James Dillon reveals a handy measurement technique for problematic ignition systems. In May's issue we looked at ignition analysis from a primary and secondary voltage perspective. But, what if the design of the ignition system or its fitting location within the vehicle makes it difficult to measure? Well, there is technique which employs the low range current clamp that can show the function of the ignition coil and its control. Figure 1 shows the clamp and its location within the circuit. The current clamp in question is a low range (0 to 60 amps), high accuracy device which has a response of 20 kHz (figure 2). This type of current clamp costs in the region of ￡100. The principle of its application tips a nod to Gustav Kirchhoff's 'Current Law' where the sum of the current flowing into a node is equal to the sum of the current flowing away from the node. This means, irrespective of whether we measure the coil supply or the coil ground, that the current flow will be the same. For the best results, the current clamp output should be analysed using an oscilloscope, as the speed and dynamics of the coil's function (approx. 6 millisecond pulse every 150 milliseconds) is far beyond the measurement and sampling capabilities of a typical multimeter. Using the current clamp to analyse the ignition coil's function provides an advantage in non-intrusive testing. If we can locate either the coil supply or its ground wire we can take a view, in real time, on the coil's function. Typically, the low voltage wires going to/from the ignition coil are accessible, whether in the engine loom, or directly at the engine ECU. The current clamp has small jaws by design, which are perfect for use in situations where access to the wiring is difficult.
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