Ignition coil


  - In this and the following pages you will learn how the ignition coil works, how diagnose problems related to its operation, and a detailed step by step guide to replace it.

 We will illustrate several vehicles from different makes and models as we get them in our shop with coil related problems.

  - How does the ignition coil work?
  The ignition coil is a type of electric transformer that changes low voltage electricity to high voltage electricity or current.
It works on the principles of magnetic induction.  Induction occurs when a magnetic field moving across a wire induces ( creates) current flow.
The coil has two windings, or coils of wire, the primary and secondary windings.
The primary winding has several turns of heavy wire; the secondary winding has many more turns of fine wire around a soft iron core.
The difference in the number of turns between the primary and secondary is what allows the coil to increase voltage.
The ignition coil by means of electromagnetism converts the low voltage from the battery to the high voltage used to fire the spark plug.
The battery in the vehicle provides the coil primary winding with low-voltage electricity. The battery current is controlled by the ignition switch.
When the ignition switch is in the ON position, current is allowed to flow trough the ignition coil, there is often a resistor between the ignition switch between the ignition switch and the other ignition system components to prevent damage to these components due to the excessive current flow.
When a current flows through a wire, ( a conductor ), a magnetic field is built up around the conductor. There are several hundred turns of wire in the primary winding, and a strong magnetic field is produced. If the current flow is interrupted after passing through the coil, and if this interruption is quick, the magnetic field will collapse.
Since the secondary winding is made up of many turns of fine wire, there is more wire ( conductor) for the magnetic lines of force to cut. Therefore, a high voltage will be induced in the secondary winding whenever the primary current is interrupted and the magnetic field collapses.

A switching device in the ignition system turns the primary winding current on and off at the proper time. When the primary current is turned off, the magnetic field in the ignition coil collapses rapidly. This collapsing field induces a voltage in the secondary winding of the ignition coil.

In this first page we will show you how to diagnose a bad ignition coil in a Toyota Camry, the particular car we will be working on is a 1993 Toyota Camry with a four cylinder engine.
The car came in to our shop with the complaint that it wouldn't start, after testing it for spark and fuel, we noticed that we didn't have any spark, we suspected either a bad ignition coil, or a bad pick-up coil, this is what we found........















  - You can use an inexpensive spark tester like the one shown in the picture to test for spark, if there is no spark, it is likely to have a problem in the ignition system.

  - To make sure that the problem was in the ignition coil only and not in the pick-up coil, we checked for signal at the fuel injectors, since we had good signal there, we moved on to test the signal at the ignition coil plug.

  - To do this, it is necessary to unplug the two wire plug attached to the distributor, it is better to do it this way to avoid a false reading caused by a bad ignition coil.
Turn the ignition switch on, and use a test light to test for continuous power at one of the terminals, next, change the other end of the test light to the positive terminal in the car battery, probe the second terminal as you have an assistant turn the car over, you should see an intermittent light in the test light, this means that all the other parts are working correctly, and in fact, the lack of spark problem is in the ignition coil.

  Now that you know that your problem is a bad ignition coil, proceed to remove the air filter tube to gain easy access to the distributor.

   Next remove the distributor cap by removing the screws that attach it to the distributor housing







  Remove the ignition rotor by pulling gently on it.






  Next pry the tabs out on the ignition coil cover and remove it from the distributor

  Mark the relationship between the distributor shaft and the ignition coil, that way can install it back pointing in the same direction





  If you don't own a timing light, mark the relationship of the distributor housing to the cylinder head, proceed to remove the two bolts that keep it in place and remove it from the vehicle.




  Secure the distributor housing to a vise, and remove the two nuts that keep the coil wires attached to the ignition coil, don't mix them one is negative and the other two are positive.






  Next remove the four screws that secure the ignition coil in place and remove it from the distributor.





  Scrape the old silicone material off the housing with a scraper, clean the surface , add a thin coat of new RTV, and install the new coil, hook the wire to their respective terminals.




  This is the perfect opportunity to replace the seal in the distributor housing, they get dry and hard as they age developing oil leaks.

   Install all the items back in the reverse order you took them off, and start the vehicle, if you did everything correctly the car should run great.

 NOTE:    We didn't show you how to test this ignition coil with an ohm meter because we tested it and its resistance was within specifications, but we knew that the coil was bad because there was no spark coming out of it, that proves that the ohms test is not a 100% accurate.