Chapter
B
: Ignition System Diagnosis and Repair
38
scribe some common secondary circuit problems that
can be detected with an oscilloscope.
Firing Section Abnormalities
Check the firing section of an oscilloscope trace for the
following:
• A large firing oscillation with little or no sparkline for
one cylinder indicates an open circuit between the
distributor rotor and the spark plug, figure 2-6. Use
the parade display to isolate the faulty cylinder.
Check for a disconnected or broken spark plug
cable. This pattern can be created deliberately to
check coil output. Remove a cable, the top of the
firing line is the maximum available voltage of the
coil. Use the kV scale on the side of the screen to
read voltage output
• One sparkline that is lower and longer than the rest
indicates low-resistance in the circuit between the
distributor cap and the spark plug, figure 2-7.
Check for carbon tracks in the distributor, poor
cable insulation, or a fouled spark plug
• A parade pattern may show a short sparkline still
exists even when the spark plug cable has been de-
liberately disconnected, figure 2-8. This indicates
that high-voltage is causing a current leak to ground
somewhere, usually through the ignition cable insu-
lation, the distributor cap, or the rotor. Carbon
tracks will often accumulate near the leakage
• There should be no more than a 20-percent differ-
ence between the highest and lowest firing spikes
when comparing cylinders in a parade pattern, fig-
ure 2-9. Variations can be caused by fuel or electri-
cal system problems. To separate the two, slowly
cover the engine air intake to richen the fuel mixture.
If the spikes go down and engine speed increases,
the problem is fuel related. If the spikes go down
and engine speed remains unchanged, the plug
gaps may be too great. If a single spike remains the
same height, the cable to that plug may be dam-
aged
• One sparkline that is higher and shorter than the
rest indicates high-resistance from an open circuit
between the distributor cap and the spark plug, fig-
ure 2-10. A damaged or loose cable or a wide plug
gap may be at fault. Corrosion on the cable termi-
nals and in the distributor cap can cause a sparkline
for one cylinder to start higher and angle downward
more sharply than the others
• A sparkline that jumps erratically or slopes up rather
than down is caused by an incorrect fuel mixture in
that cylinder. The problem may be mechanical, such
as sticking or worn valves, or caused by intake air
leaks, or fuel induction problems
LACK OF SPARKLINE
SHOWS OPEN PLUG CABLE
Fig. 2-6.
A large firing oscillation without a sparkline on a secondary
superimposed pattern indicates an open plug circuit.
LONG, LOW SPARKLINE AND SHORT
OSCILLATIONS SEPARATED FROM
OTHERS SHOWS LOW RESISTANCE
Fig. 2-7.
A low, long sparkline on a secondary superimposed pattern
indicates low-resistance in one cylinder.
Fig. 2-8.
An open plug circuit with a voltage leak in a
secondary parade pattern.
Fig. 2-9.
Look for fairly even firing line peaks when viewing a
secondary parade pattern.
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