1. Transducers Topics covered in this presentation:
Active and Passive Sensors
Common Sensor Types and Their Operation
ClassAct SRS enabled.

2. Transducers
A sensor (transducer) changes a condition into an electrical signal.
Sensor may be active or passive.
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3. Active Sensors Two wire device. Generates own voltage signal.
Voltage signal connects to ECU input.
2 wire output
Active
sensor
Trigger wheel
ECU processes input signal into digital information.
ECU operates outputs, depending upon digital information received.
Example - inductive/magnetic pickup.
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4. Passive Sensors Three wire device. Supplied with voltage from ECU.
Sensor supply
voltage
+5V 0V
Supplied with voltage from ECU.
Internal
resistance 5V 0V
Output voltage
Internal resistance changes with a change in conditions.
Sensor
Corresponding output voltage change is measured by ECU.
Example - throttle position sensor.
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5. Question 1
The main distinction between an active and a passive sensor is that an active sensor generates its own voltage, a passive sensor must be supplied with a voltage. Is this true or false?
Answer True or False.
Correct Answer = T

6. Question 2
The electronic control unit (ECU) has to convert all analogue signals to digital before processing them. Is this true or false?
Answer True or False.
Correct Answer = T

7. Passive Sensors Two wire device.
Current limiting
resistor
Supplied with voltage from ECU via resistor. 5V +V 0V W V
Internal resistance changes with a change in conditions.
Sensor
Output voltage
Internal
resistance
Corresponding output voltage change is measured by ECU.
Example - coolant temperature sensor.
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8. Current limiting resistor
Passive Sensors
Switch must be either open or closed.
Supplied with voltage from ECU via resistor.
Current limiting resistor
+12V 0V
12V
Switch
Output voltage
Switch open, ECU measures 12V.
Switch closed, ECU measures 0V.
Example - brake pedal switch.
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9. Question 3
The diagram shows a brake pedal switch and its connections to the ECU. What voltage will the ECU measure when the switch is closed?
A) 12V
Switch
+12V
B) 6V
C) 5V 0V
Correct Answer = D
D) 0V

10. Sensor Types Most common types: Intake Air Temperature (IAT).
Manifold Absolute Pressure (MAP).
Engine Coolant Temperature (ECT).
Oxygen (O2).
Mass Air Flow (MAF).
Throttle Position (TPS).
Crankshaft Position (CKP).
Camshaft Position.
Vehicle Speed (VSS).
EGR Position.
Brake Switch.
Oil Level Switch.
Knock.
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11. Oxygen Sensor (O2) Located in exhaust downpipe.
Heating
element
Sensor
Protective cap with gas intake slots
Wires to ECU
Located in exhaust downpipe.
Measures oxygen content in exhaust gas and atmosphere.
Produces output voltage corresponding to difference in oxygen levels.
0 volts = high oxygen content in exhaust gas. V
Air/fuel ratio
0.7V
0.3V
14.7:1
Rich
Lean
1.4 volts = low oxygen content.
volts = ideal oxygen content.
Heating element speeds up sensor operation from cold.
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12. Question 4
What is an IDEAL voltage for an oxygen sensor, located in the exhaust downpipe, to produce?
A) 0V
B) 0.5V
C) 1.4V
Correct Answer = B
D) 5.0V

13. Engine Coolant / Intake Air Temp Sensors
ECT located in coolant.
IAT located in incoming air.
Reference
voltage
Engine coolant temperature sensor +V V 0V W
Resistance and voltage changes with temperature
Sensor resistance changes with temperature change.
Resistance change causes voltage change at ECU input.
ECU measures voltage to determine temperature.
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14. Question 5
The ECU measures the change in current, in the resistive sensor, to monitor the intake air temperature. Is this true or false?
Answer True or False.
Correct Answer = F

15. Throttle Position Sensor (TPS)
Variable resistor, fixed to the throttle valve.
Resistance changes with throttle position.
+5V 0V
TPS
Reference
voltage
Output voltage 5V 0V
ECU supplies sensor with voltage.
Sensor output voltage changes with throttle position.
ECU measures voltage to determine throttle position.
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16. Mass Airflow Sensor (MAF)
Mounted before the throttle valve on EFI engines.
Measures mass of air entering engine.
Contains ‘hot wire’ which is heated by current flow.
Electronics module
Output to ECU
Airflow cools hot wire, changing current flow.
Air in
Electronics module outputs signal to ECU corresponding to current change.
Change in current is proportional to airflow into engine.
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17. Inductive/ magnetic pickup
Crankshaft/Camshaft/Vehicle Speed Sensor
Mounted next to trigger wheel with small air gap.
Sensor (inductive/magnetic pickup) contains magnet and coil.
Movement of tooth towards sensor disrupts magnetic field and produces voltage in coil.
Inductive/ magnetic pickup
Output
voltage
Trigger wheel
Movement of tooth away from sensor disrupts magnetic field and produces opposite polarity voltage.
Output frequency is determined by trigger wheel speed.
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18. Question 6
A vehicle speed sensor produces a signal whose frequency is proportional to the speed of the vehicle. Is this true or false?
Answer True or False.
Correct Answer = T

19. Low Oil/Brake Fluid Level Sensor
Oil level switch located in sump.
12V
Lamp on
Lamp off
ABS, ECU monitoring point
Fluid level okay
Fluid level low
Switch open
Switch closed
Brake fluid switch located in reservoir.
Switch open when fluid level normal, lamp is off.
Switch closed when fluid level low, lamp is on.
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20. Knock Sensor Mounted on the engine.+V 0V
Detects vibrations that indicate abnormal combustion (knock).
Output
voltage
Produces output voltage proportional to knock strength.
ECU retards ignition when abnormal combustion is detected.
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21. Question 7 What does a knock sensor detect? A) Gas concentration
B) Abnormal combustion
C) Wheel movement
Correct Answer = B
D) Gas level

22. Summary You should be aware of: Active and Passive Sensors
Common Sensor Types and Their Operation
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