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?
The answer is yes and no. Some of the old tests can still be
performed when there is suitable access but many of the tests focused on the
High Tension or secondary side of the coil. However access to the high tension
side is often only possible after removing the coil pack, you can use an
extension between the coil and the plug to test the HT outputs. These tests can
be performed with an oscilloscope or a spark tester.
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).
The logic or ECU, crunches the numbers and selects the
correct ignition advance, 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. 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. This 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. The chart shows the current
build up in a coil of 0.2Ω for both 12 and 8 Volts.
