Trade of Motor Mechanic - eCollege

Module 4 - Unit 1
Trade of Motor Mechanic - Phase 2 Course Notes
11.2 Inductive System Operation
2
Basic Ignition Systems
In an electronic ignition, a rotating reluctor and magnetic-pickup
coil replace the traditional cam, breaker points and condenser in the
distributors of cars equipped for electronic ignition. This system
reduces the time between tune-ups. The high spots of the reluctor
interrupt the magnetic field of the pickup coil and the permanent
magnet.
These interruptions, or pulses, are transmitted from the pickup to a
nearby electronic control unit. There, the pulses signal a transistor to
break the low-voltage sub-circuit and release high voltage from the
coil to the spark plugs. After the positive pulse is received, a dwell
control section of the control circuit determines when the primary
circuit will be switched on and how long current will flow in the
primary winding. The dwell period can thus be varied according to
engine speed, improving coil efficiency. At low engine speeds, the
dwell period is short. As engine speed rises the dwell period increases.
Ignition coils employed with electronic systems are referred to as
low inductance coils, because their primary winding resistance and
number of turns is low. Current flow is much higher than with
contact breaker systems and reaches its optimum level sooner.
A current limiting device within the control circuit limits maximum
current flow to a safe value. Ignition advance according to engine
speed and load is provided by centrifugal and vacuum advance
mechanisms.
Revision 2.0 July 2007