Temperature Type Sensors - Bosch FIX 7677 Mode D'emploi

Temperature Type Sensors

A temperature sensor is a thermistor or
a resistor whose resistance changes with
temperature. The hotter the sensor gets,
the lower the resistance becomes. Typi-
cal thermistor applications are engine
coolant sensors, intake air temperature
sensors, transmission fluid temperature
sensors, and oil temperature sensors.
Test Procedure (see Fig. 32):
Fig. 32
1. If engine is HOT let it COOL down
before proceeding.
Make sure all engine and transmis-
sion fluids are at outside air tem-
perature before proceeding with this
test!
2. Insert BLACK test lead into COM
test lead jack.
3. I n s e r t R E D t e s t l e a d i n t o
4. Disconnect wiring harness from
sensor.
5. If testing Intake Air Temperature
Sensor - Remove it from vehicle.
All other temperature sensors can
remain on vehicle for testing.
6. Connect RED test lead to either
sensor pin.
7. Connect BLACK test lead to re-
maining sensor pin.
8. Turn multimeter rotary switch to
desired OHM range.
If the approximate resistance is un-
known, start at the largest OHM
range and decrease to the appropri-
ate range as required. (See Setting
the Range on page 6)
Hair Dryer
Typical
Intake Air
Temperature
Sensor
Black
Red
test lead jack.
9. V i e w a n d r e c o r d r e a d i n g o n
display.
10. Disconnect multimeter test leads
from sensor and reconnect sen-
sor wiring.
This step does not apply to intake air
temperature sensors. For intake air
temperature sensors, leave multim-
eter test leads still connected to sen-
sor.
11. Heat up sensor.
If testing Intake Air Temperature
Sensor:
• To heat up sensor dip sensor tip
into boiling water, or...
• Heat tip with a lighter if sensor tip
is metal or a hair dryer if sensor tip
is plastic.
• View and record smallest reading
on display as sensor is heated.
• You may need to decrease the
range to get a more accurate read-
ing.
For all other temperature sensors:
• Start engine and let idle until upper
radiator hose is warm.
• Turn ignition key OFF.
• Disconnect sensor wiring harness
and reconnect multimeter test
leads.
• View and record reading on display.
12. Test Results.
Good Sensor:
• Temperature sensors HOT resis-
tance is at least 300Ω less than its
COLD resistance.
• The key point is that the COLD
resistance decreases with increas-
ing temperature.
Bad Sensor:
• There is no change between the
temperature sensors HOT resis-
tance from the COLD resistance.
• The temperature sensor is an open
or a short circuit.
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