You have a car towed into your service department that will not start. You perform a basic spark test and determine that there is no secondary spark. What do you do next? What tools do you need? How does this car's ignition system work?
It is pointless to connect the secondary ignition scope because you have no spark. Determine which ignition system you have and get a diagram that will help you identify the circuits, components and wire colors. This is your road map that can keep you from getting lost and wasting time. There are many different ignition systems, but they all need: supply voltage, ground, and crank reference signal, and a timing control signal (from the PCM). There are some ignition systems that use distributor mechanical and vacuum timing control.
The electrical diagram on the right is our map for this system and the good voltage signals on the lab scope screen are what we would expect to see. The top signal is the variable reluctance signal from the pick-up coil. The second signal is the coil current flow during saturation and discharge. The third signal is the coil primary using the 10 to 1 probe (Oxygen sensor probe).
The bottom signal is battery voltage measured at the coil B+ terminal.
The PDA ignition meter in Fig 1 can be used with one hook-up (kV Probe) to test for firing voltage. This meter will detect the intensity of the spark by recording the highest voltage detected in the MAX window.
The lab scope (Fig. 2) is setup for testing a variable reluctance crank sensor and (Fig. 3) is setup to test a Hall effect signal. If your scope screen looks like either one of these while the engine is cranking, the next step is to determine why there is no crank signal.
Figs 4 and 5 have crank reference signals, but a no-spark/no-start condition exists because the amplitude (voltage level) is too low. Some manufacturers publish minimum CKT signal values. Hall effect signals must switch from 0 volts to a predetermined voltage level. Test good signals and record them for future reference. Fig 6 is a good AC voltage signal recorded while cranking the engine. The amplitude of AC signal will increase as RPM increases.
Fig 7 is a good crank and cam sensor signal. Figs 8 and 9 are primary signals from two failed ignition coils, both coils tested good with an Ohm meter. Fig 10 is a good ignition primary signal. Figs 8, 9 and 10 were acquired using the lab scope and a 10:1 probe (oxygen sensor lead). Fig 11 displays a good ignition primary and secondary signal.
A functioning ignition needs; supply voltage, ground, RPM reference signal, timing control signal, primary coil switch (ignition module), good coil primary circuit. All of these values can be tested with your PDA lab scope.
