Ignition Coils TTC3.0
What it does
The ignition coil is a high voltage (low current)
transformer that converts the vehicle’s 12 volt
power supply to the 25-30,000 volts required to jump
the gap of the spark plug, thus instigating combustion.
How it works:
Since the advent of engine management, ignition coils
have undergone a complete redesign. Gone are the tradition
oil/asphalt filled, barrel shaped coils, now virtually
all manufacturers use resin filled plastic coils which
are manufactured in all shapes and sizes. They are
smaller, lighter and more efficient but regrettably
they are not always more reliable.
Although they appear completely different they still
rely on Faraday’s laws of electromagnetism to
generate this high voltage. For those of us who haven’t
been to school in the past few years, here’s
what happens;
When a voltage is passed through the primary windings
(from the battery (+ve), via the ignition switch,
through to ground (–ve)) a magnetic field (flux)
is generated around the windings. If the voltage is
interrupted (switched off) the magnetic field collapses,
this generates a voltage in the secondary windings.
This generated or induced voltage is dependant upon
the ratio of the windings (primary to secondary, the
design of the coil and how quickly the voltage is
turned on and off.
The switch that was used for many years was the ignition
contacts. These were mechanically driven by the engine
and timed to trigger the ignition just before the
piston reached top dead centre. During the late 70’s
and early 80’s electronic ignition replaced
the mechanically operated contacts. This enabled a
faster switching speed (hence higher voltage output)
and more accurate ignition timing settings (no mechanical
switch to wear/deteriorate). Electronic ignition used
a non-contact sensor (hall or inductive type) that
was the forerunner to today’s cam/crank sensors.
With government legislation dictating the need for
ever decreasing mixture strengths and engine manufacturers
designing more and more powerful engines, the need
arose for higher voltage ignition systems. High in-cylinder
pressures, hot combustion chamber temperatures and
lean mixture strengths all make it difficult for the
spark to jump across the plug gap, thus a higher voltage
spark is required. The most demanding applications
include turbo/super-charged, lean burn or direct petrol
injection applications. Following the advent of electronic
ignition, designers turned to developing the ignition
coil.
The traditional ignition reached its zenith. It became
“saturated” at the higher voltages demanded
from them and alternatives were sought. The higher
voltages required to trigger the spark also caused
under bonnet problems with high tension (HT) lead
insulation, both voltage and emission interference.
A new design ignition system was called for, say hello
to the resin filled plastic coils, say goodbye to
distributors and HT leads.
The development of these coil “packs”
has only been made possible with the advent of computer
aided design packages (CAD). The coils are designed
and tested before manufacture. As are virtually all
components on the modern motor vehicle, the designer’s
pencil has gone to the same bin as the contact/condenser
sets. The coil assembly is now precisely matched to
the other ignition system components (plugs, ECU driver
stage etc)all of which are integral to the engine
management system.
The secondary circuit of the coil is now completely
isolated from the primary (see diagram B), this allows
the use of “double ended” coils with two
HT outputs, these are also incorporated into multi
cylinder “packs” comprising of an assembly
of double ended coils to enable “coil per cylinder”
set ups. Also, as resin coils can be made smaller
and of any shape they can even be fitted directly
on top of the spark plugs (pencil coils).
Coil on plug units reduce the electrical capacitance
of the HT circuit and reduce the number of HT components
by removing the “weak link” the HT lead.
This also increases ignition system reliability.
They come in all shapes and sizes. Pencil Coils,
Block Coils, Cassette Coils.
Testing:
When testing ignition coils, start with the basics.
External checks for sign of HT leaks (tracking), check
for cracks to the insulating material, check for corrosion
to the terminals. Next move onto testing the internal
resistances, check primary and secondary windings,
check for the resistance (if any) between the two
circuits. Compare the readings to the relevant data
manuals. These tests will highlight many faults however;
Full testing however is very difficult as the most
common failures occur with internal insulation failure.
Cracks develop in the resin that insulates the primary
from the secondary windings. This causes HT voltage
to leak down through to the primary coils. Without
specialist equipment it is impossible to test for
this. Checking the resistance between the coils with
a conventional Ohm meter will simply show an open
circuit, it’s only when there is 30,000 volts
trying to get out that the faults show. To spell it
out, testing the resistance of the primary and secondary
windings will not guarantee an ignition coil’s
serviceability.
Caution, as the coil is now controlled by the ECU
any deterioration of the coils internal insulation
will result in ECU failure.
Ignition coils should always be replaced in sets.
All the coils on the vehicle have done the same work,
if one has failed then the others will be on their
way out, replace just one at your own risk.
Note; use caution when purchasing aftermarket copy
coils as some have been know to fail prematurely resulting
in ECU damage. Ignition coils sold by FPUK are manufactured
by OE supply sources.
For further technical information concerning wheel
speed sensors please phone our technical help line
or visit www.fpuk.co.uk
For further technical information
concerning ignition coils please phone our technical
help line 01527 839307
