Lost your spark?

Andy Crook considers using old spark tests on modern ignition systems

Published:  22 February, 2013

Modern ignition systems have evolved from contact breaker or points systems. If you look carefully you can still see some of the DNA of these systems in the latest systems fitted today. So is it reasonable to assume that some of the tests performed on older ignition systems can still be performed on these modern systems?

Some (older) readers will remember measuring dwell angles. Testing the LT or primary circuit has changed little since the days of points; a test lamp can still provide quick and effective proof of circuit integrity. However, an oscilloscope can provide extra details that can lead to more effective diagnosis. With an oscilloscope it is possible to analyse the current draw as well as the control signal/voltage at the same time. The reason why this is so important becomes clear when you consider how the modern ignition system is controlled.

Modern ignition systems consist of various inputs, logic and outputs. Inputs are from sensors such as crankshaft position (engine speed & position), Manifold Absolute Pressure or Air Mass (load) and knock sensors (abnormal combustion), see Figure 1 for a schematic diagram. The logic or ECU crunches the numbers and selects the correct ignition advance and dwell period as well as monitoring the circuit for faults and providing the circuit protection.

The outputs are the low tension circuits, fault codes and malfunction indicator lamp or in the case of amplified coils, a signal to switch the amplifier and often a conformation of ignition signal back to the ECU. These control signals can be an internal function of the ECU or in the case of amplified coils, a square waveform that is used to switch the primary coil, see the running scope image in Figure 2. The 'on time' of the coil is controlled by this signal. The output stage allows current to flow through the primary windings when the voltage is present and stops the flow of current when the voltage drops to 0V. This 'dwell' can be measured much like on the older systems. The yellow trace is the control signal and the blue trace is the current flow through the primary windings.

The cursors are measuring the 'on time' or dwell, in this case 3.16ms - a typical value for a running engine. Notice the coil has reached around 5.5 Amps then current in the circuit is limited. The primary coil windings have low resistances between 0.2 and 0.8 Ohms which allows a rapid build-up of current and can reach 60 Amps if left unchecked in around 40ms. The current in the circuit is dependent upon the voltage so to ensure good saturation, the ECU can compensate for low voltages. Figure 3 shows the current build up in a coil of 0.2? for both 12 and 8 Volts.




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