Electronic Control Units, Sensors and Actuators
Electronic Control Units, Sensors and Actuators covers ECU inputs and outputs, reference voltages, sensor signal types and actuator control. The test checks whether you can trace a fault from a sensor signal through the ECU to an actuator without guessing.
Core rules
- The ECU provides a regulated reference, often 5 V, and reads analog voltage, frequency, duty cycle or digital messages.
- Sensors include potentiometers for throttle position, thermistors for coolant temperature, Hall or inductive sensors for crank and cam position, hot-wire MAF, piezoelectric knock and wideband lambda.
- Actuators include injectors, ignition coils, solenoids, relays and stepper motors; many are PWM controlled.
- The ECU monitors plausibility, range and circuit continuity; a shorted 5 V reference can disable several sensors.
- Actuator feedback allows closed-loop control; lack of feedback can set a code.
Worked example
A three-wire throttle position sensor uses a 5 V reference, ground and signal. At closed throttle the signal is 0.5 V; at wide-open throttle it is 4.5 V. A reading of 2.5 V is (2.5 − 0.5)/(4.5 − 0.5) = 50% of the usable range. If the signal is 0.2 V with the reference at 5 V, the sensor may be shorted low or misadjusted. Check reference and ground before condemning the sensor.
Traps
Back-probing a 5 V reference with a low-impedance test lamp. Assuming a sensor is bad because its resistance is outside a table measured at another temperature. Ignoring actuator supply and ground. Forgetting that PWM averages voltage.