NABTEB Automobile Electrical Works cheat sheet

Ohm's law: V = I × R; transposed I = V/R and R = V/I. Applies to any purely resistive section of an automotive circuit.

Power formulas: P = V × I, P = I²R, P = V²/R. A 12 V lamp drawing 4 A dissipates 48 W.

Series circuit: I is common, R_total = R1 + R2 + …, voltage divides in proportion to resistance.

Parallel circuit: voltage is common, 1/R_total = 1/R1 + 1/R2 + …, current divides inversely with resistance.

Conductor resistance: R = ρL/A; doubling cable length doubles resistance, doubling cross-sectional area halves it.

Kirchhoff's laws: current into a node equals current out; sum of voltage drops around a closed loop equals the applied emf.

Conventional current flows from positive to negative externally; electron flow is opposite. Ammeters go in series, voltmeters in parallel.

A short circuit is an unintended low-resistance path; an open circuit breaks the path so no current flows.

Capacitance stores charge (Q = CV) and opposes voltage change; inductance opposes current change (V = L·di/dt).

A 12 V lead-acid battery has six cells at about 2.1 V each; open-circuit full charge is approximately 12.6–12.8 V.

Specific gravity of electrolyte: about 1.265 fully charged, 1.120 or lower indicates heavy discharge. Hydrometer readings are temperature-corrected.

Battery capacity is rated in ampere-hours (Ah) at the 20-hour rate; reserve capacity in minutes; cold cranking amps (CCA) at −18 °C.

Charging voltage from a healthy alternator is typically 13.8–14.5 V at the battery terminals with the engine running.

Alternator output is three-phase AC rectified by a six-diode bridge; the rotor field is fed through slip rings and brushes.

The voltage regulator controls field current to hold charging voltage constant as engine speed and load vary.

A warning lamp that stays lit with the engine running indicates low or no alternator output, a faulty regulator, or a broken field circuit.

Battery faults: sulphation, plate shedding, internal short, cell reversal, electrolyte stratification, terminal corrosion.

Jump-starting: connect positive to positive first, then negative to a good ground on the disabled vehicle, never to the flat battery's negative post.

Starter motor converts electrical energy to mechanical torque; key components are field coils or permanent magnets, armature, commutator, brushes, and drive.

Series-wound starters give high starting torque; permanent-magnet starters are common on small petrol engines.

Solenoid has two functions: pulls the drive into mesh with the flywheel ring gear and closes the heavy-duty contacts feeding the motor.

Typical starter current draw is 150–250 A for a small petrol engine and up to 500 A or more for a large diesel.

Voltage drop test: cranking voltage at the battery should stay above about 9.6 V at 20 °C; excessive drop indicates high-resistance cables or a failing battery.

Pre-engaged drive uses a one-way clutch (sprag) so the armature cannot be driven by the engine after start.

A clicking solenoid with no crank usually indicates low battery charge, bad earth strap, or worn solenoid contacts.

Starter will not disengage: check for stuck solenoid, welded contacts, or a jammed drive pinion.

Spark is produced when the coil's primary current is interrupted, collapsing the magnetic field and inducing high voltage in the secondary winding.

Coil turns ratio (secondary to primary) is typically 100:1; primary resistance is about 0.5–2 Ω, secondary several thousand ohms.

Dwell angle is the period, in distributor degrees, during which the primary circuit is closed; too little dwell weakens the spark, too much overheats the coil.

Ignition timing is set in degrees before top dead centre (BTDC); timing advance increases with engine speed and decreases with load.

Contact breaker points gap is typically 0.35–0.45 mm; a dwell meter or feeler gauge is used for adjustment.

Electronic ignition uses a reluctor, Hall-effect, or optical sensor to trigger the module, which switches the coil primary.

Spark plug heat range: a hot plug dissipates heat slowly and suits low-speed engines; a cold plug suits high-speed, high-load engines.

Firing order is the sequence in which cylinders receive spark; for a four-cylinder in-line engine it is commonly 1-3-4-2.

Knock (detonation) is premature uncontrolled combustion; it can be caused by over-advanced timing, low octane fuel, or a hot spot.

Headlamp circuits use relays because lamp current is high; the switch operates the relay coil, not the lamp load directly.

Sealed-beam headlamps have an integral lens, reflector, and filament; halogen lamps use a quartz envelope with halogen gas to extend filament life.

Headlamp aim: low beam should project below the horizontal to avoid dazzling oncoming drivers; aim is checked on a beam setter or against a wall.

Colour codes: white or amber for front position lamps, red for rear position and stop lamps, amber for direction indicators.

Direction indicators must flash at 60–120 flashes per minute; a fast flash usually indicates a failed bulb on that side.

Brake lamps are usually fed through the brake light switch on the pedal; a faulty switch gives no stop lamps but normal tail lamps.

Hazard warning lamps use a separate flasher unit or a combined unit that feeds all four indicators simultaneously.

Reverse lamps are switched by a gearbox-mounted switch; they should illuminate only when reverse is selected and the ignition is on.

LED lamps draw less current and last longer but may need ballast resistors to keep flasher units operating correctly.

Cable size is chosen by current-carrying capacity and permissible voltage drop; automotive cable is sized in mm² cross-section.

Voltage drop in a cable equals I × R; a 0.1 Ω cable carrying 10 A drops 1 V, which is significant on a 12 V system.

Fuses protect wiring, not equipment; the fuse rating must be below the cable's current capacity but above the load's normal current.

Blade fuse colour code: 5 A tan, 7.5 A brown, 10 A red, 15 A blue, 20 A yellow, 25 A clear, 30 A green.

A circuit breaker is resettable; a fusible link protects main feeds and is rated to blow before the main cable is damaged.

Relays allow a small current to control a large current; terminal numbers 30 (supply), 85 and 86 (coil), 87 (output), 87a (normally closed).

Harness routing must avoid sharp edges, heat sources, and moving parts; clips and grommets prevent chafing and shorts.

Earth return through the body is common; a poor earth gives dim lamps, slow cranking, or erratic operation of several circuits.

Colour coding and terminal numbering in a wiring diagram must match the harness; always check a diagram before adding accessories.

Windscreen wiper motors are usually permanent-magnet types with a park switch that returns the blades to the rest position.

Wiper circuit faults: blown fuse, faulty park switch, worn brushes, seized linkage, or a bad earth.

Horn circuits use a relay and are earthed through the steering column or a dedicated button; a single horn draws about 5–10 A.

Heated rear window elements are parallel resistances; a break in one element stops that strip only, checked with a voltmeter along the grid.

Electric fuel pumps are usually in-tank and run briefly at key-on to prime; a failed pump causes a no-start with no fuel pressure.

Central locking and alarm systems use actuators and a control module; a faulty actuator or broken wire in the door loom is a common failure.

Electric window motors use a reversing switch; a stalled motor draws high current and may be protected by a thermal cut-out.

Cigarette lighter and accessory sockets are fed through a fuse and often a relay; they should not be used above their rated current.

Six-step diagnostic routine: verify the complaint, gather information, inspect, test, repair, and confirm the repair.

Voltmeter tests measure voltage drop across a component or cable; ammeter tests measure current through a circuit in series.

Continuity test with an ohmmeter: isolate the circuit first, then check for near-zero resistance in a good conductor.

Parasitic drain test: connect an ammeter in series with the battery and measure current with all loads off; more than about 50 mA is excessive.

Load test a battery with a carbon pile or electronic tester; a battery that falls below 9.6 V under load at 20 °C is suspect.

Scan tool reads diagnostic trouble codes (DTCs); a code points to a circuit, not necessarily the component.

Voltage drop across a good earth connection should be less than 0.1 V; more indicates corrosion or a loose fastener.

Intermittent faults are found by wiggle testing harnesses and connectors while monitoring the meter or scan tool.

Disconnect the battery negative terminal first before working on the electrical system; reconnect it last.

A digital multimeter (DMM) has voltage, resistance, current, and continuity ranges; select the correct range and function before probing.

Insulated tools and a torque wrench are used on battery terminals and live circuits; never use a damaged test lead.

Battery electrolyte is dilute sulphuric acid; wear eye protection and neutralize spills with baking soda solution.

Batteries give off hydrogen during charging; keep sparks and flames away and charge in a ventilated area.

Use a fuse puller or plastic tool to remove blade fuses; never bridge a fuse with wire.

Lifting equipment and axle stands must be rated for the load; never work under a vehicle supported only by a jack.

Lockout/tagout and isolation of the battery are mandatory before working on airbag or high-voltage systems.

Personal protective equipment: safety glasses, gloves, and steel-toe boots; report damaged tools and replace them.

Wiring diagrams use standard symbols: battery, fuse, switch, relay, lamp, motor, earth, and connector.

Terminal designations follow DIN 72552: 30 = battery positive, 31 = earth, 15 = ignition, 50 = starter control, 61 = charging lamp.

A schematic shows electrical function; a wiring diagram shows physical layout and connector pin numbers.

Multimeter accuracy is stated as a percentage of reading plus digits; a 1% + 2 digit meter on a 12.6 V reading has an uncertainty of about 0.13 V.

Ohmmeter readings must be taken with the circuit isolated; parallel paths give falsely low readings.

Current measurement uses a series-connected ammeter or a clamp meter; clamp meters are safer for high-current circuits.

Torque values for electrical terminals are specified in N·m; over-tightening damages threads and under-tightening causes high resistance.

Metric-to-imperial conversion: 1 inch = 25.4 mm; 1 N·m ≈ 0.738 lb·ft.

Voltage drop is calculated from measured values: drop = source voltage − load voltage; compare with the manufacturer's limit.