Electrical and Electronic Systems
Electrical and Electronic Systems
This domain tests battery and charging behaviour, starting and lighting circuits, sensor and ECU operation, CAN bus basics, and OBD-II diagnosis. Expect Ohm law calculations and voltage-drop reasoning.
Core rules
- Ohm law: V = I x R; power P = V x I. Series resistance adds; parallel resistance reduces total resistance.
- A 12 V lead-acid battery has six cells of about 2.1 V each. Rested open-circuit voltage is about 12.6 V; charging is about 13.5 to 14.5 V.
- Voltage-drop testing under load finds high-resistance cables and grounds. A crank drop above about 0.5 V on a main cable needs attention.
- The ECU uses sensor inputs such as MAP, MAF, oxygen, crank, cam, and throttle position, then controls injectors, ignition, and actuators.
- CAN bus is a twisted differential pair. With ignition off, about 60 ohms across the DLC CAN pins indicates two 120 ohm terminators in parallel. OBD-II DLC pin 16 is battery positive.
- A DTC identifies a circuit or system, not automatically the failed part.
Worked example
A 55 W lamp operates on 12 V. Current I = 55 / 12 = 4.58 A. Resistance R = 12 / 4.58 = 2.62 ohms. Reasoning: use rated power and nominal voltage; the hot filament has higher resistance, so this is an approximation, not a precise hot value.
Traps
- Using an ohmmeter on a live circuit.
- Probing ECU pins with a high-current test lamp.
- Treating 12.6 V as proof of good battery capacity.
- Measuring CAN resistance with the ignition on.
- Replacing a component because the code names its circuit.