Chapter
C
: Fuel, Air Induction, and Exhaust Systems Diagnosis and Repair
62
reference voltage, a second wire provides the signal
voltage, and the third wire is the ground side of the
sensor and carries the return current. The sensor signal
is proportional to the movement of the potentiometer
and is unaffected by changes in temperature. Two ohm-
meter connections are required to check the operation
of a potentiometer, figure 3-15. To check, connect the
ohmmeter:
• Across the signal voltage and return current pins
• Across the signal and reference voltage pins
Both tests should show continuity, and resistance
should change with the movement of the sensor lever or
rod. However, a more precise way to evaluate a poten-
tiometer is to perform a sweep test. During a sweep
test, signal voltage is monitored on a DMM as the de-
vice is moved through its full range of travel, figure 3-16.
A sweep test can also be performed while monitoring
signal voltage with an oscilloscope.
A sensor that measures motion, such as a throttle posi-
tion (TP) sensor, or exhaust gas recirculation (EGR) valve
position sensor (EVP), can be hand-operated while
checking its resistance. A sensor that measures pres-
sure, such as a manifold absolute pressure (MAP)
sensor, can be operated with a hand vacuum or pres-
sure pump while testing.
A solenoid used as an output actuator can be checked
for circuit problems with a DMM or oscilloscope much
the same as a sensor. On actuators that use a multiplug
connector, refer to the Service Manual for test points
and values.
Many automotive solenoids are controlled by a pulsing
on and off signal. Since most solenoids are digital, they
VHz~
~
10A
RPM
+
COM
A
C F
RPM
V
%
OFF
VHz
OHMMETER
POWER
OR VREF
CIRCUIT
PCM
HARNESS
ECT
SENSOR
ECT
SIGNAL
SIGNAL
RETURN
DETACHED
CONNECTOR
Fig. 3-14.
Checking the resistance of a thermistor sensor with an
ohmmeter.
VHz~
~
10A
RPM
+
COM
A
C F
RPM
V
%
OFF
VHz
VHz~
~
10A
RPM
+
COM
A
C F
RPM
V
%
OFF
VHz
OHMMETER
OHMMETER
VREF
EVP
SIGNAL
GROUND
PCM
HARNESS
EVP
SENSOR
A
B
DETACHED
CONNECTOR
Fig. 3-15.
Ohmmeter connections for checking the resistance of a
potentiometer.
VHz~
~
10A
RPM
+
COM
A
C F
RPM
V
%
OFF
VHz
A
C
B
THROTTLE POSITION
SENSOR
REFERENCE
SIGNAL
GROUND
Fig. 3-16.
Meter connections for performing a sweep test on a
potentiometer sensor.
Summary of Contents for ASE-A8
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