470 Auto Electricity and Electronics
have to insert a feeler gauge under the throttle lever or have
the throttle plates at curb idle, for example. With the throttle
plates at the correct angle, the TPS should trigger the tester or
show a specifi ed ohms value. If an adjustment is needed and
possible, loosen the sensor mounting screws. Rotate the TPS
until the correct ohms reading is obtained. Then, tighten the
mounting screws and recheck the meter reading.
Engine Coolant Temperature Sensor Service
A bad engine coolant temperature sensor can also
affect air-fuel ratio and ignition timing by not accurately
informing the computer of the engine operating tempera-
ture. The engine coolant temperature sensor serves a simi-
lar function to a carburetor choke; it richens the mixture
when cold and leans the mixture when hot. If opened, the
coolant temperature sensor might affect cold engine drive-
ability. If shorted, it might affect warm engine driveability.
An ohmmeter is commonly used to measure coolant
temperature sensor resistance when cold and when hot.
The service manual will provide a chart listing ohms read-
ings for specifi c temperatures, Figure 24-17.
You can test the sensor while it is still in the engine by
checking ohms when the engine is cold and after it warms.
However, a more precise method is to submerse the sensor in
water and heat it on a hot plate. This will let you compare ther-
mometer readings and resistance readings with exact specs.
Figure 24-18 shows another way to test a thermal
switch or sensor. This unit controls a cold start injector. A
battery and test light are connected to the sensor. With an
increase in temperature, the sensor may switch off and the
test light will go out. The exact temperature and action of
the switching sensor can vary with the manufacturer.
Note that temperature sensor operation and ratings
can vary. When purchasing a new temperature sensor, or
any sensor, make sure you have the right one. For example,
Figure 24-19 shows how similar temperature sensors can
have different operating temperatures. If you install the wrong
one, it will upset the operation of the computer system.
When replacing a coolant temperature sensor, use a
deep socket or six-point wrench to unscrew the old unit.
Double-check that you have the correct replacement sensor.
Figure 24-13. Small jumper wires are often used when measur-
ing sensor circuit values. Make sure you do not short two jumper
wires together or to ground. This test is used to measure the
frequency signal in hertz from a MAP sensor. (Fluke)
To
Vacuum
Pump
Jumper
Wires
Harness
Connector
MAP
Sensor
Figure 24-14. Throttle position sensor is usually tested by mea-
suring the resistance or voltage across the sensor terminals at
specifi c throttle openings. If not within specs, adjust or replace
the unit. (Ford)
Throttle Body
with Throttle
at Correct
Setting
Meter Leads
TP Sensor Ohmmeter
The throttle position sensor can sometimes be tested
at a special tester terminal in the wiring harness. Some
manuals say you should measure voltage drops across the
sensor at specifi ed throttle positions. A reference voltage
is fed to the sensor by the computer. The sensor change in
resistance changes the voltage across the resistance.
Many manuals also recommend checking the resis-
tance of the throttle position sensor at different throttle
openings. The manual might have you measure ohms at
idle, half throttle, and full throttle, Figure 24-14. If resis-
tance is within specs, check the wiring leading to the TPS.
Throttle Position Sensor Removal
To remove many throttle position sensors, you must
fi le or grind off stakes (small welds) on the sensor screws,
Figure 24-15. You might also have to drill into the screws
from the bottom of the throttle body assembly. This will
let you turn and remove the screws and TPS. Refer to the
service manual for details.
Throttle Position Sensor Adjustment
Some throttle position sensors must be adjusted; some
cannot be adjusted. Many are mounted so that they can be
rotated on the carburetor or throttle body. Either a special
tester or an ohmmeter is commonly used to adjust a throttle
position sensor, Figure 24-16.
Basically, you must measure sensor resistance or note
tester output with the throttle at specifi c positions. You may