Electrical basics and units in a vehicle
- Voltage (V, volt): the electrical push. Car systems are 12 V; trucks often 24 V.
- Current (I, ampere): the flow of charge. A starter motor can draw 100–300 A; a dashboard LED a few mA.
- Resistance (R, ohm Ω): what slows the flow.
- Power (P, watt): energy each second.
Ohm's law: V = I × R. Watt's law: P = V × I. Series: R adds up and current is the same everywhere. Parallel (most car loads): each load gets full 12 V, and currents add.
The complete circuit
Every circuit needs a source, protection, a control (switch or relay), a load and a return path. In vehicles the metal body is the chassis ground return, so each load needs only one feed wire. EVs and hybrids also have a separate high-voltage system (orange cables) that only trained technicians may touch.
Using a multimeter
- Voltage: probes across (in parallel with) the part, circuit on.
- Current: meter in series, on the right range and fused input.
- Resistance / continuity: circuit OFF; a beep means a complete path.
Electronic components in vehicles
- Diodes: one-way valves; the alternator uses a diode bridge (rectifier) to turn AC into DC.
- Transistors: electronic switches inside control modules that drive injectors and lamps.
- Relays: a small switch current energises a coil that closes a heavy-current contact (horn, fan, starter).
- Sensors: temperature (thermistor), position (potentiometer, Hall effect), oxygen in exhaust, wheel speed for ABS.
- ECU (engine control unit): a computer that reads sensors and controls outputs; modules talk over a network called CAN bus.
Main vehicle electrical systems
Starting (battery, ignition switch, starter relay/solenoid, starter motor), charging (alternator, regulator, battery), ignition, lighting and signalling, accessories (wipers, windows, horn, infotainment), and electronic control (engine, ABS, airbags).
Open, short and ground faults
| Fault | What happens | Symptom |
|---|---|---|
| Open circuit | A break in the path (cut wire, blown bulb, loose connector) | No current; load does not work; fuse OK |
| Short to ground | Feed wire touches metal before the load | Very high current; fuse blows at once |
| Short to power / wire-to-wire | Two circuits touch | Things work when they should not, or together |
| High resistance | Corrosion, loose or dirty joint | Dim lamps, slow motors, hot connectors |
| Bad ground | Return path corroded | Strange behaviour, dim lights, flickering |
Protection devices and batteries
Protection devices
- Blade fuse: a thin metal strip that melts when current passes its rating (colour coded, e.g. 10 A red, 15 A blue, 20 A yellow). Replace only with the same rating.
- Circuit breaker: a bimetal strip bends when hot and opens; it resets (for motors like power windows).
- Fusible link / maxi fuse: protects big main feeds near the battery.
- Choose a fuse about 125% of normal current, and below what the wire can safely carry.
Batteries
- Flooded lead-acid: six cells, each lead and lead-dioxide plates in sulfuric acid, about 2.1 V per cell → 12.6 V full. Cheap; used for starting.
- AGM / gel: acid held in glass mat or gel; spill-proof, good for start-stop cars.
- Lithium-ion: high energy per kg; used in EVs and hybrids, with a battery management system.
- Ratings: Ah (amp-hours, capacity), CCA (cold cranking amps). State of charge: 12.6 V ≈ full, 12.2 V ≈ half, below 12.0 V ≈ flat.
Safety: batteries make hydrogen gas. No sparks or flames; wear eye protection; disconnect negative first and reconnect it last.
Testing and repairing circuits safely
- Inspect: fuses, connectors, chafed insulation, corrosion, loose grounds.
- Read the wiring diagram to know the path, colours and connectors.
- Test: voltage at the load; voltage drop across each joint with the circuit working (more than about 0.2 V on a ground or 0.5 V on a feed means too much resistance); continuity with power off; use a test light or fused jumper wire with care.
- Repair: crimp or solder with the right wire size, seal with heat-shrink, replace damaged connectors, clean and tighten grounds.
- Verify the circuit works and the fuse holds.
Never fit a bigger fuse to stop it blowing; it can start a fire.
Key formulas and definitions
- V = I × R
- P = V × I
- Series: R = R1 + R2 + …
- Parallel loads: each gets full voltage; I_total = I1 + I2 + …
- Battery voltage = cells × cell voltage (6 × 2.1 = 12.6 V)
- Voltage drop at a joint = I × R_joint
Worked examples
1. A 55 W headlamp runs on 12 V. Find the current and the hot resistance.
I = P ÷ V = 55 ÷ 12 ≈ 4.6 A. R = V ÷ I = 12 ÷ 4.58 ≈ 2.6 Ω (or R = V² ÷ P = 144 ÷ 55).
2. Two 55 W headlamps share one fuse. Choose a fuse.
Total I = 110 ÷ 12 ≈ 9.2 A. Add about 25%: ≈ 11.5 A, so use a 15 A fuse (if the wire is rated for it).
3. A battery reads 12.2 V with the engine off. Is it charged?
About half charged. A full lead-acid battery reads about 12.6 V. Charge and retest.
4. A lamp circuit draws 4.6 A. A corroded ground joint has 0.5 Ω. Find the voltage drop at the joint.
Drop = I × R = 4.6 × 0.5 = 2.3 V (in practice a bit less, since the current also falls). That is far above 0.2 V, so the ground must be cleaned.
5. The horn fuse blows every time you press the horn. What fault is likely?
A short to ground between the fuse and the horn (for example a chafed wire touching the body). Find and insulate it before fitting a new fuse.
6. A car has a 60 Ah battery. A 5 W interior lamp is left on. Roughly how long until the battery is flat?
I = 5 ÷ 12 ≈ 0.42 A. Time ≈ 60 ÷ 0.42 ≈ 144 h. But the car may not start after about half of that, so in practice within 2–3 days.
Common mistakes
- Replacing a blown fuse with a bigger one. Find the short instead.
- Measuring resistance or continuity with the circuit powered.
- Forgetting the ground side. Half of all 'dim lamp' faults are bad grounds.
- Disconnecting the positive terminal first; a spanner touching the body makes a big spark. Remove negative first.