ITI Mechanic Auto Electrical & Electronics (AITT Theory) cheat sheet

Before any high-voltage work: isolate, lock out/tag out, wait the manufacturer-specified discharge time (typically 5-10 minutes), then verify absence of voltage at the terminals with a CAT III meter rated for the pack voltage.

PPE for HV work: Class 0 insulating gloves (rated 1,000 V AC / 1,500 V DC) tested every 6 months, leather over-gloves, face shield, insulating mat; inspect gloves by air test before each use.

Battery electrolyte (H2SO4) causes burns: flush skin/eyes with water 15 minutes minimum; add acid to water, never water to acid; charge lead-acid only in ventilated areas because charging releases hydrogen (flammable at 4% concentration).

Fire classes: A solid, B flammable liquid, C energized electrical, D metal, K cooking oil. CO2 and dry powder are suitable for electrical fires; water is a shock hazard on live circuits.

Refrigerant and airbag (SRS) hazards: never probe SRS connectors with a powered ohmmeter; disconnect battery and wait the specified time (often 3-10 minutes) before touching airbag circuits.

Lifting rule: keep the load close to the body, back straight, use legs; two-person or hoist lift above roughly 25 kg; use wheel chocks and jack stands, never rely on a hydraulic jack alone.

Electrical shock severity depends on current through the body, not voltage alone: about 1 mA perception, 10-15 mA muscle tetany, 30 mA respiratory difficulty, 50-100 mA ventricular fibrillation.

Eye protection and no jewellery: rings and watches create short-circuit paths and burn hazards across battery and starter terminals; remove metal jewellery before working near exposed conductors.

Vernier caliper least count = 1 main-scale division / number of vernier divisions; a 1 mm main scale with 50 vernier divisions gives 0.02 mm. Micrometer least count = pitch / thimble divisions (0.5 mm / 50 = 0.01 mm).

Torque wrench: tighten to the specified figure in steps (e.g. 50% then 100%), use the correct drive size and socket, and return adjustable types to zero after use to preserve calibration.

Digital multimeter CAT rating: CAT II for socket outlets, CAT III for distribution/fixed installation, CAT IV for service entrance; never use a CAT II meter on a traction battery.

Feeler gauge measures valve clearances and gap settings; plastigauge measures bearing oil clearance; dial indicator with magnetic base measures runout and end play to 0.01 mm.

Insulation tester (megger) applies 250/500/1000 V DC: a healthy 12 V circuit should read above 1 MΩ; below 0.5 MΩ indicates insulation breakdown or moisture ingress.

Clamp meter measures current without breaking the circuit by sensing the magnetic field; it works on DC only with a Hall-effect type, while AC-only clamps read zero on DC.

Micrometer zero check: close the anvils and confirm the zero line; vernier caliper zero error is positive if the vernier zero sits right of the main-scale zero, and is subtracted from readings.

Hand tool rules: pull a wrench rather than push, never extend a wrench with a pipe, use a six-point socket on rusted fasteners, and use a torque-limiting screwdriver on plastic body clips.

Ohm's law: V = I × R. Power: P = V × I = I²R = V²/R. Series: R_total = R1 + R2 + ...; parallel: 1/R_total = 1/R1 + 1/R2 + ...

Kirchhoff's laws: current into a node equals current out; the algebraic sum of voltages around any closed loop is zero.

A 12 V circuit drawing 4 A has 3 Ω resistance and dissipates 48 W; a 0.05 Ω cable carrying 200 A cranking current drops 10 V and dissipates 2 kW as heat.

Magnetic field around a conductor: right-hand grip rule, field circles the wire; field strength increases with current and decreases with distance.

Faraday's law: induced EMF equals the rate of change of flux linkage; Lenz's law gives the induced current a direction that opposes the change producing it.

A relay coil of 60 Ω at 12 V draws 0.2 A; a typical automotive relay closes with about 50-100 turns and opens when coil current falls below the hold value.

Capacitance in farads: Q = C × V; capacitors oppose change in voltage, inductors oppose change in current; time constant τ = RC (seconds) or L/R.

Conductor resistance rises with temperature: R_hot = R_cold × (1 + αΔT); copper α ≈ 0.00393 per °C, so a starter cable's resistance is roughly 30% higher at 100 °C than at 20 °C.

Lead-acid cell nominal voltage is 2.1 V; a 12 V battery has six cells in series. Open-circuit voltage approximates state of charge: 12.65 V = 100%, 12.45 V = 75%, 12.24 V = 50%, 12.06 V = 25%, below 11.9 V = discharged.

Capacity is quoted in Ah at the 20-hour rate: a 60 Ah battery delivers 3 A for 20 hours. Cranking rating (CCA) is the current it delivers at -18 °C for 30 s down to 7.2 V.

Specific gravity of electrolyte correlates with state of charge: about 1.265 fully charged, 1.120 discharged (corrected 0.004 per 10 °C from 25 °C).

Charging voltage limits: 14.2-14.8 V at 25 °C for a conventional lead-acid battery; above 15 V causes gassing and water loss, below 13.8 V leaves the battery undercharged.

AGM and gel batteries require lower charge voltage (typically 14.4-14.7 V AGM, 14.1-14.4 V gel) and must never be charged above the manufacturer's gassing limit; a standard charger can damage them.

Parasitic drain test: with everything off, a healthy vehicle draws under about 50 mA after modules sleep; 50 mA over 24 h consumes roughly 1.2 Ah.

Battery load test: apply half the CCA rating for 15 s; voltage must stay above 9.6 V at 20 °C (above 9.6 V corrected for temperature, lower limit 9.6 V at 20 °C, 9.1 V at 0 °C).

Battery internal resistance rises as it ages; a battery that reads 12.6 V open-circuit but drops below 9.6 V under cranking load has high internal resistance and should be replaced.

Alternator output: three-phase stator with six diodes; ripple voltage across the battery should be under about 0.5 V AC; a failed diode causes low output and high ripple.

Alternator charge voltage: 13.8-14.8 V at idle rising slightly with rpm; the warning lamp circuit provides initial excitation current to the rotor field.

Belt-driven alternator output falls with speed; check belt tension and pulley ratio before condemning the alternator; a slipping belt causes low charge at idle and squealing.

Starter circuit voltage drop: total drop across cables and solenoid should be under 0.5 V at cranking; each cable under 0.2 V; a drop above 0.5 V means high resistance.

Starter motor draws 150-250 A typical cranking current; a seized or shorted starter can draw over 500 A and cause cable heating and battery voltage collapse.

Solenoid has two windings: pull-in (high current, both ends earthed through the motor) and hold-in (lower current); a clicking starter with no crank suggests low battery or a faulty hold-in circuit.

No-crank diagnosis order: battery state of charge and terminals, earth strap, starter relay/fuse, ignition switch signal to solenoid, then starter itself.

Charging system test: measure battery voltage at idle with loads off (should be 13.8-14.8 V), then with headlights and blower on; output should stay above about 13.5 V.

Cable current capacity depends on cross-section and length: a 1.5 mm² cable is typically rated near 15-20 A, 2.5 mm² near 25-30 A, 4 mm² near 35-40 A; always check the manufacturer's chart.

Voltage drop in a cable: V = I × R, and R = ρL/A with ρ ≈ 0.0175 Ω·mm²/m for copper. A 5 m run of 2.5 mm² carrying 20 A drops about 0.7 V.

Fuse rating rule: select a fuse above the circuit's normal current but below the cable's current capacity; a blade fuse's rating is the current it carries continuously, and it blows at roughly twice that current.

Fuse types: blade (ATO/ATC, mini, maxi), glass cartridge, ceramic, and high-current strip fuses for battery feeds; never replace a fuse with wire or a higher rating.

Connector faults: fretting corrosion from vibration, pushed-back terminals, water ingress at seals, and terminal tension loss; a voltage-drop test across a connector should read under 0.1 V under load.

Earth (ground) points: a poor chassis earth causes dim lights, slow wipers, and false sensor codes; test with a voltage drop from the component's earth pin to the battery negative under load.

Relay pin numbering: 30 = battery feed, 87 = load output, 85 and 86 = coil, 87a = normally closed contact on changeover relays.

Circuit protection also includes fusible links at the battery positive, maxi fuses in the engine bay box, and thermal circuit breakers for windows and seats.

Ignition coil primary resistance is typically 0.5-2 Ω and secondary 5-15 kΩ; a coil that fails when hot often tests normal cold, so test both states.

Spark plug gap is typically 0.8-1.1 mm; iridium plugs use a smaller gap and must not be re-gapped by levering the centre electrode.

Ignition timing is expressed in degrees before top dead centre; too much advance causes knock, too little causes overheating and power loss.

Crank position sensor: inductive type generates an AC signal (0.5-5 V peak) and its resistance is typically 500-1500 Ω; Hall-effect type needs a supply and gives a square wave.

MAP sensor output is typically 0.5-4.5 V, low voltage at high vacuum (idle) and high voltage near atmospheric pressure; a stuck sensor causes rich or lean mixture.

Oxygen sensor: zirconia type generates 0.1-0.9 V, switching about 0.45 V at stoichiometric; a heater circuit draws 1-3 A and a failed heater delays closed-loop operation.

Throttle position sensor and accelerator pedal sensor are usually dual-track potentiometers or Hall types; the ECU cross-checks both tracks and sets a code if they disagree.

Actuators: injectors (typically 12-16 Ω), idle air control valve, EVAP purge solenoid, VVT solenoid, and EGR valve; check supply voltage and ECU earth-switching before replacing.

Halogen headlamp bulbs: H4 has two filaments (main and dip), H7 has one; never touch the glass envelope with bare fingers because contamination causes hot spots and failure.

Headlamp aim: low beam cutoff should be set per manufacturer specification, typically with the vehicle on level ground and a 1% downward slope; misalignment is a common MoT/inspection failure.

Indicator flash rate is typically 60-120 flashes per minute; a fast flash indicates a failed bulb because the thermal or electronic flasher senses lower current.

Wiper circuit: fuse, relay, motor park switch, and intermittent module; a wiper that stops in the wrong position points to the park switch or motor internal contacts.

Horn circuit: relay coil earthed through the horn button; a horn that clicks but does not sound has a relay contact or horn earth fault, not a switch fault.

LED lighting retrofit: LEDs draw lower current, which can trigger bulb-failure warnings and fast indicator flash; load resistors or coding may be needed.

Central locking and power windows use reversing motors with current sensing; a window that reverses immediately indicates excess current from a jammed regulator or faulty anti-trap sensor.

Brake and reverse lamps: check the brake pedal switch adjustment first; a switch that is out of adjustment keeps brake lights on and can drain the battery.

Diode: forward drop about 0.7 V for silicon, 0.3 V for germanium, and 0.2-0.3 V for Schottky; reverse-biased it blocks current until breakdown.

Zener diode regulates by operating in reverse breakdown; the zener voltage is marked on the body (e.g. 5V1 = 5.1 V).

Transistor as a switch: NPN conducts when base is about 0.7 V above emitter; PNP conducts when base is about 0.7 V below emitter; used as low-side and high-side drivers.

MOSFET is voltage-controlled and has very low on-resistance; check for short between drain and source and for a gate that has lost its drive signal.

Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold = ±5%, silver = ±10%.

Capacitor testing: an electrolytic should show rising resistance as it charges on an analogue meter; a shorted or bulging capacitor must be replaced.

ECU output drivers: high-side drivers switch battery positive to the load, low-side drivers switch the earth; a shorted low-side driver holds the load permanently on.

Logic levels in automotive circuits: 5 V sensor reference, 12 V actuator supply, and PWM control where duty cycle sets average voltage (50% duty on 12 V gives 6 V average).

CAN bus is a differential pair, CAN-H and CAN-L, twisted to reject noise; nominal termination is 120 Ω at each end, so a healthy bus reads about 60 Ω across the pair with power off.

CAN recessive state: both lines near 2.5 V; dominant state: CAN-H rises toward 3.5 V and CAN-L falls toward 1.5 V, giving about 2 V differential.

CAN high-speed bit rates: 125 kbit/s, 250 kbit/s, 500 kbit/s (powertrain), and 1 Mbit/s; LIN is single-wire, 1-20 kbit/s, used for body switches and sensors.

A shorted CAN line reads near 0 Ω or near 2.5 V on both lines; a single module can hold the bus down, so disconnect modules one at a time to isolate the fault.

ECU inputs and outputs: analogue sensors on 5 V reference, digital switches, PWM actuators, and communication lines; a 5 V reference shorted to earth disables all sensors sharing it.

ECU power and earth: check permanent battery supply, ignition-switched supply, and dedicated earths before condemning a module; a missing earth causes multiple unrelated codes.

CAN fault codes such as U0100 (lost communication with ECM) indicate a network problem, not necessarily a failed module; check wiring and power first.

Network topology: gateway module routes messages between buses (powertrain CAN, body CAN, LIN sub-buses); a failed gateway isolates whole sections of the vehicle.

Scan tool reads DTCs, live data, freeze-frame, and performs actuator tests; a code identifies a circuit or system, not a guaranteed failed part.

OBD-II connector pinout: pins 4 and 5 chassis/signal earth, pin 16 battery positive, pins 6 and 14 CAN-H and CAN-L, pins 7 and 15 ISO 9141/ISO 14230 K and L lines.

Voltage drop testing is preferred over resistance testing on loaded circuits: measure across the component or cable while current flows; above 0.5 V on a power feed indicates high resistance.

Test light vs multimeter: a high-impedance meter can read 12 V on a circuit that cannot carry current; a loaded test light reveals a high-resistance connection.

Oscilloscope shows waveform shape, amplitude, and timing: injector pulse width, crank signal, CAN differential, and alternator ripple are best diagnosed on a scope.

Five-step diagnosis: verify the complaint, gather information and check for TSBs, inspect and test, repair the root cause, then verify the repair and clear codes.

Intermittent faults: use a wiggle test on harnesses, check for water ingress at connectors, and review freeze-frame data for the conditions when the fault set.

Current ramping with a clamp meter: a fuel pump or motor that draws rising current as it warms indicates a failing armature or bearing.

HV traction systems operate at 100-800 V DC (commonly 200-400 V); orange cable denotes high voltage and must never be cut, probed, or disconnected without HV qualification.

HV battery packs are isolated from chassis; isolation fault testing measures resistance between HV conductors and chassis, with a typical pass threshold above 100 Ω per volt (e.g. 40 kΩ at 400 V).

Service disconnect procedure: power off, remove the service plug or HV fuse, wait the specified discharge time, verify 0 V at the inverter terminals with a CAT III/IV meter, then lock out.

HV battery state of charge: lithium-ion cells are typically 3.6-4.2 V each; a 96-cell pack at 3.7 V nominal gives about 355 V.

Regenerative braking charges the pack; the inverter converts DC to three-phase AC for the motor and back again during regen; IGBT or SiC modules switch at several kHz.

HV cables are shielded and colour-coded orange; the shield is earthed at one end and the cable must be replaced, not repaired, after damage.

Battery thermal management: coolant or air cooling keeps cells within about 20-40 °C; overheating causes reduced power, and a thermal runaway event releases toxic gas and cannot be extinguished with a small extinguisher.

First aid for HV shock: isolate the supply, do not touch the casualty until isolation is confirmed, call emergency services, and begin CPR if trained and safe.

Diode: forward drop about 0.7 V for silicon, 0.3 V for germanium, and 0.2-0.3 V for Schottky; reverse-biased it blocks current until breakdown.

Zener diode regulates by operating in reverse breakdown; the zener voltage is marked on the body (e.g. 5V1 = 5.1 V).

Transistor as a switch: NPN conducts when base is about 0.7 V above emitter; PNP conducts when base is about 0.7 V below emitter; used as low-side and high-side drivers.

MOSFET is voltage-controlled and has very low on-resistance; check for short between drain and source and for a gate that has lost its drive signal.

Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold = ±5%, silver = ±10%.

Capacitor testing: an electrolytic should show rising resistance as it charges on an analogue meter; a shorted or bulging capacitor must be replaced.

ECU output drivers: high-side drivers switch battery positive to the load, low-side drivers switch the earth; a shorted low-side driver holds the load permanently on.

Logic levels in automotive circuits: 5 V sensor reference, 12 V actuator supply, and PWM control where duty cycle sets average voltage (50% duty on 12 V gives 6 V average).