Service manual

G420F(FE) Service Manual Chapter 4. Engine Electrical System 120
Misfires
COP problems can include many of the same
ailments as other ignition systems such as misfiring,
hard starting, or a no start. Spark plugs can still be
fouled by oil or fuel deposits, as well as pre-ignition
and detonation.
If the crankshaft position sensor fails, the loss of the
basic timing signal will prevent the system from
generating a spark and the engine will not start or
run. A failed driver circuit within the SECM can kill
an individual coil and prevent that cylinder from firing.
But with COP, an individual coil failure will only
cause misfiring in one cylinder.
It is important to remember that ignition misfire can
also be caused by other factors such as worn or
fouled spark plugs, loose or damaged coil connector
or terminals, dirty fuel injectors, low fuel pressure,
intake vacuum leaks, loss of compression in a
cylinder, even contaminated fuel. These other
possibilities should all be ruled out before a COP
unit is replaced.
A COP engine that cranks but fails to start, in many
cases, will often have a problem in the crankshaft or
camshaft position sensor circuits. Loss of sensor
signals may prevent the SECM from properly
synchronizing, thereby preventing the engine from
starting and running.
Figure 17. Ignition Coil and Extension Wire
Assembly for G420F(E) Engiue
COP Checks
Individual ignition coils can be tested with an
ohmmeter the same as those on a conventional
distributor or DIS ignition system. Measure primary
and secondary resistance and compare to
specifications. If resistance is out of specifications,
the coil is bad and needs to be replaced.
Also, pay close attention to the tube that wraps
around the spark pug. Cracks can allow voltage to
jump to ground causing a misfire. The spark plug
terminal should also fit tightly.
If a COP coil tests bad and is replaced, cleaning the
COP connector and wiring harness terminals can
often avoid future problems. Corrosion at either
place can cause intermittent operation and loss of
continuity, which may contribute to component
failure. Applying dielectric grease to these
connections can help prevent corrosion and assure
a good electrical connection.
Magnetic crankshaft position sensors can be tested
with an ohmmeter, and the sensor output voltage
and waveform can be read with an oscilloscope. The
output voltage of a Hall Effect crankshaft position
sensor can be checked with a voltmeter. On most
vehicles, a defective crank position sensor will
usually set a fault code that can be read with the
Service Tool.