【正文】 p. It receives the high voltage surge from the coil wire and sends it to the other end of the rotor which rotates past each spark plug terminal inside the cap. As the rotor turns on the shaft, it sends the voltage to the correct spark plug wire, which in turn sends it to the spark plug. The voltage enters the spark plug at the terminal at the top and travels down the core until it reaches the tip. It then jumps across the tip of the spark plug, creating a spark suitable to ignite the fuelair mixture inside that cylinder. The description I just provided is the simplified version, but should be helpful to visualize the process, but we left out a few things that make up this type of ignition system. For instance, we didn’t talk about the condenser that is connected to the point, nor did we talk about the system to advance the timing. Let’s take a look at each section and explore it in more detail. 3 The Ignition Switch There are two separate circuits that go from the ignition switch to the coil. One circuit runs through a resistor in order to step down the voltage about 15% in order to protect the points from premature wear. The other circuit sends full battery voltage to the coil. The only time this circuit is used is during cranking. Since the starter draws a considerable amount of current to crank the engine, additional voltage is needed to power the coil. So when the key is turned to the springloaded start position, full battery voltage is used. As soon as the engine is running, the driver releases the key to the run position which directs current through the primary resistor to the coil. On some vehicles, the primary resistor is mounted on the firewall and is easy to replace if it fails. On other vehicles, most notably vehicles manufactured by GM, the primary resister is a special resister wire and is bundled in the wiring harness with other wires, making it more difficult to replace, but also more durable. The Distributor When you remove the distributor cap from the top of the distributor, you will see the points and condenser. The condenser is a simple capacitor that can store a small amount of current. When the points begin to open the current, flowing through the points looks for an alternative path to ground. If the condenser were not there, it would try to jump across the gap of the point as they begin to open. If this were allowed to happen, the points would quickly burn up and you would hear heavy static on the car radio. To prevent this, the condenser acts like a path to ground. It really is not, but by the time the condenser is saturated, the points are too far apart for the small amount of voltage to jump across the wide point gap. Since the arcing across the opening points is eliminated, the points last longer and there is no static on the radio from point arcing. The points require periodic adjustments in order to keep the engine running at peek efficiency. This is because there is a rubbing block on the points that is in contact with the cam and this rubbing block wears out over time changing he point gap. There are two ways that the points can be measured to see if they need an adjustment. One way is by measuring the gap between the open points when the rubbing block is on the high point of the cam. The other way is by measuring the dwell electrically. The dwell is the amount, in degrees of cam rotation that the points stay closed. On some vehicles, points are adjusted with the engine off and the distributor cap removed. A mechanic will loosen the fixed point and move it slightly, then retighten it in the correct position using a feeler gauge to measure the gap. On other vehicles, most notably GM cars, there is a window in the distributor where a mechanic can insert a tool and adjust the points using a dwell meter while the engine is running. Measuring dwell is much more accurate than setting the points with a feeler gauge. Points have a life expectancy of about 10,000 miles at which time have to be replaced. This is done during a routine major tune up, points, condenser, and the spark plugs are replaced, the timing is set and the carburetor is adjusted. In some cases, to keep the engine running efficiently, a minor tune up would be performed at 5,000 mile increments to adjust the point and reset the timing. Ignition Coil The ignition coil is nothing more that an electrical transformer. It contains both primary and secondary winding circuit. The coil primary winding contains 100 to 150 turns of heavy copper wire. This wire must be insulated so that the voltage does not jump from loop to loop, shorting 4 it out. If this happened, it could not create the primary magic field that is required. The primary circuit wire goes into the coil through the positive terminal, loops around the primary windings, then exits through the negative terminal. The coil secondary winding circuit contains 15,000 to 30,000 turns of fine copper wire, which also must be insulated from each other. The secondary windings sit inside the loops of the primary windings. To further increase the coils magic field the windings are wrapped around a soft iron core. To withstand the heat of the current flow, the coil is filled with oil which helps keep it cool. The ignition coil is the heart of the ignition system. As current flows through the coil a strong magic field is build up. When the current is shut off, the collapse of this magic field to the secondary windings induces a high voltage