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Electronic Ignition

We now know two things:

A small voltage and current can be used to turn a

transistor on.

A moving magnetic field and a coil of wire can produce

a current flow.

Knowing these two facts, we will now look at the next step - removing the points

entirely - and using an electromagnetic pickup to switch the transistor.

The pickup assembly consists of a coil of wire attached next to a permanent magnet. The
distributor cam has been replaced by an armature (or reluctor) wheel on the shaft. As the
reluctor moves past the permanent magnet, it causes the magnetic field to shift slightly. This
moving magnetic field, in turn, will produce a very small voltage in the pickup coil. Two
problems -the voltage is very small, and it only occurs for a brief moment. Because of
this, an additional item must be added to the system to make it work long enough to switch
the coil primary current on and off. Note that the pickup coil is controlled by a vacuum
advance, and the armature will be controlled by a centrifugal advance.

The small voltage is sent to the Control Unit (often called the Ignition Module) which
contains a timing circuit to control magnetic field buildup and collapse, and the
large transistor used to control the primary current.

The Ignition Module contains a number of individual components (transistors, resistors,
diodes, etc.) attached to a circuit board and located in a sealed container to protect them
from vibration and moisture.

The Next Step

One of the problems with the ignition modules was the fact that they were relatively
large and the individual components had to be wired to each other to produce the
necessary circuits. With changes made in electronics technology, the next obvious step
was to use Integrated Circuits to build the modules.

Integrated Circuits

Because transistors, diodes, resistors, etc. are built up in layers - it is possible to
produce a complete circuit by simply combining layers of conductor and semi-conductor
materials all on one piece of material. This produces a circuit that is small, efficient,
and inexpensive compared to larger modules containing individual parts.

Because of the size reduction, it is possible to connect the ignition module directly to
the pickup coil in the distributor and locate it within the distributor itself (such as this
GM - HEI unit). Other manufacturers located the module on the outside of the
distributor or mounted it away from the distributor.

The main problem that still remained with all of these systems was the
inaccuracy of using centrifugal and vacuum advance mechanisms to control
spark timing. A method had to be found to produce better performance,
better fuel mileage and lower emissions through better timing control.

Next - Unit Two Assignment

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