Chapter
B
: Ignition System Diagnosis and Repair
45
ammeter to the positive (battery) coil terminal, figure 2-
19. When using an inductive ammeter, fit the inductive
pickup over the primary wire to the coil positive terminal
leaving the wire connected.
Observe the ammeter reading and compare it to speci-
fications. If there is no reading, the primary circuit is
open and the engine will not run. Higher-than-specified
current draw can be caused by:
• A short circuit in the coil
• Use of an incorrect coil
Lower-than-specified current draw results from:
• A discharged battery
• Excessive resistance in the coil primary winding
• Loose or corroded primary connections
• High-resistance in the primary wiring to the coil
Position Sensors
In order to provide the PCM with the correct data re-
garding engine RPM and cylinder position, one or more
sensors are installed to provide input information. The
sensor(s) identifies the
top dead center
(TDC) point for
cylinder number one. Once this position is established
the PCM then calculates the position of the remaining
cylinders and adjusts ignition timing and control ac-
cordingly.
Ignition System Signals
Most electronic ignition systems generally use an am-
perage-sensing ignition circuit to limit current through
the primary circuit. This current limiting function can be
seen on an oscilloscope as a hump during the dwell pe-
riod just before the module-open signal, figure 2-20.
Dwell is short at low engine speed, and increases as
RPM increases.
The primary circuit does not energize until the module
receives a verifiable engine speed signal from the crank-
shaft position (CKP) sensor. The CKP sensor provides
the ignition module or powertrain control module (PCM)
information on crankshaft speed and location. Some
systems use an additional camshaft position (CMP)
sensor, figure 2-21. A CMP sensor is used to provide
the synchronization signal on distributorless ignition and
sequential fuel injection systems, and for misfire detec-
tion on OBD-II vehicles. There are three basic designs
for automotive position or speed sensors:
• Magnetic pickup
• Hall-effect sensor
• Optical sensor
The most accurate way to diagnose speed sensor prob-
lems is to monitor the signal on an oscilloscope. A mag-
netic pickup is an analog device that produces a sine
wave pattern, while Hall-effect sensors and optical sen-
sors are digital and produce a digital, or square wave-
form. For all types, the ignition module or PCM uses the
frequency of the signal to determine the speed of the
sensed component.
Magnetic Pickup Sensor
Whether called a magnetic pickup coil, variable reluc-
tance sensor, or permanent magnet generator, this type
of sensor generates voltage as an analog signal. These
sensors are self contained and do not require an applied
LOW RPM
PRIMARY
CIRCUIT
HIGH RPM
PRIMARY
CIRCUIT
LOW RPM
SECONDARY
CIRCUIT
HIGH RPM
SECONDARY
CIRCUIT
FIRING
SECTION
INTERMEDIATE
SECTION
DWELL
SECTION
CURRENT
LIMITING
HUMP
Fig. 2-20.
Current limiting produces a hump on an oscilloscope trace
at the end of the dwell period.
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