A sensor converts a physical quantity — temperature, pressure, position, speed or gas concentration — into an electrical signal. The ECU supplies a reference, reads the signal and interprets it against a model. The sensor family determines both the signal shape and the correct test method.
Resistive sensors and dividers
Many temperature and position sensors change resistance. They are often connected in a voltage divider with a fixed resistor so the ECU can measure a voltage that changes with the physical quantity.
Divider output
Problem. A 5 V reference feeds a 1 kΩ fixed resistor in series with a 2 kΩ sensor to ground. What voltage appears across the sensor?
Solution. V_out = 5 × 2000 ÷ (1000 + 2000) = 5 × 0.6667 = 3.333 V.
Digital and speed sensors
Hall-effect and inductive sensors produce a changing signal as a toothed wheel passes. A Hall sensor typically needs a supply and produces a switching output; an inductive sensor generates its own signal and its amplitude depends on speed. That difference explains why a Hall sensor can be tested statically while an inductive sensor usually needs rotation.
- The ECU supplies a reference voltage and reads a signal.
- Resistive sensors are commonly read with a voltage divider.
- Hall sensors need a supply and produce a digital output.
- Inductive sensors generate a signal whose amplitude depends on speed.
- Always test a sensor with the method its technology requires.
- Sensors convert physical quantities into electrical signals.
- A divider turns a resistance change into a voltage change.
- Hall and inductive sensors need different tests.
- A static voltage check is not a complete sensor test.
Worked example
A sensor of 3 kΩ in the lower leg with a 1 kΩ fixed resistor and a 5 V reference gives 5 × 3000 ÷ 4000 = 3.75 V at the ECU input.