General description

Ignition system / General description

On all models except the 903 ES engine version, a mechanical contact breaker type distributor is fitted.

On 45 Super ES models which have the 903 ES engine, an electronic (Digiplex) ignition system is used which incorporates a breakerless distributor.

Mechanical contact breaker system
For 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 under high compression pressures, providing that the system is in good condition and that all adjustments are correct.

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

The low tension (sometimes known as the primary) circuit consists of the battery, the lead to the ignition switch, the lead from the ignition switch to the low tension or primary coil windings, and the lead from the low tension coil windings to the contact breaker points and condenser in the distributor.

The high tension circuit consists of the high tension or secondary coil windings, the heavy ignition lead from the centre of the coil to the centre of the distributor cap, the rotor arm, and the spark plug leads and spark plugs.

The system functions in the following manner: High tension voltage is generated in the coil by the interruption of the low tension circuit. The interruption is effected by the opening of the contact breaker points in this low tension circuit. High tension voltage is fed from the centre of the coil via the carbon brush in the centre of the distributor cap to the rotor arm of the distributor.

The rotor arm revolves at half engine speed inside the distributor cap, 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 of the contact breaker points causes the high tension 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.

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

The ignition advance is controlled mechanically, and by vacuum. 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 cam relative to the distributor shaft, and so advance the spark.

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

The vacuum advance is controlled by a diaphragm capsule connected to the carburettor venturi. The vacuum pressure varies according to the throttle valve plate opening and so adjusts the ignition advance in accordance with the engine requirements.

Digiplex ignition system This electronic system eliminates the mechanical contact breaker and centrifugal advance mechanism of conventional distributors and uses an electronic control unit to provide advance values according to engine speed and load. No provision is made for adjustment of the ignition timing.

Information relayed to the control unit is provided by two magnetic sensors which monitor engine speed and TDC directly from the engine crankshaft.

A vacuum sensor in the control unit converts intake manifold vacuum into an electric signal.

The control unit selects the optimum advance angle required and a closed magnetic circuit resin coil guarantees a spark owing to the low primary winding resistance.

Five hundred and twelve advance values are stored in the control unit memory to suit any combination of engine operating conditions.

No maintenance is required to the distributor used on this system.

Distributor drive
The mechanical breaker type distributor on 903 cc engines and the Digiplex type distributor on 903 cc ES engines are mounted on the cylinder head and driven from a gear on the camshaft through a shaft which also drives the oil pump.

The distributor on 1116 cc and 1301 cc engines is mounted on the crankcase and is driven from a gear on the auxiliary shaft as is also the oil pump.

Fig. 4.1 Typical ignition circuit (mechanical contact breaker distributor) (Sec 1)

Fig. 4.2 Digiplex electronic ignition system (Sec 1)

1 Control unit
2 Multi-plug
3 Ignition coil
4 Distributor cap
5 Crankshaft pulley
6 Flywheel
7 Battery
8 Rev counter
9 Spark plugs
10 Wiring connector
S1 Engine speed sensor
S2 TDC sensor