| Before electronic ignition systems were developed, | | | | distributor. A single small HT coil is used for each |
| ignition control of an engine was undertaken using | | | | spark plug and the igniter operates each coil |
| mechanical distributors. In distributor-based systems, | | | | separately in the correct sequence. |
| the distributor directs the output from a single | | | | Engine position information is provided by engine |
| high-tension (HT) ignition coil to the relevant spark | | | | position sensors and a timing disks that are |
| plug. Control over the timing of ignition is done | | | | accurately attached to one or two of the main |
| through a cam operated contact breaker, centrifugal | | | | engine shafts, such as the camshaft. The firing |
| weights, springs and cams, with ‘load mapping' | | | | sequence and variable spark advance is computed |
| done through using a vacuum advance unit. | | | | accurately from the pattern of teeth or pegs on the |
| These distributor-based systems are prone to | | | | timing disk. If load mapping is required, this can be |
| mechanical wear, insulation break down and contact | | | | achieved by adding a manifold pressure sensor or a |
| failure. To keep the ignition timing correct these | | | | throttle angle potentiometer to the system. A |
| systems require a periodic ‘tune up' to keep | | | | variety of extra features are available on such |
| them working correctly. | | | | systems, which can be accessed and adjusted by a |
| Modern electronic ignition systems do not require a | | | | PC. |