assessment will watch

• Never crank an engine with an HT lead removed and hanging free. Unburned fuel passes into the catalytic converter and can destroy it, and the open lead can arc to a technician or to fuel vapour.
• Use a proper spark tester with a defined gap rather than holding a lead near the block. Secondary voltage on a modern coil-on-plug system can exceed 40,000 V.
• Disable the ignition or the fuel supply using the manufacturer's method before a compression or cranking test.
• Allow coils and plugs to cool on an aluminium cylinder head before removal, so the threads are not stripped.