The ignition system's main purpose is to provide the engine with the proper spark at the right time to allow the fuel/air mixture in the combustion chamber to ignite.
As time has evolved, the ignition system has gotten more sophisticated making it a little harder to diagnose, but we will show you that even moder systems can be diagnosed with common tools.
- The old ignition system had the following parts:
- Ignition points, condenser, distributor, ignition rotor, distributor cap,ignition coil, spark plug wires, spark plugs.
This system was found in vehicles prior to 1975, very easy to diagnose, most of the time the ignition points were the item prone to malfunction, it was suggested back then to replace them every 10,000 miles to avoid breaking down due to burned ignition points, the points and condenser were replaced at the same time, these items are found inside the distributor housing.
In this early ignition system, the ignition points send directly the signal to the ignition coil to turn off the magnetic field inside the ignition coil to produce the high voltage needed to ignite the fuel mixture inside each cylinder, the high voltage is converted in to a spark in the spark plug. Each spark plug gets the high voltage at the right time thanks to the distributor, the distributor is the one that controls the ignition timing, it contains the ignition points and condenser, the points ride over several lobes in the distributor shaft, there are as many lobes as there are cylinders, for example:
A four cylinder engine has a four lobe shaft inside the distributor, a six cylinder has six etc.
The points open and close as they ride up and down on these lobes, a ground wire connects the ignition points with the negative terminal in the ignition coil, and every time the points open as they go up on each lobe the negative signal is interrupted, causing the magnetic field in the ignition coil to collapse producing a high voltage current.
This high voltage current travels through the ignition coil wire to the distributor, the coil wire is the one in the center in the distributor cap, to send this current directly to the cylinder that is in TDC and needs the spark, the distributor uses an ignition rotor, this rotor turns as the camshaft turns, and as long as the rotor is timed correctly, it will direct the current to the spark correct cylinder.
This high voltage current is converted in to a powerful spark in the spark plug, the way this is accomplished is through a gap in between the two electrodes at the end of the spark plug, as the high current is trying to jump from one electrode to the other, it creates a powerful spark, specially because the second electrode is actually a ground electrode since it is connected to the spark plug body, and the spark plug is threaded in the cylinder head that along with the engine block are connected to the negative battery terminal.
- After 1975, the manufacturers introduced the electronic ignition module to replace the points and condenser, this item is more accurate at any RPM, and has a longer life, we have seen old vehicles with over 200,000 miles that still have their original ignition module.
Note that even though the ignition module is an electronic device, those vehicles didn't have an electronic control computer yet, the ignition module received its signal from the ignition pick-up coil or hall effect switch depending on the manufacturer,mounted inside the distributor housing, the pick-up coil reads the number of turns the distributor makes sending this signal to the ignition module, this signal is sent from the ignition module to the ignition coil at the right time to send the spark to the cylinder that has reached TDC ( top dead center) and needs the spark to ignite the fuel mixture. Most distributors still have a centrifugal and vacuum advance to retard and advance the ignition timing.
- In the late 80's and early 90's, the ignition system became more sophisticated, with the introduction of the fuel injection system, there was a need from the engine control computer to receive the proper signal to determine which fuel injector was going to be turned on to ensure that each cylinder would get their fuel injected at the right time.
The auto manufacturers decided to use the ignition system to be the one sending that signal, and that cause the ignition system to undergo several changes, ending the simple to diagnose system everyone was so used to, discouraging many do it yourself individuals.
- But the truth is, you can still diagnose problems with the ignition system in your vehicle without the need to own specialized equipment, if you continue to read, we will show you how to diagnose problems related to each component, and a detailed step by step guide to their replacement.
First you need to know what items compose the modern ignition system:
- Crankshaft position sensor , Camshaft position sensor (s), electronic control module, in some systems ignition control module, knock sensor (s), ignition coil (s), ( some modern systems have the ignition coil mounted on top of the spark plug boot, eliminating the need of spark plug wires and distributor), spark plug wires, distributor, distributor cap, ignition rotor, hall effect switch, spark plugs.
The main difference between the totally electronic ignition system is that it eliminates moving parts in the ignition system, for instance:
Instead of having a distributor with the pick-up coil, ignition rotor, cap etc, the computer gets two signals:
- One comes from the Crankshaft position sensor, this sensor reads the engine revolutions, it is usually mounted near the harmonic balancer, or at the other end of the engine in the bellhousing, the way this sensor reads the engine rpm's is by reading the number of times its signal is interrupted due to the indentations on pulse ring attached to the engine either in the harmonic balancer, in front of the lower timing chain sprocket, or attached to the flywheel.
The second signal is sent to the computer by the Camshaft position sensor, this sensor as its name implies, reads the revolutions of the camshaft(s), that way the computer has an accurate way to know where each cylinder is and it is able to send the spark to the cylinder who needs it at the right time.
The way this is accomplished without having a distributor is in two ways:
Some four cylinder vehicles have an ignition coil with four terminals, each spark plug is connected to one of them, the computer sends the signal to the ignition coil and the current is sent to two cylinders at once, one is in compression stroke, and the other is in exhaust stroke. Larger engines use a series of ignition coils, six cylinder engines usually have three ignition coils, and V-8 engines usually have two.
The most common arrangement in modern engines is an ignition coil mounted on top of each cylinder, right on top of the spark plug, eliminating the need of spark plug wires, since the spark plug boot is connected directly to the ignition coil.
In either set-up, the timing is controlled directly by the engine control computer, either advancing it as it needs to or retarding it if necessary, for example:
Let's say you use a low octane gasoline and you are climbing a steep hill, the vehicle under a full throttle will start detonating, this detonation is sensed by the knock sensor(s), and the signal is sent directly to the engine control computer, the computer retards the spark signal to try to eliminate the detonation avoiding potential irreversible damage to the engine.
As you can see, modern ignition systems are more desirable than the early ones, they are extremely accurate , more reliable and last longer, and most of the time they are maintenance free, so the time that you would have spent working almost every weekend on an old car, can be spent driving to nice places during the weekend.
Please note that each make and model uses different components from the ones described above, you will learn what items are included in different makes and models as we create their pages.
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