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Chapter

: Ignition System Diagnosis and Repair

to ground and touch the other ohmmeter lead alter-
nately to each of the pickup coil connectors. The ohm-
meter should show infinite resistance at all test points.
If not, replace the pickup.

The pickup coil can also be tested using an oscilloscope
or scope. The coil has a two-wire circuit, a positive lead
and a negative lead, connecting the sensor to the PCM.
For best results, connect both scope probes directly to
the sensor leads as close to the sensor as possible. A
good trace generally sweeps up the positive slope and
drops on the negative slope. The shape of the trace
peak varies for different sensor designs, but most sig-
nals must reach a minimum amplitude before the PCM
recognizes them. Look for uniformity in the trace cycles.

Air Gap Adjustment

The magnetic sensor is designed to be positioned close
but not touching the reluctor. Most late-model CKP sen-
sors are designed to provide the correct

air gap

be-

tween the components and do not require adjustment.
Refer to the appropriate Service Manual for the correct
installation procedures.

Hall-Effect Sensors

Hall-effect sensors are found in many late-model igni-
tion system replacing the magnetic pickup type sensor.
The mounting and function of the Hall-effect sensor is
similar, but the operation and testing is different. A Hall-
effect sensor uses a microchip to switch a transistor on
and off and generate a digital signal that is transmitted
to the PCM. As a slotted or trigger wheel connected to
the crankshaft or distributor shaft, figure 2-23, passes
between the Hall element and a permanent magnet, the
magnetic field expands and collapses to produce an
analog signal. The Hall element contains a logic gate
that converts the signal into a digital signal, which trig-

gers transistor switching. The transistor transmits a dig-
ital square waveform at a variable frequency to the
PCM.

Hall-effect operation requires a three-wire circuitpower
input, signal output, and ground. The Hall element, fig-
ure 2-24, receives an input voltage from either the igni-
tion switch or the PCM to power it. As the magnetic field
is distorted by the passing of the slotted wheel the Hall
element switches the base of a transistor on and off.
This on-off signal is inverted by a transistor which in turn
opens and closes the ground-side of the primary igni-
tion coil circuit.

Hall-effect switches produce a square waveform, figure
2-25, whose frequency varies in proportion to the rota-
tional speed of the crankshaft. A Hall-effect sensor does
not generate its own voltage, and power must be pro-
vided for the device to work.

When using a scope for diagnosis remember the Hall-
effect sensor creates a waveform as the transistor
switches on and off. To get a good scope trace, attach
the positive scope probe to the transistor output signal
and connect the negative probe to the sensor ground.
For optimum results, connect both probe leads as close
as possible to the sensor.

SHUTTER BLADE

ROTATING
SHAFT

PERMANENT
MAGNET

HALL
ELEMENT

SWITCHING
TRANSISTOR

GROUND

POWER
INPUT

PCM
SIGNAL

PCM

Fig. 2-23.

Distributor-mounted Hall-effect sensor.

VANE