NITA Auto Electrician Trade Test cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. In 12 V systems, a 60 W lamp draws 5 A.

Series circuit: same current through all elements, voltages add, total resistance R = R1 + R2 + ...

Parallel circuit: same voltage across branches, currents add, 1/R = 1/R1 + 1/R2 + ...; two resistors give R = (R1R2)/(R1+R2).

Kirchhoff's current law: sum of currents into a node equals sum leaving; voltage law: sum of drops around a closed loop equals supply EMF.

Conductor resistance R = ρL/A: doubling length doubles resistance, doubling cross-section halves it; copper resistivity ≈ 0.0175 Ω·mm²/m at 20 °C.

Voltage drop on a cable: Vd = (2 × ρ × L × I)/A for a feed-and-return run; keep total drop under about 0.5 V on 12 V lighting circuits.

Capacitor opposition Xc = 1/(2πfC); inductor opposition Xl = 2πfL; in DC steady state a capacitor is open circuit and an inductor is a short.

Automotive convention: chassis/body is the return path (negative earth), so a single-wire feed plus local body ground completes the circuit.

Lead-acid cell nominal EMF is 2.1 V; a 12 V battery has six cells in series, so open-circuit rest voltage is about 12.6–12.8 V at full charge.

State of charge by rested open-circuit voltage (25 °C): 12.7 V = 100 %, 12.4 V = 75 %, 12.2 V = 50 %, 12.0 V = 25 %, below 11.8 V = discharged.

Specific gravity of electrolyte tracks charge: about 1.265 full, 1.200 half, 1.120 discharged; correct reading by 0.004 per 10 °C from 25 °C.

Cold cranking rating is amps delivered at −18 °C for 30 s while holding at least 7.2 V on a 12 V battery; reserve capacity is minutes at 25 A to 10.5 V.

Charging voltage limits: gassing begins near 14.4 V at 25 °C; apply temperature compensation of roughly −0.03 V per °C above 25 °C.

Capacity falls with discharge rate (Peukert effect); a battery rated 100 Ah at the 20 h rate delivers far less at 5 h rate.

Load test rule: apply half the CCA for 15 s; a healthy battery holds above 9.6 V at 20 °C, corrected down about 0.02 V per °C below 20 °C.

Safety: charging releases hydrogen (flammable, 4–75 % in air); vent the area, no sparks, and neutralize acid spills with bicarbonate before rinsing.

Alternator output is three-phase AC rectified by six diodes; a failed diode causes ripple and reduced output, roughly one-third loss per open diode.

Regulator target is typically 13.8–14.5 V at 25 °C; below 13.2 V means undercharge, above 15 V risks overcharge and battery damage.

Ripple test: AC volts measured at the battery with the engine running should stay under about 0.5 V AC; higher points to diode or stator fault.

Lamp/field circuit: the charge warning lamp supplies initial excitation current; if the lamp is open the alternator may not self-excite.

Starter current draw: a healthy starter pulls roughly 150–250 A for a small petrol engine; excessive draw with slow cranking suggests a shorted armature or seized bearing.

Cranking voltage drop test: measure from battery positive post to starter terminal and battery negative post to engine block while cranking; each should be under 0.5 V.

Starter solenoid has pull-in and hold-in windings; a click with no crank often means hold-in open or poor earth, not a flat battery.

Belt drive: alternator pulley ratio sets output at idle; a slipping belt gives squeal and low charge at low rpm, resolved by tension or replacement.

Headlamp beam aim: dip beam should fall about 1 % below horizontal (roughly 10 cm drop per 10 m) to avoid dazzle.

Halogen bulbs must not be touched with bare fingers; skin oils cause hot-spot failure. HID needs high-voltage ballast, typically 85 V start, and must not be tested without rated probes.

Bulb power check: a 55 W halogen at 12 V draws about 4.6 A; two lamps on one 10 A fuse is acceptable, three is not.

Indicator flash rate is set by the flasher unit; rapid flashing usually means a lamp out on that side, changing load and hence timing.

LED retrofits draw far less current; a parallel load resistor or LED-compatible flasher is needed to keep the lamp-failure warning and flash rate correct.

Horn and wiper motors are inductive loads; suppression diodes or capacitors across them prevent switch arcing and radio interference.

Wiper circuit: park switch keeps the motor powered until the park position, so a motor that stops mid-screen indicates park contact fault.

Trailer socket wiring (7-pin): 1 left indicator, 2 fog, 3 earth, 4 right indicator, 5 right tail, 6 brake, 7 left tail.

Ignition coil energy E = ½LI²; primary current and dwell time set the spark energy available at the plug.

Dwell angle is the crank degrees the primary circuit is closed; too little dwell weakens spark, too much overheats the coil.

Spark plug gap and heat range must match the engine; a hotter plug in a high-load engine risks pre-ignition and electrode melting.

Coil-on-plug and wasted-spark systems fire in pairs; a misfire on two paired cylinders points to one coil or its driver, not two faults.

Knock sensor detects detonation via cylinder block vibration; the ECU retards timing, so a failed sensor usually causes reduced power and poorer economy.

Cranking and camshaft sensor signals synchronise injection and ignition; loss of crank signal normally means no start, loss of cam signal means long cranking.

Ignition timing is measured in degrees before top dead centre; base timing is checked with the timing connector or service mode enabled.

Misfire diagnosis: compare primary current waveform and secondary burn time; a short burn time with high voltage suggests an open plug lead.

Fuse rating must be above the circuit's normal load current but at or below the cable's current-carrying capacity; never uprate a fuse to stop it blowing.

Metric cable sizes: 0.5, 0.75, 1.0, 1.5, 2.5, 4, 6, 10, 16, 25, 35 mm²; choose by load current plus voltage-drop check, not by fit alone.

Automotive wire colour codes use a main colour with a tracer; document any repair and keep colour continuity across a joint.

Crimp, do not solder, in high-vibration harness runs; use the correct crimp tool and pull-test each terminal.

Heat-shrink and loom tape protect against abrasion; route harnesses away from exhaust and moving parts and secure at intervals under about 300 mm.

Relays keep high-current loads off thin switch wiring: terminal 30 supply, 87 output, 86 coil feed, 85 coil earth, 87a normally closed.

Circuit protection types: blade fuses for general circuits, maxi fuses for high current, fusible links at the battery feed, and circuit breakers for repeat loads.

Earth points must be clean bare metal with star washers; a corroded earth raises resistance and causes dim lamps and erratic ECU behaviour.

Multimeter use: volts in parallel with the load, amps in series, ohms only on a dead circuit; a meter on amps across a battery will blow its fuse.

Voltage-drop testing is more reliable than resistance testing because it is done under load; spec is usually under 0.1 V per connection, 0.5 V total.

Continuity test confirms a path but not capacity; a single strand of wire reads 0 Ω yet fails under load.

Parasitic drain test: measure current with everything off and modules asleep; typical limit is 30–50 mA. Above that, clamp-meter each fuse circuit.

Oscilloscope shows injector and ignition waveforms; look at amplitude, burn time and shape, not just presence of a signal.

Fault-finding order: verify the complaint, check obvious and known-good items, then work from supply to load, testing each point under load.

Intermittent faults: wiggle-test harnesses and connectors while monitoring the meter; record conditions (temperature, vibration, moisture) that trigger them.

Insulation resistance is measured with a megger at a test voltage above the circuit rating; never megger an ECU or connected electronics.

ECU inputs are analogue (temperature, pressure, position) or digital (crank, cam, switch); outputs drive relays, injectors, coils and motors.

NTC temperature sensors drop resistance as temperature rises; a common reference pair is about 2.5 kΩ at 20 °C and 300 Ω at 90 °C.

Potentiometer throttle and pedal sensors give a voltage ratiometric to a 5 V reference; two-track sensors are compared by the ECU for plausibility.

Hall-effect and inductive crank sensors differ: Hall needs a supply and gives a square wave; inductive generates its own AC signal and needs a gap spec.

Mass air flow hot-wire sensors output a voltage rising with airflow; contamination causes lean running and poor response.

Oxygen sensor output switches 0.1–0.9 V; a lazy sensor or a fixed reading indicates a failed sensor or an air/fuel fault upstream.

Actuator tests: injector resistance typically 12–16 Ω; a shorted or open coil sets a driver fault code.

ECU adaptation: after replacing a sensor or throttle body, clear codes and perform a relearn or the system may run on default values.

DC series motor (starter) has high starting torque and no load-speed limit; never run a starter motor off-load for long.

Permanent-magnet motors are used for wipers, windows and blowers; reversing is done by swapping polarity through a relay pair or a changeover switch.

Motor current rises as load torque rises; a stalled motor draws locked-rotor current several times rated and will overheat quickly.

Pulse-width modulation controls blower and fan speed; average voltage equals duty cycle × supply, so 50 % duty on 12 V gives about 6 V average.

Brushless motors use an electronic commutator and Hall sensors; loss of a sensor phase causes rough running or failure to start.

Back-EMF opposes supply and limits motor speed; a motor that runs fast and weak often has worn brushes or a shorted field.

Suppression: fit a capacitor or diode across motor terminals to absorb inductive spikes that damage switches and ECUs.

Motor control checks: supply at the motor, earth at the motor, then switch/relay operation; a motor that runs with direct 12 V proves the control circuit faulty.

Disconnect the battery negative before working on electrical circuits; wait for airbag systems to discharge per manufacturer time, commonly several minutes.

High-voltage (HV) systems on hybrids and EVs use orange cable; only trained personnel with rated gloves and meters may work on them.

Battery charging area needs ventilation; hydrogen ignites at 4 % in air, so no flames, grinding or smoking near a gassing battery.

Use insulated tools and remove metal jewellery when working near live 12 V feeds; a ring across a battery post causes severe burns.

Lifting: use rated stands and chocks, never rely on a jack alone; keep the work area clear of leads and spills.

Waste segregation: used oil, coolant, batteries, solvents and refrigerant each have separate disposal streams; refrigerant must be recovered, not vented.

Eye and hand protection for battery acid, brake fluid and grinding; acid on skin is flushed with water for at least 15 minutes.

Lock-out: tag and isolate equipment before repair, and check test lamps and meters are working before trusting a 'no fault' result.

Schematic symbols: battery long line positive, fuse rectangle, relay coil rectangle with contacts, earth the three-line chassis symbol, motor circle with M.

Wiring diagram shows physical routing and connector pin numbers; schematic shows electrical function. Use the right one for the question asked.

Terminal numbering (DIN): 30 permanent positive, 15 ignition positive, 31 earth, 50 starter control, 61 charge lamp, 85/86 relay coil, 30/87 contacts.

Scale reading: a drawing at 1:20 means 1 mm on paper equals 20 mm actual; multiply the measured length by the scale denominator.

Cable size calculation: A = (2 × ρ × L × I)/Vd; for 12 V, 4 m run, 10 A, 0.5 V drop, A ≈ (2 × 0.0175 × 4 × 10)/0.5 = 2.8 mm², so use 4 mm².

Power and energy: P = VI, energy in watt-hours = watts × hours; a 5 W lamp on for 4 h uses 20 Wh.

Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold is 5 % tolerance.

Series/parallel reduction: simplify from the far end inward, then apply Ohm's law to find branch currents and voltage drops.