Car Care

Understanding Your Car

Understanding your car /Ignition system

Checking battery | Ignition system | Ignition coil | Spark plug leads | Distributor | Spark plugs Reading spark plugs | Distributor, points, timing | Adjusting Electronic Ignition

Checking and maintaining the battery

In a car in good order and condition having regular services, a battery should work well for at least 3 years. A battery that loses its charge, terminals that are corroded, wires that are frayed, is topped up using tap water, the hold down clamp is missing or loose, or the casing is cracked, is a battery that will let you down. If the
electrolyte level is low, requiring frequent top ups, it is a sign that the electrical system is faulty.

Maintaining the coil, distributor and spark plugs

The ignition system, sometimes called high tension system, converts the relatively low charging system voltage and battery voltage into very high voltage needed to jump across the spark plug gap. When the spark jumps across the gap it ignites the air and fuel mixture in the engine cylinder. The high tension system consists of a distributor, a transformer coil, high tension leads and spark plugs.

Ignition Coil

The ignition coil is a transformer. It transforms the car's 12 volts to many thousands of volts. It is firmly fixed to the engine or cars body under the bonnet. The clamp holding the coil must be clean and tight with a good electrical contact to metal surfaces. Inside the casing there is two coils of wire, the 12 volt wire is wound about 120~200 times around a thinner wire that is wound 20,000~120,000 times around an metal centre core. When connected to an electric circuit a strong magnetic field is concentrated around the coils, when the electricity is interrupted the magnetic field collapses inducing itself from one set of coils into the other set of coils. As we want to increase the voltage our battery voltage is connected to the lesser number of turns and the high tension (spark plug) leads are connected to the higher number of turns of wire. As the lesser number of coils field induces into the many, many turns of the other, the voltage is multiplied over and over again, causing the sharp rise in voltage necessary to jump the plug gap. The plastic tower on top of the coil forms the
insulator that stops the high voltage sparks from leaking into the car's body. The Ignition coil must be clean and free of damage.

HT Leads

High-Tension Leads are usually suppression or radio leads, which have a delicate carbon centre that can easily be damaged if pulled, kinked or handled roughly. When the leads centre carbon is damaged they are open circuit which means they have a gap in them, which the spark will have to jump before it can jump the spark plug gap. The electrical terminals may build up corrosion that increases the resistance of electricity. Usually the engine will misfire intermittently when this happens. When these leads are oil soaked or wet, the electricity can short along it and leak the electricity to ground also causing a misfire to occur.


The job of a
distributors is to distribute the high voltage spark to each individual spark plug in turn at the correct time of engine speed and rotation. The time it takes the air/fuel mixture to burn remains the same, but the engine rotates at different speeds, so to get the best performance, power and economy the mixture will need to be ignited at different times before the piston reaches 'top dead centre'. The distributor has a mechanical and a vacuum advance mechanism to advance the contact points or cam lobes to achieve this. The coloured, brittle cap on top of the distributor is made of plastic or Bakelite. It separates and insulates the individual leads, so that spark plugs can fire one at a time, in the correct order. The coil lead is connected to the centre, electricity passes through the cap via a terminal to the inside where it passes to a rotating button. The terminal on top of this rotor button passes the electricity to each plug lead in turn.

Spark plugs

Spark plugs seal off the compression gasses and allow an insulated terminal with a close gap to protrude into the combustion chamber of the engine cylinder. The spark plugs come in different heat ranges. Using the wrong heat range can make the engine run poorly and use excessive amounts of fuel, or worse it could severely damage the engine. The expected life expectancy of a normal spark plug is 20,000 - 40,000 km. Spark plugs should be cleaned and adjusted at regular intervals recommended in the vehicle handbook. Extreme care must be taken if using a cheap box spanner to remove & install spark plugs as they are usually too tight in the engine and the plug insulator is brittle and easily broken, also the hex can round off at the edges. If the spark plugs are too loose when installed, carbon will find its way up the threads and cause burning away or seizing of the threads. When cleaning spark plugs you will need a sandblasting plug cleaner and a plug gapping tool, also a thread cleaner could be handy in case of slightly damaged threads. Care of spark plugs is still very much under the control of the driver; hard driving from cold, often driving less than 25 klms, over-revving the engine, laboring the engine, can lead to shortened spark plug life.
NOTE:-Care is needed when removing spark plugs not to allow loose foreign material (dirt & grit) to enter the open plugholes when removing them or while they are removed. This can be disastrous.

Reading spark plugs

Reading spark plugs means looking closely to determine what conditions the plugs have been operating under.

If you suspect a fuel or ignition problem, look closely at the firing end of spark plugs under a strong light, particularly right down inside the edges alongside the centre electrode. The different deposits can help diagnose the problem. There are three basic indicators when looking at spark plugs:

Good | Fouled | Overheated
The borderline between the fouling and optimum operating temperatures (450 deg C) is called spark plug self-cleaning temperature. This temperature is the point that accumulated carbon is cleaned off. Temperatures above 850 deg C cause Pre-ignition and plug overheating. top

What you will see on the spark plug end

  • Normal plug: the normal plug is a matt finish, with reasonably sharp electrode edges insulator coloured brown or grey, the condition can be judged to be good and the spark plug is functioning correctly.
  • Worn plug: worn spark plugs will have terminal ends that are round and the gap between the terminals will be much larger than specifications.
  • Sooty fouling: If there is a black powdery finish on the electrode and surrounding metal, which can be wiped away, it indicates an excess of unburnt fuel.
  • Oily fouling: A wet oily deposit, possibly covered in grit, indicates faulty piston rings or valve guides.
  • Ash fouling: The accumulation of ashy deposits on the firing end is influenced by oil leakage, fuel quality and engine operating periods. Causes of fouling: the fouling indicated above could still operate a spark plug without misfiring if the resistance from the centre electrode to the shell is over 10MŮ. If the insulation resistance drops to 0, the firing end is either fouled by wet or dry carbon.

    Lead fouling: Lead fouling usually appears as yellowish brown deposits on the insulator nose and this can not be detected by a resistance tester at room temperature. Lead compounds combine at different temperatures; those formed at 370 ~ 420 deg C having the greatest influence on the resistance.
    Molten: Beads of molten metal indicate excessive heat, the electrodes may even be welded together.
    Causes of molten and lead fouled plugs: Worn out spark plugs: A soft ash deposit of grey or light brown that can be easily chipped away, electrode edges are rounded with a larger than normal gap. A worn out spark plug not only wastes fuel but also strains the whole ignition system because the expanded gap and rounded edges require higher voltages before it will operate. top

    Distributor, points, engine timing

    The ignition
    distributors is about the size of a can of beans with a plastic cap with thick leads running from the top of it across to the spark plugs. Older type cars have a basic distributor containing ignition contact points and a rotating shaft with a cam lobe for each cylinder. As the engine rotates so does the cam lobe, which pushes the contact points open, interrupting the flow of current in the ignition coil. This is the trigger that interrupts the coil circuit causing the high voltage spark. The high voltage has an unwanted effect which causes the current flowing across the contact point to continue to flow, so a condenser is fitted to the circuit to soak-up the excess current, stopping the current flow as the points open. The distributor is connected to the coil by two circuits, a thin wire with low voltage at the contact points and high voltage lead from the coil tower to the centre post of the distributor cap. Apart from keeping the distributor cap clean and corrosion free, it requires only inspection when tuning the engine. Ignition points on the other hand need to be inspected, adjusted and replaced regularly to keep the engine running at it's best. top

    Adjusting electronic ignition systems

    distributors don't have the same wearing parts as older points systems at the trigger device. And so do not require the same regular adjustment as points systems do. Electronic systems, once correctly timed are very reliable and relatively robust, and do not require further adjustment. Electronic systems are not easily diagnosed without specialised equipment. Because specialised equipment is needed to check electronic systems they are not servicable on the side of the road, usually requiring towing when they breakdown. top

    Introduction | Safety | Battery | Understanding Your Car | Checking your own car | Fuel Systems | Glossary of Terms