9.1 Ignition System Fundamentals: Primary & Secondary Circuits

Why the Ignition Circuit Is Its Own Competency

The Skill Verification Program occupational standard for Automobile Electrician lists Repair Ignition System as a separate competency unit inside the Repair Electrical Systems of Vehicle standard. Its performance criteria require you to check and replace the ignition switch, ignition coil and resistor, inspect high-tension (HT) cables for damaged insulation and out-of-specification resistance, verify the electrical power source and charging system, clean or replace spark plugs, replace fuses with correct ratings, check the distributor, distributor cap and rotor, and test ignition system sensors.

An auto electrician owns this circuit end to end because almost every failure in it is electrical rather than mechanical.

The Two Circuits Inside Every Ignition System

The coil bridges the two. Its primary winding is a few hundred turns of relatively heavy wire; its secondary winding is tens of thousands of turns of very fine wire. The turns ratio of roughly 100:1 to 150:1 multiplies voltage when the primary magnetic field collapses.

Typical coil winding resistances

Always compare against the manufacturer's repair manual figure. Many coil-on-plug units contain the switching transistor inside the coil body, so an ohmmeter across the connector proves nothing. A resistance test on those coils is a common technician trap.

The Ignition Switch and the Ballast Resistor

Using DIN 72552 terminal designations, the ignition switch feeds:

• Terminal 30 - permanent unswitched battery positive into the switch.
• Terminal 15 - switched positive, live in RUN and START. This is the normal coil feed, routed through the ballast resistor where one is fitted.
• Terminal 15a - the output from the ballast resistor to the coil positive terminal.
• Terminal 50 - starter solenoid control, live only in START.

The ballast resistor solves a cranking problem. During cranking, battery voltage collapses to perhaps 9-10 V, which would weaken the spark exactly when the engine needs it most. The system therefore uses a coil with a low-resistance primary winding and inserts a ballast resistor in series for normal running. During cranking, a bypass circuit fed from the starter solenoid shorts the ballast out and applies full available voltage directly to the coil.

Two classic faults follow from this design:

1. Engine starts while cranking but dies the instant the key returns to RUN - the ballast resistor is open-circuit. The bypass carried the coil during cranking; when the bypass dropped out, the coil lost its feed.
2. Engine will not start when cold but runs once hot - the bypass wire from the solenoid is open, so the coil runs on reduced voltage during cranking.

A ballast resistor is a resistance wire or ceramic block and must never be replaced with ordinary wire, which would over-drive the coil and burn the points or the driver transistor.

Contact Breaker Points and the Condenser

Older vehicles still common in light commercial fleets use a contact breaker (CB) point set inside the distributor, driven by a cam on the distributor shaft. The competency standard explicitly lists C.B Point and Condenser as required knowledge.

• Point gap is typically 0.40-0.50 mm at full cam lift.
• Dwell angle is the number of degrees of distributor rotation for which the points remain closed and the coil is charging. Point gap and dwell angle are inversely related: widening the gap makes the points open earlier and close later, so dwell decreases; closing the gap increases dwell.
• The condenser (capacitor) sits in parallel with the points, typically 0.20-0.30 microfarads. It absorbs the inductive surge at the moment the points open, which stops the points arcing and makes primary current collapse faster, raising secondary voltage.

Failure signatures worth memorising:

Distributor, Cap, Rotor and HT Leads

On a points system the distributor performs three jobs: it triggers the primary circuit, it distributes secondary voltage to the correct cylinder, and it advances timing through centrifugal weights and a vacuum diaphragm.

Inspection checklist required by the competency standard:

• Cap - check for hairline cracks, carbon tracking lines between towers, corroded or eroded brass segments, and moisture ingress. A carbon track is a permanent low-resistance path, so the cap must be replaced rather than cleaned.
• Rotor - check the tip for erosion and the centre carbon contact for wear. Measure rotor resistance where a suppression rotor is used, commonly about 1,000-5,000 ohms.
• Centre carbon brush and spring inside the cap - a stuck or worn brush causes a total no-spark condition.
• Shaft end float and side play - excessive play changes dwell and timing together.

HT lead testing. Modern suppression leads use a carbon-impregnated or wound ferrite core to limit radio interference. Measure each lead end to end with a digital multimeter on the ohms range:

• Carbon-core leads: roughly 5,000-15,000 ohms per metre.
• Most manufacturers reject any single lead above about 25,000 ohms, and any lead reading open circuit.
• Always compare leads of similar length against each other. One outlier is usually the failed lead.

Check insulation in a darkened bay with the engine running: visible blue arcing to the block, the manifold or a bracket confirms breakdown. Never pull a live HT lead off by the wire; use insulated pliers on the boot only, and remember that secondary voltage can exceed 30,000 V.

Spark Plugs

Reading plugs is a diagnostic skill the practical assessment rewards. Light tan or grey deposits indicate normal combustion. Dry black soot indicates a rich mixture or weak ignition. Wet black oily deposits indicate oil control failure. A white, blistered or melted tip indicates overheating or pre-ignition. Torque plugs to specification - typically 20-30 Nm for a 14 mm gasket-seat plug and less for taper-seat designs - because both under-torque and over-torque ruin heat transfer.

Required Voltage Versus Available Voltage

Every spark event is a race between two figures:

• Required voltage is the voltage needed to ionise the gap. It rises with a wider plug gap, higher cylinder pressure under heavy load or boost, a lean mixture, and worn electrodes.
• Available voltage is the maximum the coil can deliver. It falls with a weak coil, high primary resistance, a low battery, high HT lead resistance, or insulation leakage.

A misfire under load with a perfect idle is the classic signature of a small margin between the two. At idle the required voltage is low enough for a tired coil to cope; under load the required voltage climbs past what the system can supply.