Ignition probes measure the voltages across automobile ignition systems. When connected to oscilloscopes, these probes reveal activities and misfires in spark plugs and other ignition components. In increasing numbers of current model automobiles, spark-plug wires are being replaced by ignition coils. Manufacturers have taken advantage of this shift in components, offering specialized ignition probes. However, measuring these systems does not require a specialized tool. The process can be emulated with two probes and either an oscilloscope or a software-developed oscilloscope emulator.
Locate the automobile's ignition system. Identify the spark plugs in the ignition system. Examine the mechanical components around them and locate the ignition coil. Identify the positive and negative terminals in the ignition coil modules. If not immediately evident, much of this information is available in either the vehicle's owner's manual or in wiring diagrams available through automotive repair manuals.
Connect the two probes to two channels on the oscilloscope. Depending upon models, oscilloscopes have the ability to perform a difference between two channels and present this information on the screen. Consult the oscilloscope's manual for details about setting this process up. If an oscilloscope is not available, then the appropriate software and hardware can be used to create a facsimile scope. For example, National Instrument's LabVIEW graphical programming language has the ability to interface with signal input hardware and present information in real time. Create a simple routine in this versatile software to read the results from the two probes and calculate the difference. Such a program will require a While Loop, the Initialize Analog Channel, Read Analog Channel, the subtraction operator and a graph display.
Affix the positive probe to the ignition coil's positive terminal. Affix the negative probe to the ignition coil's negative terminal. Run the vehicle. The screen shows the ignition coil activity as a series of peaks separated by relative inactivity. These peaks will be in kiloVolts. Adjust the signal readout as necessary to make these peaks visible. Record the data, if necessary.
