GM Corvette 1963-1982 Repair Guide

Conventional Ignition System



The distributor is geared to the camshaft of the engine, and in the case of a Chevrolet V8, is also used to drive the engine oil pump in addition to its usual "spark sorting" duties. As the distributor shaft turns, the lobes of the distributor cam actuate the breaker point set, causing the points to open and close. The points are attached to the breaker plate, which on models using a vacuum advance system, is capable of rotating around the distributor shaft a small amount. This rotation of the breaker plate changes the relationship between the distributor shaft and the point set. The change in this relationship alters the time at which the points open, thereby changing the ignition timing. The amount of the ignition timing change, in degrees, is controlled by the vacuum advance diaphragm which senses the engine load through engine vacuum changes. The vacuum advance diaphragm is controlled by either "ported" or "manifold" vacuum. A ported vacuum source is one that is located above the throttle plates of the carburetor, whereas a manifold vacuum source is located below the carburetor throttle plates (or attached to a fitting directly on the intake manifold). At a given open throttle position, the vacuum signal to the distributor will be basically the same, whether it is from a ported or manifold vacuum source. During idle, however, manifold vacuum is high; ported is low. As vacuum to the advance unit is increased, the amount of degrees which the unit moves (advances) the breaker plate is also increased, and vice-versa (retard). It is because of this that you must disconnect the vacuum advance unit before adjusting the initial timing on engines which have a manifold vacuum source to the distributor. The most important function of the vacuum advance is to advance the ignition timing during cruise conditions for improved fuel economy. As engine load increases, engine vacuum drops, which allows the ignition timing to retard, thereby preventing detonation due to over-advanced timing. Some special high performance models such as the 427/L88 do not have vacuum advance systems. Fuel economy was not designed into such "off-road" engines.

Do not remove the vacuum advance for street usage-fuel economy will suffer, along with engine performance if not set-up properly.

Another timing feature of the distributor is the centrifugal advance mechanism. This mechanism alters the ignition timing by rotating the distributor cam (a small amount) independent of the distributor shaft. Again, the relationship between the distributor shaft and the point opening is changed due to this rotation. The centrifugal advance mechanism consists basically of two small springs and two specially shaped weights attached to both the distributor cam and the shaft. These components are mounted underneath the ignition rotor. Instead of vacuum, this mechanism uses engine rpm as a guide for advancing the ignition timing. As engine rpm (and distributor shaft rpm) increases, the weights are moved gradually outward (against the spring tension) by centrifugal force. As the weights move outward, the distributor cam is moved to a more advanced timing position. When the engine rpm drops, the springs gradually pull the weights back to their low-speed position, thereby reducing the amount of timing advance.


The point set, or breaker points as they are sometimes called, is the switching device of a conventional ignition system. The "primary" side of the ignition coil is connected to the battery, through the ignition switch, a resistor, and various wiring. When the ignition switch is in the "run" position, voltage from the ignition switch passes through a resistor, which lowers the battery voltage to the coil from 12V down to about 6-8V.

If this resistor is defective, the full 12V from the battery will quickly burn the breaker point contacts and shorten the life of the ignition coil.

When the ignition switch is in the "start" position, the resistor is bypassed. This provides a full 12V to the ignition system to aid in starting the engine.

Voltage flow after the resistor (or bypass wire) is through the primary (low voltage) windings of the coil, to the breaker point set. When the points are closed (grounded), a magnetic field is produced within the primary windings of the ignition coil. When the points open, the coil voltage no longer has an easy path to ground. The magnetic field collapses and transfers the voltage from the primary windings (outer) of the coil to the "secondary" windings (inner) of the coil, through induction. Since the number of secondary windings in the ignition coil is much greater than the primary, the voltage is multiplied to roughly 20-25,000 volts. This high voltage then travels out of the center tower of the ignition coil, through the high tension lead (secondary coil wire) to the center tower of the distributor cap. The voltage is then distributed by the ignition rotor to the outer terminals of the distributor cap (in the "firing order"), through the plug wires and finally, the voltage jumps the gap of the spark plug, causing ignition of the air/fuel mixture in the cylinder.

The condenser, which is also attached to the breaker plate (grounded), acts as a temporary voltage storage unit when the points are closed. This helps to prevent arcing between the point contacts when the points are opened. Rapid metal transfer between the point contacts is indicative of a bad condensor.