Chapter 25 Computer System Service 413
OFF
2 200m
DIGITAL MULTIMETER
2
2 20
20 00
10A
MAX
MAX MAX
600 AC 600mAC 0mAC
600mDC 600mDC
10
COM
500
2 200
2K
20 0K
20 00K K
2M
2 20M
V
V
A
A
OFF
2 200m
DIGITAL MULTIMETER
2
2 20
20 00
10A
MAX
MAX MAX
600 AC 600mAC
600mDC 600mDC
10
COM
500
2 200
2K
20 0K
20 00K K
2M
2 20M
V
V
A
A
OFF
2 200m
DIGITAL MULTIMETER
2
2 20
20 00
10A
MAX
MAX MAX
600 AC 600mAC
600mDC 600mDC
10
COM
500
2 200
2K
20 0K
20 00K K
2M
2 20M
V
V
A
A
Digital
thermometer
Temperature
sensor
Compare
reading
to specs
Heat
gun
A
5V ref. from computer
Read
voltage
Activate
(rotate)
resistor
sensor and
note its
position
Measure voltage drop
across resistor
Ignition
key on
B
Switching
sensor
Zero to
infinite
ohms
Move switch
on and off
C
Figure 25-4. There are many ways to test passive sensors.
A—An ohmmeter can be used to measure internal resistance.
Some sensors can be heated with a heat gun while measuring
resistance. A digital thermometer may also be used to compare
actual temperature with sensor resistance. B—A voltmeter can
be used if sensor specifi cations are given only as a voltage
drop. The sensor must have reference voltage while testing
occurs. C—Switching sensors may be tested with an ohmmeter
or a high-impedance test light. The meter or light should show
a change in condition when the switch is turned on and off.
OFF
2 200m
DIGITAL MULTIMETER
2
2 20
20 00
10A
MAX
MAX MAX
600mAC 600mAC
600mDC 600mDC
10
COM
500
2 200
2K
20 0K
20 00K K
2M
2 20M
V Ω
V Ω
A
A
OFF
2 200m
DIGITAL MULTIMETER
2
2 20
20 00
10A
MAX
MAX MAX
600 AC 600mAC 0mAC
600mDC 600mDC
10
COM
500
2 200
2K
20 0K
20 00K K
2M
2 20M
V Ω
V Ω
A
A
Measure reference
voltage to sensor
Wires to
sensor
Computer
Test leads
Passive (resistive)
sensor; wires
disconnected
A
Typical
reading
Wires disconnected
at computer
Measure VR
at computer
connector
If no or low reference
voltage to sensor,
check at computer B
Computer
Figure 25-5. Passive sensors rely on a computer reference
voltage for proper operation. A—Use a digital voltmeter
to measure the amount of voltage supplied to the sensor.
Typical reference voltage might be 5 volts, but refer to the
service manual specifi cations. B—If the reference voltage is
not correct, test for voltage at the computer. If the reference
voltage at the computer is correct, a problem with the wiring
harness is indicated.