Chapter 24 Sensor, Actuator, Computer Service 473
Airfl ow Sensor Service
A bad airfl ow sensor will normally cause the system
to go into limp-in mode, as will several other sensors. If
shorted or opened, the computer will begin to operate on
preprogrammed values. The car will perform poorly and
get lower fuel economy.
Since there are various types of airfl ow sensors,
you must refer to the shop manual for exact procedures.
Many manufacturers have you use an ohmmeter to check
the airfl ow sensor when it is a variable resistance type,
Figure 24-21. Others have integrated circuits on the sensor
and special testing methods are required to prevent poten-
tial sensor damage. Specifi c terminals on the sensor must
be touched to make readings, Figure 24-22.
If faulty, you must usually remove and replace the air-
fl ow sensor. During replacement, make sure that you have
the correct unit. Also, tighten all fi ttings carefully. An air
leak after the airfl ow sensor will upset its operation and can
trigger trouble codes, Figure 24-23.
Fuel Sending Unit Service
A bad fuel sending unit can open, short, or develop high
or low resistance and cause the fuel gauge to read improperly.
It can also upset the operation of the trip computer system.
This sensor is simply a variable resistor operated by a fl oat.
The fl oat follows the fuel level in the tank so that resistance
changes proportionately with the level of the fuel.
Many manufacturers give ohm values for specifi c
fl oat arm positions, Figure 24-24. Your ohmmeter must
read close within specs for each fl oat arm position. Special
testers are also available for checking the gauge circuit.
Figure 24-20. Intake air temperature sensor is usually a therm-
istor. Use an ohmmeter to compare resistance for specifi c tem-
perature. (Honda Motor Co.)
Air Cleaner
Ohmmeter Leads Intake Air
Temperature Sensor
Figure 24-21. Depending on the airfl ow sensor type, different test-
ing methods are required. This type requires an ohmmeter to check
the internal resistance change as a fl ap is opened and closed.
VOM
Specified
Test Point
Ground
Airflow
Meter or
Sensor
Test Leads
Figure 24-22. A—Note the internal diagram of one type airfl ow
sensor. When testing an airfl ow sensor, refer to the service man-
ual. It will tell you which terminals to touch with the meter and
what the test results should be. Some airfl ow sensors have inter-
nal integrated circuits that could be damaged by an improper
testing method. B—Specs for just one airfl ow sensor. (Toyota)
E2 E2 E1 Vc THA Fc Vs
A
Between
Terminals
Resistance Temperature
E2 – Vs
E2 – Vc
E2 – THA
E1 – Fc
20 – 400 Ω
200 – 400 Ω
10 – 20 KΩ
4 – 7 KΩ
2 – 3 KΩ
0.9 – 1.3 KΩ
0.4 – 0.7 KΩ
Infinity
—
—
–20°C ( 4°F)
0°C ( 32°F)
20°C ( 68°F)
40°C (104°F)
60°C (140°F)
—
B
Figure 24-23. When replacing an airfl ow sensor, make sure
there are no air leaks to upset operation.
Seal
Mass Airflow
Sensor
Clamp
Flexible Air
Intake Duct
Clamp
Throttle
Body
Air Cleaner
Duct
Lock