General information and precautions
General information
Pre-1986 models
The electronic system fitted to Fiesta models manufactured before 1986 consists of a breakerless distributor driven from the end of the camshaft, an electronic amplifier module mounted on the bulkhead on the lefthand side, and a high output type ignition coil fitted next to the amplifier module on the bulkhead.
The breakerless distributor is of Bosch manufacture and is distinguishable from conventional systems by its blue distributor cap. The unit has no mechanical contact breaker or condenser, these components being replaced by a trigger wheel, a trigger coil and a stator. The action of the distributor is to provide a pulse to the electronic module which in turn actuates the ignition coil to ignite the fuel/air mixture via the HT leads and spark plugs.
The electronic amplifier module is a sealed unit located on the left-hand side of the engine compartment bulkhead. The function of the module is to sense the trigger pulse from the distributor and amplify its voltage sufficiently to operate the module’s output transistor. On receipt of this amplified voltage the module shuts off the ignition coil primary circuit allowing HT voltage to build up within the coil in the conventional manner and fire the appropriate spark plug via the distributor and HT leads. On completion of the firing cycle the primary circuit is then switched on again by the module and the cycle is repeated for the next cylinder.
The ignition coil operates on conventional principles but with a higher output voltage. The unit is rated at 8 volts and is supplied via a ballast resistor wire during normal running. When starting the engine the ballast resistor wire is bypassed and the coil receives full battery voltage. The coil used on electronic breakerless systems is distinguished by a yellow label on the case.
The spark plugs used are the copper cored resistor type with a metric thread form. Only this type of plug is suitable for use in the electronic ignition system.
Repair and overhaul operations should be limited to those described in this Chapter as only the distributor cap, rotor arm, and HT leads are available as repair parts, all other items are sealed and only renewable as complete units. Should a fault in the system develop or be suspected, the advice of your dealer should be sought. Fault diagnosis procedures are lengthy and must follow a systematic approach using sophisticated test equipment. For these reasons fault diagnosis and repair are considered to be beyond the scope of the average owner.
1986 models onwards
From 1986 onwards, all Fiesta models are fitted with breakerless electronic ignition systems. 1.6 litre CVH models retain the same basic system as described above for the pre 1986 models, except that the electronic module is integral with the distributor. 1.0 and 1.1 litre OHV and 1.4 litre CVH models are equipped with a new system also incorporating an electronic module integral with the distributor. The new system operates in the following way.
The ignition system is divided into two circuits, low tension (primary) and high tension (secondary). The low tension circuit consists of the battery, ignition switch, primary coil windings, electronic amplifier module and the signal generating system inside the distributor. The signal generating system comprises the trigger coil, trigger wheel, stator, permanent magnets and stator pick-up. The high tension circuit consists of the secondary coil windings, the heavy ignition lead from the centre of the distributor cap to the coil, the rotor arm and the spark plug leads and spark plugs.
When the system is in operation, low tension voltage is changed in the coil into high tension voltage by the action of the electronic amplifier module in conjunction with the signal generating system. As each of the trigger wheel teeth pass through the magnetic field created around the trigger coil in the distributor, a change in the magnetic field force (flux) is created which induces a voltage in the trigger coil. This voltage is passed to the electronic amplifier module which switches off the ignition coil primary circuit. This results in the collapse of the magnetic field in the coil which generates the high tension voltage. The high tension voltage is then fed via the carbon brush in the centre of the distributor cap to the rotor arm. The voltage passes across to the appropriate metal segment in the cap and via the spark plug lead to the spark plug where it finally jumps the spark plug gap to earth.
The distributor used on the electronic ignition system of OHV engines is of Bosch manufacture, whereas on CVH engines either a Bosch or Lucas unit may be used. Although the components of the signal generating system differ between the Bosch and Lucas distributors, the principles of operation of both are as just described. The distributor is driven by a skew gear from the camshaft on the OHV engine and by an offset dog on the end of the camshaft on CVH engines.
The ignition advance is a function of the distributor and is controlled both mechanically and by a vacuum-operated system. The mechanical governor mechanism consists of 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 trigger wheel 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 the springs which is largely responsible for correct spark advancement.
The vacuum control consists of a diaphragm, one side of which is connected via a small bore hose to the carburettor, and the other side to the distributor. Depression in the inlet manifold and carburettor, which varies with engine speed and throttle position, causes the diaphragm to move, so moving the baseplate and advancing or retarding the spark. A fine degree of control is achieved by a spring in the diaphragm assembly. Additionally, one or more vacuum valves and temperature sensitive control valves may be incorporated in the vacuum line between inlet manifold or carburettor and the distributor. These control the duration of the vacuum felt at the distributor and are part of the vehicle emission control systems.
Precautions
General
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. In addition to the precautions given in the “Safety first!” Section at the beginning of this manual, take note of the following points when working on the system:
Always remove rings, watches, etc before working on the electrical 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.
Do not reverse the battery connections. Components such as the alternator or any other having semi-conductor circuitry could be irreparably damaged.
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.
Never disconnect the battery terminals, or alternator multi-plug connector, when the engine is running.
The battery leads and alternator multiplug must be disconnected before carrying out any electric welding on the car.
Never use an ohmmeter of the type incorporating a hand cranked generator for circuit or continuity testing.
Ignition and engine management systems
Warning: The HT voltage generated by an electronic ignition system is extremely high, and in certain circumstances could prove fatal. Take care to avoid receiving electric shocks from the HT side of the ignition system. 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.
Engine management modules are very sensitive components, and certain precautions must be taken to avoid damage to the module when working on a vehicle equipped with an engine management system as follows.
When carrying out welding operations on the vehicle using electric welding equipment, the battery and alternator should be disconnected.
Although underbonnet-mounted modules will tolerate normal underbonnet conditions, they can be adversely affected by excess heat or moisture. If using welding equipment or pressure washing equipment in the vicinity of the module, take care not to direct heat, or jets of water or steam at the module. If this cannot be avoided, remove the module from the vehicle, and protect its wiring plug with a plastic bag.
Before disconnecting any wiring, or removing components, always ensure that the ignition is switched off.
On models with underbonnet-mounted modules, do not run the engine with the module detached from the body panel, as the body acts as an effective heat sink, and the module may be damaged due to internal overheating.
Do not attempt to improvise fault diagnosis procedures using a test lamp or multimeter, as irreparable damage could be caused to the module.
After working on ignition/engine management system components, ensure that all wiring is correctly reconnected before reconnecting the battery or switching on the ignition.
On some early Bosch distributors it is possible that with the distributor cap removed, if the engine is cranked, the cap securing clips may fall inward and jam the trigger wheel/vane, knocking it out of alignment. If this happens, the distributor will have to be renewed as the trigger wheel/vane cannot be repositioned. Care should therefore be taken not to crank the engine with the distributor cap removed. Later distributors have redesigned clips which eliminate the problem.
Ignition coil
The LT connections to the coil used with electronic ignition cannot be confused as the terminals are of different size. Never fit a coil from a conventional ignition system into an electronic ignition system otherwise the amplifier module may be damaged.