Volkswagen Golf/Jetta 2 manual

General information and precautions
Ignition system - contact breaker type / General information and precautions


General information
The ignition system covered in this Chapter is of the conventional contact breaker type.

On 1.05 and 1.3 litre engines, the distributor is mounted on the left-hand (gearbox) end of the cylinder head and is driven direct from the camshaft (see illustrations). On 1.6 and 1.8 litre engines, the distributor is mounted at the front (radiator) end of the engine and it is driven by a skew gear in mesh with the intermediate shaft of the engine (see illustration).

1.1a Contact breaker ignition system components - Bosch distributor, 1.05 and
1.1a Contact breaker ignition system components - Bosch distributor, 1.05 and 1.3 litre engines

1.1b Contact breaker ignition system components - Ducellier distributor, 1.05
1.1b Contact breaker ignition system components - Ducellier distributor, 1.05 and 1.3 litre engines

1.1c Contact breaker ignition system components - 1.6 litre engine
1.1c Contact breaker ignition system components - 1.6 litre engine

To enable the engine to run correctly, it is necessary for an electrical spark to ignite the fuel/air mixture in the combustion chamber at exactly the right moment in relation to engine speed and load. The ignition system is based on feeding low tension voltage from the battery to the coil, where it is converted to high tension voltage. The high tension voltage is powerful enough to jump the spark plug gap in the cylinders many times a second under high compression, providing that the system is in good condition.

The ignition system is divided into two circuits, the low tension (LT) circuit and the high tension (HT) circuit.

The LT (primary) circuit comprises the battery, a lead to the ignition switch, a lead from the ignition switch to the LT coil windings (terminal +) and a lead from the LT coil windings (terminal -) to the contact breaker points and condenser in the distributor. The condenser is fitted in parallel with the contact points and its purpose is to reduce arcing between the points and also to accelerate the collapse of the coil low tension magnetic field.

The HT circuit comprises the HT (secondary) coil windings, the heavy ignition lead from the coil to the distributor cap, the rotor arm and the spark plug leads and spark plugs.

The system functions in the following manner. LT voltage is changed in the coil into HT voltage by the opening and closing of the contact breaker points. HT voltage is then fed via the carbon brush in the centre of the distributor cap to the rotor arm of the distributor and each time it comes in line with one of the four metal segments in the cap, which are connected to the spark plug leads, the opening and closing of the contact breaker points causes the HT voltage to build up, jump the gap from the rotor arm to the appropriate metal segment, and so via the spark plug lead to the spark plug, where it finally jumps the spark plug gap before going to earth.

Ignition timing is advanced and retarded automatically, to ensure that the spark occurs at just the right instant for the particular load at the prevailing engine speed.

Ignition advance is controlled both mechanically and by a vacuum-operated system. The mechanical governor mechanism comprises two weights, which move out from the distributor shaft as the engine speed rises due to centrifugal force. As they move outwards, they rotate the cam relative to the distributor shaft and so advance the spark.

The weights are held in position by two light springs and it is the tension of these springs which is largely responsible for correct spark advancement.

The vacuum control comprises a diaphragm, one side of which is connected via a small bore pipe to the inlet manifold and the other side to the contact breaker plate.

Depression in the inlet manifold, which varies with engine speed and throttle opening, causes the diaphragm to move, so moving the contact breaker plate and advancing or retarding the spark. A fine degree of control is achieved by a spring in the vacuum assembly.

The system incorporates a ballast resistor or resistive wire in the low tension circuit, which is in circuit all the time that the engine is running. When the starter is operated, the resistance is bypassed to provide increased voltage at the spark plugs for easier starting.

Precautions
It is necessary to take extra care when working on the electrical system to avoid damage to semi-conductor devices (diodes and transistors) and to avoid the risk of personal injury. Take note of the following points:
a) Before disconnecting any wiring, or removing components, always ensure that the ignition is switched off.

b) Always remove rings, watches, etc.

before working on the ignition system.

Even with the battery disconnected, capacitive discharge could occur if a component live terminal is earthed through a metal object. This could cause a shock or nasty burn.

c) Do not reverse the battery connections.

Components such as the alternator or any other having semi-conductor circuitry could be irreparably damaged.

d) If the engine is being started using jump leads and a slave battery, connect the batteries positive to positive and negative to negative. This also applies when connecting a battery charger.

e) Never disconnect the battery terminals, or alternator multi-plug connector, when the engine is running.

f) The battery leads and alternator multiplug must be disconnected before carrying out any electric welding on the vehicle.

g) Never use an ohmmeter of the type incorporating a hand cranked generator for circuit or continuity testing.

h) The HT voltage generated by an electronic ignition system is extremely high and in certain circumstances, could prove fatal. Persons with surgicallyimplanted cardiac pacemaker devices should keep well clear of the ignition circuits, components and test equipment.

i) Do not handle HT leads, or touch the distributor or coil when the engine is running. If tracing faults in the HT circuit, use well insulated tools to manipulate live leads.


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