Chapter A: General Diagnosis
10
to level off in a relatively flat horizontal line. Dwell ends
with the abrupt upward stroke of the first firing oscilla-
tion.
Use the raster pattern to compare the dwell period for
each cylinder. Dwell sections should not vary by more
than four to six degrees between cylinders. Variations
can be caused by a worn distributor, timing chain, or
faulty crankshaft position sensor (CKP).
Secondary Ignition Patterns
Secondary voltage traces also move left to right across
the screen in firing, intermediate, and dwell sequence.
Oscillations are displayed on a high-voltage scale. Nor-
mal firing voltage with an electronic ignition system can
exceed 40 kV.
Firing Section
The firing section of the secondary pattern begins with
a straight vertical line that indicates the amount of volt-
age required to create an arc across the spark plug air
gap. This is called the firing line or voltage spike. When
the arc is established, less voltage is required to main-
tain it and the trace drops to about one-quarter the
height of the voltage spike, then continues horizontally
as the sparkline. The sparkline, which represents con-
tinued current across the spark plug gap, may have a
series of very small oscillations.
Intermediate Section
The secondary intermediate trace indicates excess coil
voltage being dissipated and is similar to that of the pri-
mary ignition pattern. Look for a short vertical rise from
the sparkline followed by diminishing oscillations. Os-
cillations should be of relatively even width and taper
down gradually to a near horizontal line. The intermedi-
ate section ends as the primary circuit is switched on.
Dwell Section
The dwell section begins as the primary circuit is
switched on and continues until the firing section be-
gins. A typical trace pattern shows the switching as a
sharp downward spike followed by a series of small d
minishing oscillations that level off to a near flat line.
The length of the dwell section and the trace pattern
may be engine speed dependent, and varies for diffe
ent systems, figure 1-10. Some switching signals pro-
duce a series of oscillations; others do not. All begin
with a sharp vertical drop and continue as a nearly flat
horizontal line.
SPARK BEGINS
FIRING SECTION
SPARK ENDS
FIRING LINE
DIMINISHING
OSCILLATIONS
Fig. 1-8.
A normal primary circuit firing trace.
PRIMARY CIRCUIT
SWITCHES ON
INTERMEDIATE
SECTION
SPARK ENDS
Fig. 1-9.
The intermediate section begins as the spark extinguishes
and continues until the primary circuit is switched on.
Fig. 1-10.
Dwell time increases with engine speed on some
electronic control systems.
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