Classical or conventional ignition


Distributor ignition systems have been with us for approximately 90 years. The classical or conventional ignition system consists of the following components: ignition coil, distributor, spark plugs, high-voltage wires and some means of controlling the primary ignition circuit. The primary circuit of the ignition coil can contain: points, points controlling a transistor, the transistor being controlled by some other means (breaker less) or electronic ignition. In point-type ignition systems the current in the primary circuit is controlled by a mechanical switch (or breaker). The mechanical points may control a switching transistor which opens and closes the primary circuit of the ignition coil. In breaker less transistor and electronic ignition a Hall effect, VRS (Variable Reluctance Sensor) or an optical sensor may be used to control the switching transistor.

Current flows from the positive terminal of the battery, through the ignition switch and/or relay, through a fuse and on to the positive terminal of the ignition coil. The current returns to the battery through the negative terminal of the ignition coil, on through the switching device (points or a transistor) through the vehicle chassis, and to the negative terminal of the battery. While current is flowing in the primary circuit a magnetic field builds up in the ignition coil. Due to the inductance of the ignition coil it takes some time (1-6 mS, depending on design) for the primary current to reach its nominal value. When the primary current flow is interrupted, the magnetic field collapses rapidly (in about 20µS) and a high voltage is induced in the primary winding (CEMF Counter electro motive Force). This voltage is transformed in to a very high voltage in the secondary winding. The amplitude of this voltage depends on the turns ratio (commonly 100:1). A 300V primary voltage, therefore, will be 30 000V in the secondary winding. The voltage will only build until the break down voltage of the spark gap is reached - the firing voltage of the spark plug.


Primary ignition diagnosis.

In order to diagnose the primary ignition it is necessary to connect one oscilloscope probe to the ignition coil negative terminal. The probe must be connected to an analog input №5 on the USB Autoscope. Select "Control => Load user preset => Ignition => Ignition_Primary" in the USB Oscilloscope program. Start the engine. A primary ignition system waveform will be displayed.


Secondary ignition diagnosis.

Connections for secondary waveform diagnosis.
  • Disconnect all leads from the USB Autoscope.
  • Connect power supply wiring for the ignition system adapter Ignition Adapter to the vehicle battery (red to positive, black to negative).
  • Connect the ignition system adapter to the USB Autoscope.
  • Connect the capacitive pick up Cx (red) to the coil wire, as close to the ignition coil as possible. Connect the capacitive pick up lead to the In Synchro on the ignition system adapter.
  • Connect the sync probe Sync (black) to the plug wire №1, as close to the spark plug as possible. Connect the sync probe lead to the In Synchro on the ignition system adapter.



Connection of high-voltage probes.

  • Start the engine.
  • In the USB Oscillocope program, select "Control => Load user preset => Ignition => Ignition_Classic".


    • Once the engine is started, the red LED on the ignition system adapter should be lit. If the green LED is lit, the primary connections are backwards. Swap the connection on the ignition coil to the other terminal. The connections on the ignition coil may have been connected erroneously during a previous repair, or the ignition system is CDI (Capacitive Discharge Ignition).

    If all connections are correct, the USB Oscillocope program will display a parade waveform, showing firing voltage, spark voltage and burn time for each cylinder.

    It may be necessary for correct display to modify the synchronization of the capacitive probe. In the left bottom corner of the USB Oscillocope window press the icon of a hammer and a screw-driver and in the opened window "Setup" to modify the parameter "Cap. sensor synchronization level" so that the waveform is stable.

Normal operating parameters for an ignition system are as follows:
- Firing voltage - also known as break down voltage - on the average 4-18 kV;
- Spark voltage - also known as burn voltage 1-4 kV;
- Spark duration - also known as burn time 1-2 ms.
The above values will change according to the air/fuel ratio and cylinder pressure.

Typical waveforms form from classic or conventional ignition systems.



Wiring diagram for a conventional (classic) ignition system.
This system uses points to control the primary current.

1. Attachment point for the capacitive probe Cx.
2. Attachment point for the sync probe Sync.
3. Connection point for obtaining a primary waveform.
4. Battery.
5. Ignition switch.
6. Ignition coil.
7. Distributor equipped with points.
8. Spark plugs.




Typical secondary waveform from a classic or
conventional ignition system, equipped with points.

1. The points close, primary current starts to flow and a magnetic field is being built up in the ignition coil.
2. The points open and a high voltage is induced. The height of the firing line represents the voltage needed to initiate a spark. (Cause ionization or break down of the spark gap).
3. The spark line. The height represents the voltage needed to sustain the spark and the length of the line the duration.
4. The spark is extinguished. The beginning of the intermediate section with some dampened oscillations.




Typical primary waveform from a classic or
conventional ignition system, equipped with points.

1. The points close, primary current starts to flow and a magnetic field is being built up in the ignition coil.
2. The points open and a high voltage is induced. The height of the firing line represents the voltage needed to initiate a spark. (Cause ionization or break down of the spark gap).
3. The spark line. The length of the line represents the spark duration.
4. The spark is extinguished. The beginning of the intermediate section with some dampened oscillations.

3-4. The oscillations seen are the result of current flow between he capacitor in parallel with the points and the primary windings. These oscillations actually strengthens the spark and may be absent if the coil is connected backwards.




Typical waveform from the sync probe attached to the timing cylinder.

1. This probe is a synchronization probe and will, therefore, show the strobe waveform.



Distributor type electronic ignition


The connections are the same as for conventional ignition.



The connections for an electronic ignition system.


Typical waveforms from electronic ignition systems.



Wiring diagram for distributor ignition. This system
switches the primary circuit using a transistor.

1. Attachment point for the capacitive probe Cx.
2. Attachment point for the sync probe Sync.
3. Connection point for obtaining a primary waveform.
4. Battery.
5. Ignition switch.
6. Ignition coil.
7. Distributor containing a Hall sensor.
8. Spark plugs.
9. Ignition control module, also known as the ICM.
10. Connection point for observing the signal from the Hall sensor in the distributor.




Distributor ignition secondary waveform.

1. The transistor in the ICM turns on, primary current starts to flow and a magnetic field is being built up in the ignition coil.
2. The ICM limits the primary current once a predetermined value is reached. Commonly 6-8A. 2 is often called the "current limiter hump".
3. The transistor turns off and a high voltage is induced. The height of the firing line represents the voltage needed to initiate a spark (cause ionization or break down of the spark gap).
4. The spark line. The height represents the voltage needed to sustain the spark and the length of the line the duration.
5. The spark is extinguished. The beginning of the intermediate section with some dampened oscillations.




Distributor ignition primary waveform.

1. The transistor in the ICM turns on, primary current starts to flow and a magnetic field is being built up in the ignition coil.
2. The ICM limits the primary current once a predetermined value is reached. Commonly 6-8A. 2 is often called the "current limiter hump".
3. The transistor turns off and a high voltage is induced. The height of the firing line represents the voltage needed to initiate a spark (cause ionization or break down of the spark gap).
4. The spark line. The height represents the voltage needed to sustain the spark and the length of the line the duration. Notice that there are very few oscillations in this part of the waveform, compared to point-type ignition. This is because there is no or very little capacitance across the switching device.
5. The spark is extinguished. The beginning of the intermediate section with some dampened oscillations.




Typical waveform from the sync probe attached to the timing cylinder.

1. This probe is a synchronization probe and will, therefore, show the strobe waveform.




Hall sensor waveform.

Make sure the Hall switch will pull the waveform all the way to ground. Intermittent problems can be traced to failures to completely pull the signal to ground.



Distributor ignition with the ignition coil built in to the distributor


Other than the built in ignition coil, this system is similar to normal distributor ignition. Some Asian built cars used this arrangement.



Connection of secondary ignition pick-ups.

Connections for secondary waveform diagnosis.
  • Disconnect all leads from the USB Autoscope.
  • Connect power supply wiring for the ignition system adapter Ignition Adapter to the vehicle battery (red to positive, black to negative).
  • Connect the ignition system adapter to the USB Autoscope.
  • Attach the capacitive plate pick up as close to the ignition coil as possible.
  • Connect the sync probe (black) to the plug wire for the cylinder №1, as close to the spark plug as possible.
  • Start the engine.
  • In the USB Oscillocope program, select "Control => Load user preset => Ignition => Ignition_Classic".


    • Once the engine is started, the red LED on the ignition system adapter should be lit. If the green LED is lit, the primary connections are backwards. Swap the connection on the ignition coil to the other terminal. The connections on the ignition coil may have been connected erroneously during a previous repair.

    If all connections are correct, the oscilloscope will display a parade waveform, showing firing voltage, spark voltage and burn time for each cylinder.

    It may be necessary for correct display to modify the synchronization of the capacitive probe. In the left bottom corner of the USB Oscillocope window press the icon of a hammer and a screw-driver and in the opened window "Setup" to modify the parameter "Cap. sensor synchronization level" so that the waveform is stable.



Two-distributor ignition


Some V8 and V12 engines manufactured in the 90's by Audi and BMW were equipped with two distributors, one for each cylinder bank. These two systems operated independently of each other.

Diagnosis is the same as for a normal distributor ignition, except one, then the other ignition system must be examined.



Double plug distributor ignition


Some engines were equipped with double plug distributor ignition (some Nissan's equipped with the "Z" engines). The double plug distributor ignition uses two ignition coils, but one distributor equipped with a special cap and rotor.

These engine were so equipped to improve combustion and lower emissions, especially at idle. The engines were equipped with one "intake" and one "exhaust" spark plug. The two spark plugs does not fire simultaneously, the exhaust plug fires a few mS later. The exhaust spark plugs does not fire once a higher RPM is reached, to minimize noise.

Except for the points made above, diagnosis is the same as for normal distributor ignition. Do remember that the exhaust side does not fire at higher RPM.