414 Section 4 Computer Control Technology
Testing Active Sensors
As mentioned, an active sensor produces its own
voltage and sends it back to the computer. The volt-
age produced by an active sensor is very low, often
under 1 volt. This makes sensor wiring harness conti-
nuity very critical. One poor electrical connection can
keep the low voltage from returning to the computer.
Figure 25-6 shows several ways to test an active
sensor. In Figure 25-6A, an ohmmeter is connected
to a common magnetic sensor. The ohmmeter will
measure the resistance of the coil winding. Replace
the sensor if the resistance is high or low.
In Figure 25-6B, an ac voltmeter is connected to a
magnetic sensor. The trigger wheel must be rotated
(engine cranked over, wheel or hub in an ABS sys-
tem turned, etc.) to make the sensor generate volt-
age. A magnetic sensor should typically produce
about 1.5–3 volts ac. A magnet can also be passed by
a coil to make it produce a voltage.
In Figure 25-6C, a digital voltmeter is connected
to an oxygen sensor. With the engine running in
closed loop, the voltmeter should show the sensor’s
output voltage. If the output voltage from the sensor
is low or high, the sensor may require replacement.
Figure 25-7 shows how to use small jumper wires
to connect a meter to check a sensor while it is still
functioning in the circuit.
Tech Tip
Whenever a sensor tests good, check the
wiring leading to the sensor. Bad wiring may
be blocking current fl ow back to the computer.
Replacing Sensors
When replacing a sensor, there are several general
rules you should remember:
• Always purchase an exact sensor replacement.
Even though two sensors may look identical,
their internal resistance or circuitry may be dif-
ferent. See Figure 25-8.
• Release the sensor connector properly. Most con-
nectors have positive locks that must be released.
If you damage the connector, intermittent prob-
lems may result from a loose connection.
• Use special tools as needed. Some sensors, such
as oxygen sensors, require the use of a sensor
socket. This socket has a deep pocket and a cut-
out that will fi t over the sensor and any wires.
Conventional deep sockets may not fi t over the
wiring or the sensor head.
OFF
2 200m
DIGITAL MULTIMETER
2
2 20
20 00
10A
MAX
MAX MAX
600mAC 600mAC
600mDC 600mDC
10
COM
500
2 200
2K
20K 0K
20 00K K
2M
2 20M
V Ω Ω
VΩ
A
A
OFF
2 200m
DIGITAL MULTIMETER
2
2 20
20 00
10A
MAX
MAX MAX
600mAC 600mAC
600mDC 600mDC
10
COM
500
2 200
2K
20K 0K
20 00K K
2M
2 20M
V Ω Ω
VΩ
A
A
OFF
2 200m
DIGITAL MULTIMETER
2
2 20
20 00
10AMAX
MAX MAX
600 AC 600mAC 0mAC
600mDC 600mDC
10
COM
500
2 200
2K
20K 0K
200K 00
2M
2 20M
V Ω Ω
VΩ
A
A
Wires to sensor
disconnected
Magnetic
sensor
Compare coil resistance to specs
A
B
C
Passing magnet by
sensor should also
generate a voltage
Magnetic
sensor
Trigger wheel spinning (engine
running, wheel spinning, etc.)
Reading should be about
1.5 to 3.0 volts AC
Oxygen
sensor
Exhaust from
running engine
Reading should be within specs
(vary from 0.1 to 0.9 volts)
Wires to sensor
disconnected
Figure 25-6. Since an active sensor generates its own output
signal, testing methods for active sensors are slightly different
from those for passive sensors. A—Magnetic sensor internal
coil resistance can be measured with an ohmmeter. B—An
ac voltmeter can be used to check a magnetic sensor while it
is operating. With the triggering device moving, an ac voltage
signal should be generated. C—Sometimes it is recommended
to connect a meter directly to the sensor. You can compare the
operating sensor’s output to specifi cations. Be sure to test the
harness leading to the sensor.