IGNITION SYSTEM
GENERAL INFORMATION
The ignition system consists of a primary (low voltage) and a secondary (high voltage) circuit (Fig. 98).
The primary circuit consists of the
- Battery
- Ignition switch.
- Primary circuit resistor.
- Primary windings of the ignition coil.
- Breaker points.
- Condenser.
The secondary circuit consists of the
- Secondary windings of the ignition coil
- Distributor rotor.
- Distributor cap.
- High tension wires.
- Spark plugs.
When the ignition is switched on and the breaker points are closed, the primary or low voltage current flows from the battery through the ignition switch to the primary windings in the coil, then to ground through the closed breaker points. When the breaker points in the distributor open the magnetic field built up in the primary windings of the coil collapses and the secondary windings of the coil produce high voltage current. High voltage current is produced each time the breaker points open.
The high voltage flows through the coil tension lead to the distributor cap where the rotor distributes it to one of the spark plug terminals in the distributor cap. This process is repeated for every power stroke of the engine. The condenser is connected to the insulated contact breaker point and earth. Its purpose is
- To absorb the current surge in the primary feed circuit and the current Induced in the coil primary winding, as the magnetic field in the coil collapses, when the contact breaker points open. This action prevents excessive arcing at the contact breaker points.
- To discharge this stored current through the coil primary winding to earth in, the OPPOSITE direction to the primary current flow. This ensures that the maximum output of high tension current is given by the coil secondary windings, when the contact breaker points open.
The ignition coil supply is 6-7 volts. During normal running the excess voltage supplied by the 12 volt battery is reduced by a ballast resistor, which is con-nected in series with the lead from the Ignition switch to
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the coil. When the starter motor is operating, additional contact in the starter solenoid switch shorts out the ballast resistor, thus ensuring that the voltage at the coil primary feed terminal, and hence its performance remains unaffected by the drop In battery voltage which occurs while the starter motor is in operation.
The accurate timing of the high tension current to jump spark plug gaps is determined by —
- The static ignition timing which is the relationship of the contact breaker opening position to the crankshaft position. This, in degrees before top dead centre is given under “Ignition” in the Data Section.
- The correct action of the centrifugal advance mechanism, situated below the contact breaker, and the vacuum advance mechanism diaphragm unit on the outside of the distributor body.
The centrifugal and vacuum advance mechanism is necessary to meet the varying ignition timings imposed by the varying engine speeds, and varying engine loadings at varying speeds. But to do this the STATIC (or basic) ignition timing MUST BE CORRECT.
IGNITION FAULT DIAGNOSIS
Test equipment
Ignition system troubles are caused by a failure in the “primary and/or secondary circuit or incorrect ignition timing.
Owing to the greater number of parts in a Vee 8 ignition system, faults are sometimes more difficult to locate quickly. For this reason TEST EQUIPMENT INCOR-PORATING AN OSCILLOSCOPE (SCOPE) IS STRONGLY RECOMMENDED. This equipment is particularly useful for tracing faults quickly in the ignition high tension circuits, checking the coil performance, and testing the distributor performance up to an engine speed of 3000 r.p.m. with the distributor in position on the engine. The manufacturers of this equipment such as Crypton Equipment Ltd., Bridgewater, England, provide very com-plete instructional manuals explaining its advantages, operation and method of use.
The ignition timing should be checked with a strobos-copic timing light as it is rather difficult to turn the engine if a lamp only is used to check the contact breaker opening point.
If the high speed operation of the distributor is suspect, the distributor should be removed from the engine and tested on a distributor tester over its whole speed range.
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