SOLID WIRE- Solid wire spark plug wires are used on older vehicles. The wire conductor is simply a strand of metal wire. Solid wires cause radio interference and are no longer used on vehicles.

RESISTANCE WIRE- Resistance spark plug wires consist of carbon-impregnated strands of rayon braid. They are used on modern vehicle because they contain internal resistance that prevents radio interference. Also known as radio interference wires, they have approximately 10,000 ohms per foot. This prevents high-voltage- induced popping or cracking of the radio speakers.

On the outer ends of the spark plug wires, boots protect the metal connectors from corrosion, oil, and moisture that would permit high voltage to leak across the terminal to the shell of the spark plug.

CONTACT POINT IGNITION SYSTEM

Before studying today's electronic ignition systems, you should have a basic understanding of the contact point ignition system. The two systems use many of the same components. These include the battery, the ignition coil, the ignition distributor, the spark plugs, and wires and cables that connect them.

Contact Point Ignitions System Components

The internal components of the distributor for a contact point ignition consist of the following:

DISTRIBUTOR CAM- The distributor cam is part of, or is attached to, the distributor shaft and has one lobe for each cylinder. As the cam rotates with the shaft at one half of engine speed, the lobes cause the contact points to open and close the primary circuit.

CONTACT POINTS- The contact points, also called breaker points, act like spring-loaded electrical switches in the distributor. Its function is to cause intermittent current flow in the primary circuit, thus causing the magnetic field in the coil to build up and collapse when it reaches maximum strength. Wires from the condenser and ignition coil primary circuit connect to the points.

CONDENSER- The condenser, also known as a capacitor, is wired in parallel with the contact points and grounded through the distributor housing. The condenser prevents arcing or burning at the distributor contact points when the points are first open. The condenser provides a place where current can flow until the contact points are fully open.

Contact Point Ignition System Operation
With the engine running, the distributor shaft and distributor cam rotate. This action causes the distributor cam to open and close the contact points.

With the contact points wired to the primary windings of the ignition coil, the contact points make and break the ignition coil primary circuit. With the contact points closed, the magnetic field builds up in the coil. As the points open, the magnetic field collapses and voltage is sent to the spark plugs.

With the distributor operating at one half of engine speed and with only one cam for each engine cylinder, each spark plug only fires once during a complete revolution of the distributor cam.

To ensure that the contact points are closed for a set time, point dwell, also known as cam angle, is set by using a dwell meter. Point dwell is the amount of time given in degrees of distributor rotation that the points remain closed between each opening.

A dwell period is required to assure that the coil has enough time to build up a strong magnetic field. If the point dwell is too small, the current will have insufficient time to pass through the primary windings of the ignition coil, resulting in a weak spark. However, if the point dwell is too great, the contact points will not open far enough, resulting in arcing or burning of the points.