Chevrolet TrailBlazer, Service Manual

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Ignition System
WARNING
To avoid personal injury and/or vehicle damage, refer to the service precautions at the
beginning of this section.
General Information
NOTE: For information on understanding electricity and troubleshooting electrical circuits,
please refer to chassis electrical.
Coil on Plug (COP) System
The coil over plug system was developed so that spark and spark timing could be better
controlled on an individual cylinder basis. Each cylinder has an ignition coil mounted
directly above the spark plug on the cylinder head cover. A short suppresser/connector
replaces the spark plug wire and links the coil to the plug. There are different methods
used for primary triggering. Some manufacturers use a combination coil/module, which
means each coil has its own control circuit that is activated by the PCM. Others use
remote mounted modules to trigger the coils.
Each individual coil is allowed to saturate while all other cylinders fire. For a V-8 engine,
this allows a period of seven firing events for coil saturation, compared to three events for
the same V-8 engine with a waste spark system. The coil over plug system also benefits
from a minimum amount of energy lost, due to the resistance of spark plug wires.
Magnetic Sensor / Pick-Up Coil
The magnetic sensor in electronic ignition system is made up of a small coil of wire
wrapped around an iron core, a permanent magnet and a toothed wheel called a reluctor.
These sensors can be found mounted in a distributor, or at the front, middle, or rear of the
crankshaft or camshaft, and are two-wire sensors.
The permanent magnet produces a magnetic field that passes thru the center of the pick-
up coil. As the reluctor turns, the small teeth enter the magnetic field. Because the metal is
a better conductor for the field than the air between the magnet and reluctor, the field
strength begins to increase and reaches its maximum when the reluctor teeth are closest
to the sensor. An increase in magnetic field induces a positive voltage to the module. As
the teeth leave the magnetic field, the decrease in pole strength induces a negative
voltage into the module. This alternating positive and negative voltage causes a small AC
current. This alternating current after passing through an analog/digital converter is used
by the module or engine controller to trigger the primary circuit.
Hall-Effect Device
Another device that can be used to create a triggering signal is a hall-effect device. A hall-
effect device can be thought of as a solid-state On/Off switch. The hall-effect switch is a
three-wire device that must receive a power and ground. The hall-effect switch is used in
conjunction with an interrupter ring with a series of slots or openings cut into it. Depending