Electronic Control Units
What the exam tests
ECU architecture, inputs and outputs, power supplies, memory and replacement procedures.
Core
- An ECU contains a microprocessor, ROM/flash for programs, RAM for temporary data, EEPROM for adaptive and immobiliser data, an A/D converter, and output drivers.
- Inputs are analogue voltages, digital switches, frequency signals or PWM. Outputs are low-side drivers (switch to ground), high-side drivers (switch to positive) or PWM.
- The ECU needs permanent battery power for memory and ignition-switched power for operation, plus clean grounds. A missing ground can make the ECU appear dead.
- Sensors are fed a regulated 5 V reference and a separate sensor ground; voltage drops in either distort every reading.
- Adaptive learning compensates for wear and deposits. Disconnecting the battery may clear adaptations and cause rough running until relearn.
- Replacement ECUs usually need coding, immobiliser matching and configuration.
Worked example
An engine will not start and the scan tool cannot communicate with the ECU. Do not condemn the ECU. Check battery voltage, ECU fuses, main relay, and both power feeds and grounds at the ECU connector. If 12 V and ground are present but the 5 V reference at a sensor is 0 V, the ECU or its internal supply is suspect. If the 5 V reference is correct, diagnose communication wiring and the network before replacement.
Traps
Replacing an ECU without checking powers and grounds; back-probing with a high-current test light; fitting a used ECU without coding; assuming all ECUs are interchangeable; clearing adaptations and calling it a fault.