Automotive Electrical and Electronic Systems
This domain tests the storage, generation, starting, ignition and control electronics of a modern vehicle, plus the fault-finding arithmetic an inspector uses on the road.
Battery, charging and starting
- A 12 V lead-acid battery has six cells of about 2.1 V each, giving roughly 12.6 V at rest; fully charged specific gravity is near 1.265.
- The alternator generates alternating current, rectifies it with a diode bridge and regulates charging to about 13.8–14.4 V. A starter motor is series-wound, drawing 150–300 A briefly to produce high starting torque.
Ignition and electronics
- Ignition evolved from contact-breaker coil systems to electronic and distributorless (waste-spark) types, and now coil-on-plug. Key sensors include crankshaft position, camshaft position, manifold absolute pressure, mass air flow, coolant temperature and the zirconia oxygen (lambda) sensor.
- The ECU controls fuel and spark; CAN bus carries data between modules; OBD-II stores powertrain (P) diagnostic trouble codes. ABS uses inductive or Hall-effect wheel-speed sensors.
Worked example
A starter draws 200 A through a cable and earth path of 0.02 Ω total. Voltage drop = I × R = 200 × 0.02 = 4 V. At a 12.6 V rest voltage only 8.6 V reaches the motor, so it cranks slowly - a voltage-drop test locates the bad earth or cable better than an open-circuit check.
Traps
- Assuming a battery reading 12.0 V is fully charged (it is roughly half discharged).
- Treating an alternator like a dynamo that needs a cut-out, or confusing an oxygen sensor's voltage output with a resistance signal.
- Overlooking earth-path resistance when diagnosing slow cranking.