Halderman J.D., Linder J. Automotive Fuel and Emissions Control Systems
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181
COM
A
mA A
mV
V
V
mA
A
A
V
DIGITAL MULTIMETER
RECORD
MAX MIN
HZ
%
10
2
345
6
7
8
9
0
HZ
MAXMIN
040.5
KΩ
FIGURE 13–5 Measuring the resistance of the ECT sensor.
The resistance measurement can then be compared with
specifications.
ECT SENSOR
GROUND CONNECTION
ENGINE
BLOCK
ECT SIGNAL CIRCUIT
ECT GROUND CIRCUIT
10k OHMS
1k OHMS
SIGNAL
IN
A/D CONVERTER
5V VREF
UNDER 122°F
5V VREF
OVER 122°F
FIGURE 13–6 When the voltage drop reaches approxi-
mately 1.2 volts, the PCM turns on a transistor. The
transistor connects a 1-kΩ resistor in parallel with the
10-kΩ resistor. Total circuit resistance now drops to around
909 ohms. This function allows the PCM to have full control
at cold temperatures up to approximately 122°F and a sec-
ond full control at temperatures greater than 122°F.
4.80
4.48
4.16
3.84
3.52
3.20
2.88
2.56
2.24
1.92
1.60
1.28
0.96
0.64
0.32
0.00
-40° -18° 5° 25° 45° 65° 85° 105° 135° 155° 175° 195° 215° 235° 255° 275°
ECT VOLTAGE
ABOVE 120°
ECT VOLTAGE
BELOW 120°
DC
VOLTS
DEGREES F
FIGURE 13–4 The transition between steps usually occurs at a temperature that would not interfere with cold engine starts
or the cooling fan operation. In this example, the switch point between the two resistors occurs when the sensor voltage is about
1 volt and rises to about 3.6 volts.
182 CHAPTER 13
Chrysler ECT Sensor Without Pull-Up Resistor
°F °C Voltage (V)
130 54 3.77
140 60 3.60
150 66 3.40
160 71 3.20
170 77 3.02
180 82 2.80
190 88 2.60
200 93 2.40
210 99 2.20
220 104 2.00
230 110 1.80
240 116 1.62
250 121 1.45
Chrysler ECT Sensor with Pull-Up Resistor
°F °C Volts
20 29
4.70
10 23
4.57
0
18
4.45
10
12
4.30
20
7
4.10
30
1
3.90
40 4 3.60
50 10 3.30
60 16 3.00
70 21 2.75
80 27 2.44
90 32 2.15
100 38 1.83
Pull-Up Resistor
Switched by PCM
110 43 4.20
120 49 4.10
130 54 4.00
140 60 3.60
150 66 3.40
160 71 3.20
170 77 3.02
180 82 2.80
190 88 2.60
200 93 2.40
210 99 2.20
220 104
2.00
230 110 1.80
240 116 1.62
250 121 1.45
General Motors ECT Sensor Without
Pull-Up Resistor
°F
°C
Ohms
Voltage Drop
Across Sensor
40 40
100,000 5
22 30
53,000 4.78
4 20
29,000 4.34
14
10
16,000 3.89
32 0 9,400 3.45
50 10 5,700 3.01
68 20 3,500 2.56
86 30 2,200 1.80
104 40 1,500 1.10
122 50 970 3.25
140 60 670 2.88
158 70 470 2.56
176 80 330 2.24
194 90 240 1.70
212 100 177 1.42
230 110 132 1.15
248 120 100 .87
Ford ECT Sensor
°F °C Resistance (Ω) Voltage (V)
50 10 58,750 3.52
68 20 37,300 3.06
86 30 24,270 2.26
104 40 16,150 2.16
122 50 10,970 1.72
140 60 7,600 1.35
158 70 5,370 1.04
176 80 3,840 0.80
194 90 2,800 0.61
212 100 2,070 0.47
230
110 1,550 0.36
248 120 1,180 0.28
TEMPERATURE SENSORS 183
Honda ECT Sensor (Voltage Chart)
°F °C Voltage (V)
0
18
4.70
10
12
4.50
20
7
4.29
30
1
4.10
40 4 3.86
50 10 3.61
60 16 3.35
70 21 3.08
80 27 2.81
90 32 2.50
100 38 2.26
110 43
2.00
120 49 1.74
130 54 1.52
140 60 1.33
150 66 1.15
160 71 1.00
170 77 0.88
180 82 0.74
190 88 0.64
200 93 0.55
210 99 0.47
Nissan ECT Sensor
°F °C Resistance (Ω)
14
10
7,000–11,400
68 20 2,100–2,900
122 50 680–1,000
176 80 260–390
212 100 180–200
Mercedes ECT
°F °C Voltage (DCV)
60 20 3.5
86 30 3.1
104 40 2.7
122 50 2.3
140 60 1.9
158 70 1.5
176 80 1.2
194 90 1.0
212 100 0.8
European Bosch ECT Sensor
°F °C Resistance (Ω)
32 0 6,500
50 10 4,000
68 20 3,000
86 30 2,000
104 40 1,500
122 50 900
140 60 650
158 70 500
176 80 375
194 90 295
212 100 230
Honda ECT Sensor (Resistance Chart)
°F °C Resistance (Ω)
0
18
15,000
32 0 5,000
68 20 3,000
104 40 1,000
140 60 500
176 80 400
212 100 250
Normal operating temperature varies with vehicle make
and model. Some vehicles are equipped with a thermostat
with an opening temperature of 180°F (82°C), whereas other
vehicles use a thermostat that is 195°F (90°C) or higher. Before
replacing the ECT sensor, be sure that the engine is operating
at the temperature specified by the manufacturer. Most manu-
facturers recommend checking the ECT sensor after the cooling
fan has cycled twice, indicating a fully warmed engine. To test
for voltage at the ECT sensor, select DC volts on a digital meter
and carefully back probe the sensor wire and read the voltage.
SEE FIGURE 13–7 .
NOTE: Many manufacturers install another resistor in
parallel inside the computer to change the voltage drop
across the ECT sensor. This is done to expand the scale
of the ECT sensor and to make the sensor more sensi-
tive. Therefore, if measuring voltage at the ECT sensor,
check with the service manual for the proper voltage at
each temperature.
TESTING THE ECT SENSOR USING A SCAN TOOL
Follow the scan tool manufacturer’s instructions and connect
a scan tool to the data link connector (DLC) of the vehicle.
Comparing the temperature of the engine coolant as displayed
on a scan tool with the actual temperature of the engine is an
excellent method to test an engine coolant temperature sensor:
184 CHAPTER 13
GENERIC LOGGING FORM
REMARKS: ECT Voltage
2001 Jeep Wrangler Warm-up Cycle
AUTO 202 - Fuel and Emissions Systems
SHOW DATA: ALL GRAPH VIEW: ALL
FORM SAVED TIME: 2/18/04 4:11:55 PM
UPLOAD TIME: 2/18/04 4:09:05 PM
METER ID: FLUKE 189 V2.02 0085510089
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
15:27
15:28 15:29 15:30 15:31 15:32 15:33 15:34 15:35 15:36 15:37 15:38 15:39
V DC
TIME
FIGURE 13–7 An ECT sensor being tested using a digital meter set to DC volts. A chart showing the voltage decrease of the
ECT sensor as the temperature increases from a cold start. The bumps at the bottom of the waveform represent temperature
decreases when the thermostat opens and is controlling coolant temperature.
1. Record the scan tool temperature of the coolant (ECT).
2. Measure the actual temperature of the coolant using an
infrared pyrometer or contact-type temperature probe.
NOTE: Often the coolant temperature gauge in the
dash of the vehicle can be used to compare with the
scan tool temperature. Although not necessarily ac-
curate, it may help to diagnose a faulty sensor, espe-
cially if the temperature shown on the scan tool varies
greatly from the temperature indicated on the dash
gauge.
The maximum difference between the two readings
should be 10°F (5°C). If the actual temperature varies by more
than 10°F from the temperature indicated on the scan tool,
check the ECT sensor wiring and connector for damage or
corrosion. If the connector and wiring are okay, check the sen-
sor with a DVOM for resistance and compare to the actual
engine temperature chart. If that checks out okay, check the
computer.
NOTE: Some manufacturers use two coolant sen-
sors,one for the dash gauge and another one for the
computer.
INTAKE AIR
TEMPERATURE SENSOR
PURPOSE AND FUNCTION The intake air temperature (IAT)
sensor is a negative temperature coefficient (NTC) thermistor that
decreases in resistance as the temperature of the sensor increases.
The IAT sensor can be located in one of the following locations:
In the air cleaner housing
In the air duct between the air filter and the throttle body,
as shown in
FIGURE 13–8
Built into the mass airflow (MAF) or airflow sensor
Screwed into the intake manifold, where it senses the
temperature of the air entering the cylinders
NOTE: An IAT installed in the intake manifold is the
most likely to suffer damage because of an engine back-
fire, which can often destroy the sensor.
The purpose and function of the intake air temperature
sensor is to provide the engine computer (PCM) the temperature
TEMPERATURE SENSORS 185
ability. One such condition is a warm engine being stopped in
very cold weather. In this case, when the engine is restarted,
the ECT may be near normal operating temperature such as
200°F (93°C), yet the air temperature could be 20°F (30°C).
In this case, the engine requires a richer mixture because of the
cold air than the ECT would seem to indicate.
of the air entering the engine. The IAT sensor information is used
for fuel control (adding or subtracting fuel) and spark timing,
depending on the temperature of incoming air:
If the air temperature is cold, the PCM will modify the
amount of fuel delivery and add fuel.
If the air temperature is hot, the PCM will subtract the
calculated amount of fuel.
Spark timing is also changed, depending on the tempera-
ture of the air entering the engine. The timing is advanced
if the temperature is cold and retarded from the base-
programmed timing if the temperature is hot.
Cold air is more dense, contains more oxygen, and
therefore requires a richer mixture to achieve the proper
air-fuel mixture. Air at 32°F (0°C) is 14% denser than air
at 100°F (38°C).
Hot air is less dense, contains less oxygen, and therefore
requires less fuel to achieve the proper air-fuel mixture.
The IAT sensor is a low-authority sensor and is used by the
computer to modify the amount of fuel and ignition timing as
determined by the engine coolant temperature sensor.
The IAT sensor is used by the PCM as a backup in the
event that the ECT sensor is determined to be inoperative.
NOTE: Some engines use a throttle-body temperature
(TBT) sensor to sense the temperature of the air entering
the engine, instead of an intake air temperature sensor.
Engine temperature is most accurately determined by
looking at the engine coolant temperature (ECT) sensor. In cer-
tain conditions, the IAT has an effect on performance and drive-
IAT
SENSOR
FIGURE 13–8 The IAT sensor on this General Motors 3800
V-6 engine is in the air passage duct between the air cleaner
housing and the throttle body.
Quick and Easy ECT Test
To check that the wiring and the computer are func-
tioning, regarding the ECT sensor, connect a scan
tool and look at the ECT temperature display:
STEP 1 Unplug the connector from the ECT sen-
sor. The temperature displayed on the
scan tool should read about 40.
NOTE: 40° Celsius is also 40°
Fahrenheit. This is the point where
both temperature scales meet.
STEP 2 With the connector still removed from the
ECT sensor, use a fused jumper lead and
connect the two terminals of the connec-
tor together. The scan tool should display
about 285°F (140°C).
This same test procedure will work for the IAT and
most other temperature sensors.
TECH TIP
Poor Fuel Economy? Black Exhaust Smoke?
Look at the IAT
If the intake air temperature sensor is defective,
it may be signaling the computer that the intake
air temperature is extremely cold when in fact it is
warm. In such a case the computer will supply a
mixture that is much richer than normal.
If a sensor is physically damaged or electrically
open, the computer will often set a diagnostic trou-
ble code (DTC). This DTC is based on the fact that
the sensor temperature did not change for a certain
amount of time, usually about eight minutes. If, how-
ever, the wiring or the sensor itself has excessive
resistance, a DTC will not be set and the result will
be lower-than-normal fuel economy and, in serious
cases, black exhaust smoke from the tailpipe during
acceleration.
TECH TIP
186 CHAPTER 13
General Motors
Transaxle Sensor—Temperature to Resistance
(approximate)
°F °C Resistance Ohms
32 0 7,987–10,859
50 10 4,934–6,407
68 20 3,106–3,923
86 30 1,991–2,483
104 40 1,307–1,611
122 50 878–1,067
140 60 605–728
158 70 425–507
176 80 304–359
194 90 221–259
212 100 163–190
P
C
M
SIGNAL HIGHER VOLTAGE
WHEN SENSOR
IS COLD
LOWER VOLTAGE
WHEN SENSOR
IS HOT
5V
TEMPERATURE
SENSOR HIGH
RESISTANCE
COLD LOWER
RESISTANCE
HOT
FIGURE 13–9 A typical temperature sensor circuit.
TESTING THE INTAKE AIR
TEMPERATURE SENSOR
If the intake air temperature sensor circuit is damaged or faulty,
a diagnostic trouble code (DTC) is set, and the malfunction in-
dicator lamp (MIL) may or may not turn on, depending on the
condition and the type and model of the vehicle. To diagnose
the IAT sensor follow these steps:
STEP 1 After the vehicle has been allowed to cool for sev-
eral hours, use a scan tool, observe the IAT, and
compare it to the engine coolant temperature (ECT).
The two temperatures should be within 5°F of each
other.
STEP 2 Perform a thorough visual inspection of the sensor and
the wiring. If the IAT is screwed into the intake mani-
fold, remove the sensor and check for damage.
STEP 3 Check the voltage and compare to the following
chart.
What Exactly Is an NTC Sensor?
A negative temperature coefficient (NTC) thermis-
tor is a semiconductor whose resistance decreases
as the temperature increases. In other words, the
sensor becomes more electrically conductive as the
temperature increases. Therefore, when a voltage is
applied, typically 5 volts, the signal voltage is high
when the sensor is cold because the sensor has a
high resistance and little current flows through to
ground.
SEE FIGURE 13–9 .
However, when the temperature increases, the
sensor becomes more electrically conductive and
takes more of the 5 volts to ground, resulting in a
lower signal voltage as the sensor warms.
?
FREQUENTLY ASKED QUESTION
TRANSMISSION FLUID
TEMPERATURE SENSOR
The transmission fluid temperature (TFT), also called trans-
mission oil temperature (TOT), sensor is an important sensor
for the proper operation of the automatic transmission. A TFT
sensor is a negative temperature coefficient (NTC) thermistor
that decreases in resistance as the temperature of the sensor
increases.
Intake Air Temperature Sensor Temperature Versus
Resistance and Voltage Drop (Approximate)
°F
°C
Ohms
Voltage Drop
Across the Sensor
40 40
100,000 4.95
18 8
15,000 4.68
32 0 9,400 4.52
50 10 5,700 4.25
68 20 3,500 3.89
86 30 2,200 3.46
104 40 1,500 2.97
122 50 1,000 2.47
140 60 700
2.00
158 70 500 1.59
176 80 300 1.25
194 90 250 0.97
212 100 200 0.75
TEMPERATURE SENSORS 187
NOTE: Check service information for the exact shift
strategy based on high and low transmission fluid tem-
peratures for the vehicle being serviced.
Chrysler
Sensor Resistance (Ohms)—Transmission
Temperature Sensor
°F °C Resistance Ohms
40 40
291,490–381,710
4 20
85,850–108,390
14
10
49,250–61,430
32 0 29,330–35,990
50 10 17,990–21,810
68 20 11,370–13,610
77 25 9,120–10,880
86 30 7,370–8,750
104 40 4,900–5,750
122 50 3,330–3,880
140 60 2,310–2,670
158 70 1,630–1,870
176 80 1,170–1,340
194 90 860–970
212 100 640–720
230 110 480–540
248 120 370–410
Ford
Transmission Fluid Temperature
°F °C Resistance Ohms
40 to 4 40 to 20
967K–284K
3 to 31 19 to 1
284K–100K
32 to 68 0 to 20 100K–37K
69 to 104 21 to 40 37K–16K
105 to 158 41 to 70 16K–5K
159 to 194 71 to 90 5K–2.7K
195 to 230 91 to 110 2.7K–1.5K
231 to 266 111 to 130 1.5K–0.8K
267 to 302 131 to 150 0.8K–0.54K
The transmission fluid temperature signal is used by the
powertrain control module (PCM) to perform certain strategies
based on the temperature of the automatic transmission fluid.
For example:
If the temperature of the automatic transmission fluid is
low (typically below 32°F [0°C]), the shift points may be
delayed and overdrive disabled. The torque converter
clutch also may not be applied to assist in the heating of
the fluid.
If the temperature of the automatic transmission fluid is
high (typically above 260°F [130°C]), the overdrive is dis-
abled and the torque converter clutch is applied to help
reduce the temperature of the fluid.
CYLINDER HEAD
TEMPERATURE SENSOR
Some vehicles are equipped with cylinder head temperature
(CHT) sensors:
VW Golf
14°F (⫺10°C) ⫽ 11,600 Ω
68°F (20°C) ⫽ 2,900 Ω
176°F (80°C) ⫽ 390 Ω
Some vehicles, such as many Ford vehicles that are equipped
with an electronic returnless type of fuel injection, use an
engine fuel temperature (EFT) sensor to give the PCM infor-
mation regarding the temperature and, therefore, the density
of the fuel.
ENGINE FUEL
TEMPERATURE (EFT)
SENSOR
Some engines, such as Toyota, are equipped with exhaust
gas recirculation (EGR) temperature sensors. EGR is a well-
established method for reduction of NO
X
emissions in inter-
nal combustion engines. The exhaust gas contains unburned
hydrocarbons, which are recirculated in the combustion pro-
cess. Recirculation is controlled by valves, which operate
as a function of exhaust gas speed, load, and temperature.
The gas reaches a temperature of about 850°F (450°C) for
which a special heavy-duty glass-encapsulated NTC sensor
is available.
The PCM monitors the temperature in the exhaust pas-
sage between the EGR valve and the intake manifold. If the
temperature increases when the EGR is commanded on, the
PCM can determine that the valve or related components are
functioning.
EXHAUST GAS
RECIRCULATION (EGR)
TEMPERATURE SENSOR
188 CHAPTER 13
ENGINE OIL
TEMPERATURE SENSOR
Engine oil temperature sensors are used on many General
Motors vehicles and are used as an input to the oil life moni-
toring system. The computer program inside the PCM calcu-
lates engine oil life based on run time, engine RPM, and oil
temperature.
1. The ECT sensor is a high-authority sensor at engine start-
up and is used for closed-loop control as well as idle
speed.
2. All temperature sensors decrease in resistance as the
temperature increases. This is called negative temperature
coefficient (NTC).
3. The ECT and IAT sensors can be tested visually as well as
by using a digital multimeter or a scan tool.
4. Some vehicle manufacturers use a stepped ECT circuit
inside the PCM to broaden the accuracy of the sensor.
5. Other temperature sensors include transmission fluid
temperature (TFT), engine fuel temperature (EFT), ex-
haust gas recirculation (EGR) temperature, and engine oil
temperature.
SUMMARY
1. How does a typical NTC temperature sensor work?
2. What is the difference between a stepped and a nonstepped
ECT circuit?
3. What temperature should be displayed on a scan tool if the
ECT sensor is unplugged with the key on, engine off?
4. What are the three ways that temperature sensors can be
tested?
5. If the transmission fluid temperature (TFT) sensor were to
fail open (as if it were unplugged), what would the PCM do
to the transmission shifting points?
REVIEW QUESTIONS
Diagnostic
Trouble Code
Description
Possible Causes
P0112 IAT sensor low voltage • IAT sensor internally shorted-to-ground
• IAT sensor wiring shorted-to-ground
• IAT sensor damaged by backfire (usually associated with
IAT sensors that are mounted in the intake manifold)
•
Possible defective PCM
P0113 IAT sensor high voltage •
IAT sensor internally (electrically) open
•
IAT sensor signal, circuit, or ground circuit open
• Possible defective PCM
P0117 ECT sensor low voltage • ECT sensor internally shorted-to-ground
•
The ECT sensor circuit wiring shorted-to-ground
•
Possible defective PCM
P0118 ECT sensor high voltage •
ECT sensor internally (electrically) open
•
ECT sensor signal, circuit, or ground circuit open
• Engine operating in an overheated condition
•
Possible defective PCM
The OBD-II diagnostic trouble codes that relate to temperature
sensors include both high- and low-voltage codes as well as
intermittent codes.
TEMPERATURE
SENSOR DIAGNOSTIC
TROUBLE CODES
TEMPERATURE SENSORS 189
1. The sensor that most determines fuel delivery when a fuel-
injected engine is first started is the ______________ .
a. O2S c. Engine MAP sensor
b. ECT sensor d. IAT sensor
2. What happens to the voltage measured at the ECT sensor
when the thermostat opens?
a. Increases slightly
b. Increases about 1 volt
c. Decreases slightly
d. Decreases about 1 volt
3. Two technicians are discussing a stepped ECT circuit.
Technician A says that the sensor used for a stepped
circuit is different than one used in a nonstepped circuit.
Technician B says that a stepped ECT circuit uses differ-
ent internal resistance inside the PCM. Which technician is
correct?
a. Technician A only
b. Technician B only
c. Both Technicians A and B
d. Neither Technician A nor B
4. When testing an ECT sensor on a vehicle, a digital multi-
meter can be used and the signal wire tested with the con-
nector attached the ignition on (engine off). What setting
should the technician use to test the sensor?
a. AC volts c. Ohms
b. DC volts d. Hz (hertz)
5. When testing the ECT sensor with the connector discon-
nected, the technician should select what position on the
DMM?
a. AC volts c. Ohms
b. DC volts
d. Hz (hertz)
6. When checking the ECT sensor with a scan tool, about
what temperature should be displayed if the connector is
removed from the sensor with the key on, engine off?
a. 284°F (140°C) c. 120°F (50°C)
b. 230°F (110°C) d. 40°F (40°C)
7. Two technicians are discussing the IAT sensor. Techni-
cianA says that the IAT sensor is more important to the
operation of the engine (higher authority) than the ECT
sensor. Technician B says that the PCM will add fuel if the
IAT indicates that the incoming air temperature is cold.
Which technician is correct?
a. Technician A only
b. Technician B only
c. Both Technicians A and B
d. Neither Technician A nor B
8. A typical IAT or ECT sensor reads about 3,000 ohms when
tested using a DMM. This resistance represents a tem-
perature of about ______________ .
a. 40°F (40°C)
b. 70°F (20°C)
c. 120°F (50°C)
d. 284°F (140°C)
9. If the transmission fluid temperature (TFT) sensor indicates
cold automatic transmission fluid temperature, what would
the PCM do to the shifts?
a. Normal shifts and normal operation of the torque con-
verter clutch
b. Disable torque converter clutch; normal shift points
c. Delayed shift points and torque converter clutch
disabled
d. Normal shifts, but overdrive will be disabled
10. A P0118 DTC is being discussed. Technician A says that
the ECT sensor could be shorted internally. Technician B
says that the signal wire could be open. Which technician
is correct?
a. Technician A only
b. Technician B only
c. Both Technicians A and B
d. Neither Technician A nor B
CHAPTER QUIZ
190 CHAPTER 14
chapter
THROTTLE POSITION
SENSORS
14
OBJECTIVES: After studying Chapter 14 , the reader should be able to: • Prepare for ASE Engine Performance (A8) certification
test content area “E” (Computerized Engine Controls Diagnosis and Repair). • Discuss how throttle position (TP) sensors work.
• List the methods that can be used to test TP sensors. • Describe the symptoms of a failed TP sensor. • List how the
operation of the TP sensor affects vehicle operation. • Discuss TP sensor rationality tests.
KEY TERMS: Potentiometer 190 • Skewed 193 • Throttle position (TP) sensor 190
THROTTLE
BODY
THROTTLE
POSITION
SENSOR
FIGURE 14–1 A typical TP sensor mounted on the throttle
plate of this port-injected engine.
TP
S
EN
SO
R
0.5-VOLT SIGNAL
GROUND RETURN
5-VOLT REFERENCE
4.5-VOLT SIGNAL
GROUND RETURN
A
C
B
A
C
B
WOT
IDLE
WOT
IDLE
5-VOLT REFERENCE
TP SENSOR
FIGURE 14–2 The signal voltage from a throttle position
increases as the throttle is opened because the wiper arm is
closer to the 5-volt reference. At idle, the resistance of the
sensor winding effectively reduces the signal voltage output to
the computer.
THROTTLE POSITION
SENSOR CONSTRUCTION
Most computer-equipped engines use a throttle position (TP)
sensor to signal to the computer the position of the throttle.
SEE FIGURE 14–1 . The TP sensor consists of a potentio-
meter, a type of variable resistor.
POTENTIOMETERS A potentiometer is a variable-
resistance sensor with three terminals. One end of the resistor
receives reference voltage, while the other end is grounded.
The third terminal is attached to a movable contact that slides
across the resistor to vary its resistance. Depending on whether
the contact is near the supply end or the ground end of the resis-
tor, return voltage is high or low.
SEE FIGURE 14–2 .
Throttle position (TP) sensors are among the most com-
mon potentiometer-type sensors. The computer uses their
input to determine the amount of throttle opening and the rate
of change.
A typical sensor has three wires:
A 5-volt reference feed wire from the computer
Signal return (a ground wire back to the computer)
A voltage signal wire back to the computer; as the throt-
tle is opened, the voltage to the computer changes
Normal throttle position voltage on most vehicles is about
0.5 volt at idle (closed throttle) and 4.5 volts at wide-open throt-
tle (WOT).
NOTE: The TP sensor voltage at idle is usually about
10% of the TP sensor voltage when the throttle is wide
open but can vary from as low as 0.3 volt to 1.2 volts,
depending on the make and model of vehicle.