Service manual

G420F(FE) Service Manual Chapter 4. Engine Electrical System 119
Ignition System
Spark-ignited engines require accurate control of
spark timing and spark energy for efficient
combustion. The MI-07 ignition system provides this
control. The system consists of the following
components:
• SECM
• Ignition coil drivers *
• Ignition coil(s) *
• Crankshaft position sensor *
• Crankshaft timing wheel *
• Cam position sensor *
(for sequential ignition or fuel injection only)
• Cam timing wheel *
(for sequential ignition or fuel injection only)
• Spark plugs *
The SECM, through use of embedded control
algorithms and calibration variables, determines the
proper time to start energizing the coil and fire the
spark plug. This requires accurate crank/camshaft
position information, an engine speed calculation,
coil energy information, and target spark timing. The
SECM provides a TTL compatible signal for spark
control. The coil must contain the driver circuitry
necessary to energize the primary spark coil
otherwise an intermediary coil driver device must be
provided. The SECM controls spark energy (dwell
time) and spark discharge timing.
Figure 16. Coil-On-Plug Ignition System
Coil-On-Plug Ignition System
Coil-on-plug (COP) is a type of distributorless
ignition system where individual ignition coils are
mounted directly over each spark plug. No spark
plug wires are used. On most engines, the plugs
and coils are located on top of the cylinder head for
easy mounting of the coils. A topside location is best
because it keeps the coils away from the heat of the
exhaust.
COP Components
In a typical COP ignition system, a crankshaft
position sensor generates a basic timing signal by
reading notches on the crankshaft, flywheel, or
harmonic balancer. The crank sensor signal goes to
the small engine control module (SECM), where it is
used to determine firing order and turn the individual
ignition coils on and off.
The operation of the ignition system is essentially
the same as any other ignition system. Each coil has
a low primary resistance (0.4 to 0.6 ohms) and steps
up the primary system voltage from 12 volts to as
much as 40,000 volts to produce a spark for the
spark plug.
The primary difference between COP and other
ignition systems is that each COP coil is mounted
directly atop the spark plug so the voltage goes
directly to the plug electrodes without having to pass
through a distributor or wires. It is a direct
connection that delivers the hottest spark possible.
Resistor plugs are generally used to suppress
electromagnetic interference (EMI).