Chapter
B
: Ignition System Diagnosis and Repair
37
For any ignition system to operate properly, the follow-
ing conditions must be met:
• Battery cranking voltage must be over 9 volts
• Primary wiring must be in good condition, and all
connections need to be clean and tight
• Spark must be available at the spark plugs while the
engine is cranking
The air gap between the pickup coil and trigger wheel is
adjustable on some systems. Gap has no effect on the
dwell period; dwell is determined by the control module.
The air gap must be set to a specific clearance when a
new pickup unit is installed. During use, the air gap
should not change. However, it should be checked be-
fore performing troubleshooting tests.
SECONDARY CIRCUIT DIAGNOSIS AND
REPAIR
Often, it is useful when diagnosing a no-start condition
to first verify that secondary voltage is available from the
coil by performing a spark test. Begin by disconnecting
a spark plug cable from the plug. Connect a
spark
tester
to the plug wire and attach the ground clip of the
tester to a bare metal spot on the engine, figure 2-5. Ad-
just the spark gap according to Service Manual specifi-
cations. If no specifications are available, start with a
setting of approximately 1/4" (6mm). Crank the engine
while observing the spark tester. A bright blue spark
should be clearly visible. Do not crank the engine with
the spark plug wires disconnected as this may create
high open-circuit voltage and cause damage to the ig-
nition coil or other electronic systems.
Inspection
Begin your evaluation with a visual inspection of the
secondary components. Look for:
• Cracked, burned, or brittle insulation on the ignition
cables and boots
• Distributor cap defects, such as a sticking or worn
carbon button, cracks, carbon tracks from arcing
current, burned or corroded terminals, and corro-
sion inside the cap towers
• Rotor damage, such as a bent or broken contact
strip, burned or eroded tip, cracked or broken posi-
tioning lug, and carbon tracks or cracks on the body
Oscilloscope Testing
Oscilloscope testing is the most efficient method of lo-
cating secondary circuit malfunctions. Variations in the
secondary trace will indicate if parts are malfunctioning
or incorrectly adjusted. The following paragraphs de-
Fig. 2-3.
A crankshaft position sensor provides the input used to
initiate plug firing on a direct ignition system.
Fig. 2-4.
Some systems use an additional sensor on the camshaft to
properly phase the firing sequence.
Fig. 2-5.
Using a spark tester to check for secondary voltage
available from the coil.
Summary of Contents for ASE-A8
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