chap55 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 55, the reader should
be able to:

•

•
•
•

Prepare for ASE Engine Performance (A8)
certification test content area “E”
(Computerized Engine Controls Diagnosis and
Repair).
Explain the purpose and function of the ECT
and IAT temperature sensors.
Describe how to test temperature sensors.
Discuss how MAP sensors work.
2

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 55, the reader should
be able to:

• List the methods that can be used to test MAP
•
•
•
•

sensors.
List how the operation of the MAP sensor
affects vehicle operation.
Discuss MAP sensor rationality tests.
Discuss how MAF sensors work.
List the methods that can be used to test MAF
sensors.
3

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 55, the reader should
be able to:

• List how the operation of the MAF sensor
affects vehicle operation.
Discuss MAF sensor rationality tests.
Discuss how oxygen sensors (O2S) work.

•
•
• Describe the symptoms of a failed oxygen
sensor.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
barometric manifold absolute pressure (BMAP) •
barometric pressure (BARO) sensor • bias voltage
cylinder head temperature (CHT) • closed-loop operation
• cross counts
engine coolant temperature (ECT)
false air • false lean indication • false rich indication • fuel
trim
hot film sensor • hot wire sensor • intake air temperature
(IAT)
5

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
Karman vortex sensor
lean air–fuel ratio sensor (LAF) • linear air–fuel ratio
sensor
manifold absolute pressure (MAP) • mass airflow sensor
(MAF)
negative temperature coefficient (NTC)
open-loop operation • oxygen sensor (O2S)
piezoresistivity • pressure differential

6

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
skewed • speed density
tap test • throttle-body temperature (TBT) • throttle
position (TP) • transmission fluid temperature (TFT) •
transmission oil temperature (TOT)
vacuum • vane airflow sensor (VAF)
wide-band O2S

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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ENGINE COOLANT TEMPERATURE SENSORS
Computer­equipped vehicles use an engine coolant temperature 
(ECT) sensor. The computer controls spark timing and fuel 
mixture, and needs to know the engine temperature. 

An engine coolant 
temperature sensor 
(ECT) screwed into 
the coolant passage 
will provide this 
information. 
Figure 55–1 A typical engine

coolant temperature (ECT)
sensor. ECT sensors are
located near the thermostat
housing on most engines.
Continued
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Ignition timing can be tailored to coolant  temperature. A hot engine 
cannot have the spark timing as far advanced as can a cold engine. 
The ECT sensor is also used as an input for:
Idle air control (IAC) position

Oxygen sensor closed­loop 
status

Canister purge on/off times
Idle speed

Coolant temperature sensors are constructed of a semi­conductor 
material that decreases in resistance as sensor temperature increases.
If the sensor has a poor connection (high resistance) the computer 
will supply a richer­than­normal mixture based on resistance of 
the coolant sensor. Poor fuel economy and a possible­rich code 
can be caused by a defective sensor or high resistance in wiring.   
If too low a resistance, a leaner­than­normal fuel mixture would be 
supplied to the engine
Continued
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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 55–2
A typical ECT sensor temperature versus voltage curve.

Coolant sensors have 
very high resistance 
when the coolant is cold 
and low resistance when 
the coolant is hot. 
This is referred to as a 
negative temperature 
coefficient (NTC),
which is opposite to the 
situation with most other 
electrical components. 
10

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STEPPED ECT CIRCUITS
Some manufacturers use a step­up resistor to effectively broaden 
the range of the ECT sensor. Chrysler and General Motors vehicles 
use the same sensor as a non­stepped ECT circuit, but instead 
apply the sensor voltage through two different resistors.

When the temperature is cold, usually below 120°F (50°C), 
the ECT sensor voltage is applied through a high­value 
resistor inside the PCM.
When the temperature is warm, usually above 120°F (50°C), 
the ECT sensor voltage is applied through a much lower 
resistance value inside the PCM. 
See Figure 55­3.
11

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By James D. Halderman

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Figure 55–3 A typical two-step ECT circuit showing that when the coolant temperature is low, the
PCM applies a 5-volt reference voltage to the ECT sensor through a higher resistance compared
to when the temperature is higher.

The purpose of this extra circuit is to give the PCM a more accurate 
reading of the engine coolant temperature compared to the same 
sensor with only one circuit. See Figure 55­4.
Continued
12
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 55–4 The transistors between steps usually occur at a temperature that would not interfere
with cold engine starts or the cooling fan operation. In this example, the transition occurs when
the sensor voltage is about I volt and rises to about 3.6 volts.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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TESTING THE ENGINE COOLANT
TEMPERATURE SENSOR
Testing the Engine Coolant Temperature by Visual Inspection The 
correct functioning of the engine coolant temperature (ECT) sensor 
depends on the following items:

Properly filled cooling system  Ensure radiator reservoir bottle 
is full and the radiator filled to top. The ECT sensor must be 
submerged in coolant to indicate proper coolant temperature.
Proper pressure maintained by the radiator cap  If the 
radiator cap is defective and can’t allow the system to 
pressurize, air pockets could develop. These pockets could cause 
the engine to operate hotter­than­normal, and prevent proper 
temperature measurement, especially if pockets occur around the 
sensor.
Continued
14

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Proper antifreeze–water mixture  Most manufacturers 
recommend a 50/50 mixture of antifreeze and water for best 
freezing protection and heat transfer ability.
Proper operation of the cooling fan  If the cooling fan does 
not operate correctly, the engine may overheat.
WARNING:  Be sure that the radiator is cool before removing the 
radiator cap to avoid being scalded by hot coolant.

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By James D. Halderman

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Testing the ECT Using a 
Multimeter  Both resistance (in 
ohms) and voltage drop across 
the sensor can be measured and 
compared
with specifications. 

Figure 55–5 Measuring the
resistance of the ECT
sensor. The resistance
measurement can then be
compared with
specifications.

Continued

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Figure 55–6 When the voltage drop reaches approximately 1.20 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
binary control at cold temperatures up to approximately 122°F, and a second full binary control
at temperatures greater than 122°F.

Some vehicles use a PCM to attach another resistor in the circuit to 
provide a more accurate measure of the engine temperature. 
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

By James D. Halderman

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ECT TEMPERATURE CHARTS

See these charts on 
Page 627 & 628 of 
your textbook. 
Continued
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See these charts on 
Page 627 & 628 of 
your textbook. 
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See these charts on 
Page 627 & 628 of 
your textbook. 
Continued
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See these charts on 
Page 627 & 628 of 
your textbook. 
Continued
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See these charts on 
Page 627 & 628 of 
your textbook. 
Continued
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If resistance values match the approximate coolant temperature and 
there is still a coolant sensor trouble code, the problem is generally 
in the wiring between the sensor and the computer. 

Normal operating temperature varies. Some vehicle thermostats 
have an opening temperature of 180°F (82°C), where other vehicles 
use a thermostat that is 195°F (90°C) or higher. 
Most manufacturers recommend checking the ECT sensor after the 
cooling fan has cycled twice, indicating a fully warmed engine.
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 
sensitive. Therefore, if measuring voltage at the ECT sensor, check with 
the service manual for the proper voltage at each temperature.

23

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Figure 55–7 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.

24

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Testing the ECT Sensor Using a Scan Tool  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.
1. Record the scan tool temperature of the coolant (ECT).
2. Measure the actual temperature of the coolant using an infrared 
pyrometer or contact­type temperature probe.
HINT: Often the coolant temperature gauge in the dash of the vehicle can 

be used to compare with the scan tool temperature. Although not 
necessarily accurate, it may help to diagnose a faulty sensor, especially if 
the temperature shown on the scan tool varies greatly from the temperature 
indicated on the dash gauge.

25

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