Chapter
B
: Ignition System Diagnosis and Repair
46
voltage to produce a signal. Since these sensors pro-
duce their own signal, it is possible to get a waveform
with correct amplitude and frequency, even though
there are problems on the ground circuits.
The pickup coil is wound around a permanent magnet,
whose field expands and collapses as the teeth of a ro-
tating notched or slotted wheel known as the reluctor or
trigger wheel pass by it, figure 2-22. The magnetic field
generates an alternating current (AC) voltage signal and,
as the speed of the engine increases, so does the fre-
quency and amplitude of the signal. The PCM uses the
frequency of the signal to determine rotational speed.
The magnetic sensor may be mounted in several loca-
tions:
• In the distributor
• Part of the harmonic balancer
• Near the flywheel at the rear of the engine
• In the side of the cylinder block
Depending on the system, the reluctor design varies to
meet the design requirements. Refer to the appropriate
Service Manual for the exact description of the system
used.
Testing a Magnetic Sensor
Regardless of system design, the resistance of a pick-
up coil can be checked with an ohmmeter. All pickup
coils operate at a specific resistance; however, specifi-
cations and test points will vary between manufacturers
and models.
In general, measurements are taken by connecting the
ohmmeter leads to opposite sides of the coil. If the read-
ing is not within specifications, replace the coil. To
check for a grounded coil, connect one ohmmeter lead
ONE CAMSHAFT REVOLUTION
(720 OF CRANKSHAFT ROTATION)
CMP
SIGNAL
ONE CRANKSHAFT
REVOLUTION (360 )
CKP
SIGNAL
30 GAP
(MISSING TEETH)
CAMSHAFT
TIMING
ROTOR
TOOTH
CAMSHAFT POSITION
(CMP) SENSOR
TIMING
ROTOR
CRANKSHAFT POSITION
(CKP) SENSOR
MISSING
TEETH
Fig. 2-21.
Some systems use both a crankshaft position (CKP) sensor and camshaft position (CMP) sensor to control ignition coil firing.
PICKUP COIL
SENSOR
–
+
0
TYPICAL SIGNAL
VOLTAGE WAVEFORM
ROTATING TRIGGER
WHEEL (RELUCTOR or
ARMATURE)
Fig. 2-22.
A pickup coil is self-powered and generates an analog AC
signal as the magnetic field expands and collapses.
Summary of Contents for ASE-A8
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