Chapter
B
: Ignition System Diagnosis and Repair
39
Intermediate Section Abnormalities
Use the intermediate section of the trace to check coil
function. A faulty coil, primary circuit, or coil wire be-
tween the coil and distributor cap may cause a lack of
oscillations.
• Absent coil oscillations with a normal dwell section
are the result of a faulty coil or a high-resistance
short to ground in the primary circuit
• A pattern displayed upside-down indicates that coil
polarity is reversed, usually because of reversed pri-
mary connections at the coil
• The entire pattern jumping on the screen results
from an intermittent open in the coil secondary
winding
• Reduced coil oscillations along with a missing mod-
ule-on signal are caused by an open between the
coil and the distributor cap
• Variation in the firing signals at the end of a raster
pattern is caused by worn ignition signal parts or a
faulty module. This variation indicates timing differ-
ences among cylinders. Note that this is normal for
those ignition systems that use computer control of
timing to regulate idle speed
Dwell Section Abnormalities
The dwell section of a secondary trace should be a rel-
atively flat line with the possible exception of a current
limiting hump. Be aware, dwell length increases with en-
gine speed on some systems, figure 2-11. Dwell varia-
tion between cylinders can be checked using the raster
display. A dwell variation of more than 4 to 6 degrees
between cylinders indicates mechanical wear in the dis-
tributor. Use the degree scale at the bottom of the
screen to read dwell angles.
SPARK PLUG AND IGNITION CABLE
SERVICE
Spark plugs are routinely replaced at specific service in-
tervals as recommended by the vehicle manufacturer.
However, these intervals are guidelines and actual spark
plug service life will vary. Spark plug life depends upon:
• Engine design
• Type of driving
• Kind of fuel used
• Types of emission control devices
Because the spark plugs are the final component in all
secondary circuits, the remainder of the circuit cannot
perform properly if they are not in good condition.
Spark Plug Removal
Spark plug access is limited on many late-model en-
gines due to a maze of air conditioning and emission
control plumbing and engine-driven accessory mount-
ing. Engine accessories may have to be loosened from
their mountings and moved to get to the plugs. Air con-
ditioning compressors, air pumps, and power steering
pumps are frequent candidates for relocation during
spark plug service. Whenever you must move one of
these accessories, be careful of its plumbing and wiring.
The spark plugs on some engines are most easily
reached from underneath the engine. To remove the
spark plugs:
1. Disconnect cables at the plug by grasping the boot
and twisting gently while pulling. Do not pull on the
cable. Insulated spark plug pliers provide a better
grip and are recommended when working near hot
manifolds, figure 2-12.
HIGH, SHORT SPARKLINE
Fig. 2-10.
A high, short sparkline on a secondary superimposed
pattern indicates high-resistance in one cylinder.
DWELL AT
800 RPM
DWELL AT
1700 RPM
DWELL AT
3000 RPM
Fig. 2-11.
Dwell length increases in proportion to engine speed on
some systems.
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