DESCRIPTION AND OPERATION
See Figures 1, 2 and 3Conventional Spark Control Ignition System
A conventional electronic ignition system with vacuum/centrifugal spark control is used on carbureted 1985-88 Chevrolet Sprint and 1989-91 Geo Metro LSi models.
The basic components of this ignition system are the ignition coil, the distributor, the spark plugs and spark plug wiring. The distributor consists of a signal generator (signal rotor and pick-up coil), igniter, rotor, ignition module, vacuum advancer and centrifugal advancer.
When the distributor shaft rotates, a fluctuating magnetic field is generated due to changes in the air gap between the pick-up coil and signal rotor. As a result, an alternating current (AC) voltage is induced in the pick-up coil. This induced AC voltage peaks when a ridge on the signal rotor is adjacent to the ridge on the pick-up coil. When the voltage peaks, the igniter breaks the circuit to ground from the negative side of the coil primary winding. With the circuit broken, the magnetic field in the ignition coil, which has been generated by the electrical current passing through it, collapses. The high voltage induced by the collapsing field is then forced to find a ground through the secondary coil wire, the distributor cap, the rotor, the spark plug wire and finally across the spark plug air gap to the engine block.
Spark timing is mechanically controlled by a vacuum advance system which uses engine manifold vacuum and a centrifugal advance mechanism.
An electronic ignition system with knock controller is used on turbocharged 1987-88 Chevrolet Sprint vehicles only.
This system functions in much the same manner as a conventional electronic ignition system except it utilizes an igniter with knock controller which electrically controls the ignition timing based on signal received from a knock sensor. The system ensures that optimum ignition timing is always obtained for best engine output and drive train protection. When there is no knocking, the ignition system operates in the same way as an ordinary ignition system. When knocking occurs, the system can retard the ignition timing up to 12°, according to the degree of knocking.
The distributor uses a rotor, signal generator, vacuum advance unit and centrifugal advancer. The vacuum advance unit controls the ignition timing as follows. The throttle body bore and surge tank internal pressure is delivered into the vacuum advance unit through hoses. When the pressure is negative (vacuum), it moves the breaker clockwise (in reverse direction to rotor rotation) to advance the ignition timing. On the other hand, the positive pressure causes the breaker to move counterclockwise to retard the ignition timing.
The igniter with knock controller is located at the left side of the engine compartment. It has two main circuits, one serving as a fully transistorized igniter and the other as a knock controller. Based on the engine speed and signals from the Electronic Control Module (ECM) and knock sensor, the knock controller gives the igniter an instruction to retard the ignition timing so that engine knocking is suppressed.
The controller also has a fail safe circuit built-in. It give an instruction to retard ignition timing by a certain amount if the knock sensor fails or a short or open circuit occurs in the output leads.
The knock sensor is located on the cylinder block at the intake manifold side. It detects engine vibration and converts it into an electrical signal which is transmitted to the igniter.
An electronic spark control ignition system is used on 1989-91 Geo Metro Base and XFi models, and all 1992-93 Geo Metro models.
The ignition circuit consists of the battery, distributor, ignition switch spark plugs, primary and secondary wiring. The ESC system is monitored and controlled by the engine control module (ECM). The distributor used in this system consists of a signal generator (signal rotor and pick-up coil), and rotor. The igniter is located in the ECM.
All spark timing changes in the distributor are performed electronically by the ECM. After receiving signals indicating engine speed, manifold vacuum, coolant temperature and other engine functions, the ECM selects the most appropriate timing setting from memory and signals the distributor to change the timing accordingly. No vacuum or mechanical advance mechanisms are used.
The ECM controls a driver to ground which is connected to negative side from the coil's primary circuit. When this ground is interrupted, the field around the primary coil collapses and a high voltage is induced in the secondary coil. The high voltage induced in the secondary coil is then forced to find a ground through the coil wire, distributor cap, rotor, spark plug wire and across the spark plug air gap to the engine block.