Chapter 24 Sensor, Actuator, Computer Service 465
engine speed sensor can be mounted in the distributor
(pickup coil) or on the engine (crank position sensor) so
that it can detect crankshaft rotation.
Most speed sensors are magnetic and produce a weak
voltage signal. You can use a VOM or scope to check the
speed sensor. An ohmmeter can also be used to measure
the internal resistance of the sensor coil windings.
An ohmmeter will produce a static (nonrunning) test
of a speed sensor. Disconnect the sensor wires, and mea-
sure the internal resistance of the sensor coil, Figure 24-5A.
Resistance specs will vary from about 250 to 1500 ohms,
so refer to the manual for an accurate value.
To use a digital VOM, set the meter on AC volts and
connect the test leads to the sensor wires. Crank the engine
and read the meter. Typically, a good sensor will produce
an AC voltage output of about 1.5 to 3 volts. Again, check
the specifi cations before condemning a speed sensor.
An oscilloscope can also be used to measure the voltage
output from most speed sensors. As shown in Figure 24-5B,
connect the engine analyzer leads to the pickup coil. Crank
the engine and read the AC voltage output waveform on the
oscilloscope screen, Figure 24-5C.
A faulty speed sensor will have high resistance, low
resistance, or a low-voltage or no voltage output. Replace
the sensor if it is not within factory specs.
A nonmagnetic feeler gauge may be needed to adjust
some distributor-mounted pickup coils or speed sensors.
The gap between the sensor and its trigger wheel must be
correct. However, most speed sensors lock in place and do
not require adjustment. Figure 24-6 shows how to adjust
one type of speed sensor.
Vehicle Speed Sensor Service
A bad vehicle speed sensor will usually reduce engine
performance and fuel economy but will not normally keep
the engine from running. It provides data for precise con-
trol of fuel metering, ignition timing, transmission/transaxle
shift points, and such. A bad vehicle speed sensor might
also affect transmission torque converter lockup.
Figure 24-5. A—Magnetic speed sensor or pickup coil can be tested with an ohmmeter. Coil windings should have spec resistance.
Wiggle the wires while testing to check for internal breaks in the wires. B—AC voltmeter or an oscilloscope can also be used to
measure the small voltage output of a magnetic sensor while cranking engine. C—Oscilloscope pattern. (Chrysler Corporation and
Snap-on Incorporated)
~V
OFF
V
Ω
mA
mA
A
~
Wiggle
Pickup
Coil
Leads
while
Testing
Note! Crank
Engine to Use
AC Voltmeter.
Do Not Crank
Engine to Use
Ohmmeter.
Tap on Pickup Coil
with Screwdriver
while Testing
Pickup
Lead
Scope or
Analyzer
Leads
Sensor or
Pickup
Lead
Ohmmeter or
AC Voltmeter
Ohmmeter or
Voltmeter Leads
A
B
C
0V
10V
Sensor or
Pickup Coil
Waveform
Positive Peak
Negative Peak
Figure 24-6. This camshaft speed sensor uses a paper spacer
to aid in adjustment. Mounting screw is loosened and the spacer
is placed between the sensor tip and trigger wheel tooth on the
camshaft. After the sensor is reinstalled and the mounting screws
are tightened, the spacer is removed. (Chrysler Corporation)
Paper
Spacer
Rib Material
(for Sensor Depth
Positioning)
Camshaft
Position
Sensor