412 Section 4 Computer Control Technology
To test a passive sensor, either measure its internal
resistance with an ohmmeter or measure the volt-
age drop across the sensor with its reference voltage
applied. Depending on the sensor or manufacturer,
both tests may be performed. You can also read com-
puter data stream values with your scan tool. How-
ever, the scan tool cannot isolate the sensor, wiring,
or computer for individual tests. Any unusual sen-
sor readings by a scan tool should be verifi ed using
a multimeter.
Testing Variable Resistance Sensors
Service Procedure
To test a variable resistance sensor with an
ohmmeter:
1. Disconnect the sensor wires.
2. Connect the ohmmeter’s test leads to
the sensor terminals, Figure 25-4A.
3. Compare the ohmmeter reading to the
manufacturer’s specifi cations.
The sensor’s resistance must be within factory
specifi cations. For example, if you are testing a tem-
perature sensor, connect an ohmmeter to the sensor
terminals and then measure coolant temperature
with a digital thermometer. Note the resistance
readings at various coolant temperatures and com-
pare these readings to service manual specifi cations.
If the sensor’s resistance is within specifi cations,
check for opens or shorts in the wires going to the
sensor. If the wires are good, suspect the computer
itself. The service manual will give detailed instruc-
tions for testing the computer.
Service Procedure
To check a variable resistance sensor with
a voltmeter:
1. Connect the voltmeter in parallel with
the sensor. Be sure to leave the com-
puter wires connected to the sensor,
Figure 25-4B.
2. Measure the voltage drop across the
sensor with the computer reference
voltage applied.
3. Compare your measurements to
specifications.
Testing Switching Sensors
With a switching sensor, such as a power steering
pressure switch, you can use an ohmmeter to check
that the switch is opening and closing. As shown in
Figure 25-4C, connect your ohmmeter and move the
switch opened and closed. Your meter should reg-
ister infi nite ohms and then zero ohms. Replace the
sensor if it is defective. You could also use a high
impedance test light to quickly check the operation
of a switching sensor.
Note that some auto manufacturers do not give
resistance specifi cations for passive sensors, they
only give voltage drops. You may have to use a test
harness to connect the meter in parallel with the
sensor. You can make or purchase this type of test
harness.
Testing Reference Voltage
A reference voltage (typically 5 volts) is fed to
passive sensors. Then, when conditions and sensor
resistance change, the amount of voltage fl owing
back to the computer also changes. The reference
voltage is needed so a signal returns to the computer.
Service Procedure
To measure reference voltage to a passive
sensor:
1. Disconnect the wires leading to the
sensor.
2. Connect a digital voltmeter to the wires.
3. Turn the ignition key on and note your
readings.
4. Compare your voltage readings to
specifications.
Typically, the open circuit voltage should be about
5 volts. Refer to Figure 25-5.
If the reference voltage to the sensor is low, check
the wiring harness for high resistance, as something
is preventing the full reference voltage from reach-
ing the sensor. Low reference voltage would cause a
sensor to produce erroneous readings. It is possible
for the reference voltage to be too high if a current-
carrying wire is shorted into the circuit. High volt-
age may also be caused by a computer malfunction,
but this is rare.