472 Auto Electricity and Electronics
Figure 24-17. Engine temperature or coolant temperature sensor
testing is normally done with an ohmmeter. A—One example of sen-
sor ohms for specifi ed temperatures. B—Heating the sensor while
testing the action. C—Cooling the sensor to check the action.
Temperature to Resistance Values
for Temperature Sensor (Approximate)
Deg. F Deg. C Ohms
210
160
100
70
40
20
0
-40
100
70
38
20
-4
-7
-18
-40
185
450
1,800
3,400
7,500
13,500
25,000
100,700
Hot Plate
Water
Temperature
Sensor
Thermometer
Ohmmeter
Ohmmeter
Water
Thermometer
Temperature or
Coolant Sensor
Ice
A
B
C
Figure 24-18. Connecting a voltage source and test light to a
sensor is another testing method for a thermal sensor. As the
temperature or conditions change, the test light should turn on
or off. This sensor operates a cold start injector. (Peugeot)
Test Light On
Thermal
Switch
Sensor
Jumper
Cold
No Current
Flow
Hot
Battery or
Voltage Source
Test Light Off
40°
30°
40°
30°
20°
Figure 24-19. Make sure you always have the correct replace-
ment sensor. For example, note how these temperature sensors
have different operating temperatures. If you install the wrong
one, the computer system may not function properly. (Pontiac)
Switch
Pin
Sender
Pin
Calibration
Temperature 125°C
Operating
Temperature 258°F
Single Pin Single Pin
You might want to coat the sensor threads with approved
sealer. Then, hand start and tighten the sensor in the engine.
Do not overtighten the coolant sensor, or it could bottom out
in the engine and be ruined.
Intake Air Temperature Sensor Service
A bad intake air temperature sensor will usually not
have a pronounced effect on vehicle operation. It normally
allows the computer system to make fi ne adjustments of
air-fuel ratio and timing with changes in outside air tem-
perature. If the sensor fails, it will normally trigger a self-
diagnosis code and you would know to test the sensor and
its circuit.
An ohmmeter is commonly recommended for check-
ing an air temperature sensor. As with an engine coolant
temperature sensor, the unit is frequently a thermistor that
changes internal resistance with temperature. The sensor
should have spec ohms for certain temperatures. If not,
replace the sensor. Look at Figure 24-20.