NAVTTC Auto Electrician cheat sheet

Battery electrolyte is dilute sulfuric acid (about 35% H2SO4); flush skin or eyes with clean water for at least 15 minutes, then seek medical aid.

Lead-acid batteries emit hydrogen gas, which is explosive between roughly 4% and 75% concentration in air; keep flames, sparks and grinding dust away from charging areas.

Disconnect the battery negative (earth) terminal first and reconnect it last, so a spanner touching the bodywork cannot complete a short circuit.

A 12 V battery can deliver several hundred amperes into a short circuit; remove rings, bracelets and metal watch straps before working on live circuits.

Use a carbon-dioxide or dry-powder extinguisher on electrical and fuel fires; never apply water to a live electrical fire.

Jack stands or axle stands carry the vehicle load; a hydraulic jack alone is a lifting device only and must not support a raised vehicle.

Petrol vapour is heavier than air and ignites below 40 degrees C flash point; drain and store fuel only in approved closed containers in a ventilated area.

Before welding or grinding on a vehicle, disconnect the battery and the ECU connectors to protect electronic modules from stray currents and voltage spikes.

Lockout of a rotating engine or a live test bench means removing the ignition key and tagging the isolator; treat any tagged isolator as live until personally verified.

Multimeter DC voltage range must be selected above the expected value; a 12 V system needs at least the 20 V range, a 24 V system at least the 200 V range.

Current measurement requires the meter in series with the load; placing the meter in parallel on a current range blows the internal fuse or the shunt.

Resistance and continuity tests are valid only on a de-energised circuit; an ohmmeter reading in a live circuit is meaningless and may damage the meter.

A clamp meter measures current by magnetic field around a conductor and must clamp a single conductor; clamping both feed and return cancels the reading to zero.

Torque wrench accuracy is typically plus or minus 4% of setting; store it at its lowest scale value to avoid spring fatigue.

Feeler gauge blades are marked in millimetres or inches; a plug gap of 0.8 to 1.1 mm is set by the blade that drags slightly, not the one that fits loosely.

Crimping is preferred over soldering for vehicle harness repairs because a correct crimp resists vibration; a soldered joint must be supported by heat-shrink to avoid fatigue fracture.

Test lamp and logic probe are suitable for power and earth presence checks; only a high-impedance digital meter or oscilloscope should probe ECU signal lines.

Wire size is chosen by current and volt drop: a 1.0 mm2 cable is typical for 10 A accessory loads over short runs; long runs need larger cross-section to hold drop under 0.5 V.

Ohm's law: V = I x R; power: P = V x I = I squared x R = V squared / R.

Series circuit: total resistance adds, current is common, voltages divide in proportion to resistance.

Parallel circuit: voltage is common, currents add, total resistance is less than the smallest branch resistance.

Kirchhoff's voltage law: the sum of voltage drops around any closed loop equals the supply voltage; Kirchhoff's current law: current into a node equals current out.

In a negative-earth system the chassis is the return conductor; a poor earth raises resistance and causes dim lamps and slow motors.

Volt drop in a cable equals current times cable resistance; acceptable total circuit drop is typically under 0.5 V at full load for a 12 V system.

A relay uses a small coil current to switch a large contact current; coil terminals are usually 85 and 86, contact terminals 30 (supply), 87 (load) and 87a (normally closed).

Alternating current in an alternator stator is rectified by six diodes; a failed diode produces ripple visible on a scope and reduces charging output by about one third.

Capacitance opposes voltage change and inductance opposes current change; a capacitor suppresses contact arcing and radio interference, an inductor smooths current.

A fully charged 12 V lead-acid battery at rest reads about 12.6 to 12.8 V; 12.4 V is roughly 75% state of charge, 12.0 V about 25%.

Open-circuit voltage must be read after at least two hours rest, or after a surface charge is removed with a 10 to 15 second load.

Specific gravity of electrolyte in a fully charged cell is about 1.265; 1.120 indicates a discharged cell and a spread over 0.050 between cells indicates a failed cell.

Cold cranking amps (CCA) is the current a battery delivers for 30 seconds at minus 18 degrees C while staying above 7.2 V.

A load test applies half the CCA rating for 15 seconds; a good battery holds above 9.6 V at 20 degrees C.

Battery capacity in ampere-hours is the current a battery delivers for 20 hours to 10.5 V; a 60 Ah battery gives 3 A for 20 hours.

Charging voltage above 14.8 V causes gassing and water loss; below 13.5 V leaves the battery undercharged and sulfated.

A maintenance-free calcium battery needs a charging voltage about 0.2 to 0.4 V higher than an antimony battery to reach full charge.

Batteries in parallel add capacity and keep voltage; in series add voltage and keep capacity; mixed age or capacity batteries in either arrangement cause circulating currents.

Starter motor draws 150 to 250 A on a typical passenger car; cranking voltage should stay above 9.6 V at the battery terminals.

Starter circuit volt drop: measure from battery positive to starter terminal and from battery negative to starter body; each should be under 0.5 V during cranking.

A starter solenoid has a pull-in winding (high current, engages drive) and a hold-in winding (lower current, holds contacts closed); a failed hold-in causes chatter.

Alternator output is regulated to about 13.8 to 14.4 V at 20 degrees C; a reading under 13.2 V at 1500 rpm indicates a charging fault.

Belt condition and tension affect output: a slipping belt gives low output at idle and a squeal under load, not a flat battery symptom alone.

Alternator ripple test with a meter set to AC volts should show under 0.5 V AC; higher ripple indicates a failed rectifier diode.

The charge warning lamp is usually excited through the alternator's field circuit; if the lamp does not glow with the key on, the alternator may not start charging.

A no-crank condition with bright headlamps points to the starter control circuit (ignition switch, relay, neutral or clutch switch, immobiliser), not the battery.

A slow crank with dimming lamps points to high resistance in the battery cables, earth strap or a failing battery; check volt drop before replacing the starter.

Ignition coil primary resistance is typically 0.5 to 2 ohms; secondary resistance 6 to 15 kilo-ohms depending on coil type.

Spark plug gap is typically 0.8 to 1.1 mm; a worn plug with rounded electrodes needs a higher voltage to fire and misfires under load.

Ignition timing is set in degrees before top dead centre; too much advance causes pinging, too little causes overheating and poor power.

A distributorless or coil-on-plug system fires each coil from the ECU; a single misfiring cylinder points to that coil, plug or injector, not the crank sensor.

The crank position sensor provides engine speed and position; a failed signal stops both spark and injection, so the engine cranks but does not start.

Suppression plug leads are typically 5 to 15 kilo-ohms; a lead reading open or above 30 kilo-ohms causes misfire and radio interference.

A spark tester set to a 6 to 8 mm gap confirms spark energy under compression load better than a plug laid on the engine block.

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

Knock sensor detects detonation and the ECU retards timing; a loose or faulty sensor causes reduced power and poor economy rather than a no-start.

Headlamp bulb types: H4 has two filaments (dip and main) with a common earth; H7 has one filament and needs a separate earth return.

A sealed-beam or halogen lamp drawing 55 W at 12 V takes about 4.6 A; use P = V x I to size fuses and wiring.

Fuse rating is chosen at about 1.5 times the circuit's normal current; a 10 A circuit is protected by a 15 A fuse.

A blown fuse indicates an overload or short; replacing it without finding the cause will blow the new fuse.

The flasher unit is a thermal or electronic relay; fast flashing usually means a bulb is open circuit, not a failed flasher.

Wiper motors have park switches that keep the motor running until the park position; a wiper that stops mid-screen has a faulty park switch or earth.

Horn circuits use a relay because the horn draws 5 to 10 A; a horn that clicks but does not sound has a relay contact or horn earth fault.

LED lamps draw less current than the bulb they replace; a flasher designed for bulbs may flash fast or not at all with LEDs unless it is LED-compatible.

Reverse lamps are switched by a gearbox-mounted switch; brake lamps by a pedal switch; a single non-working brake lamp is usually a bulb or earth, not the switch.

Six-step diagnostic sequence: verify the complaint, gather information, inspect, test, repair, verify the repair.

Symptom-to-circuit logic: a fuse that blows immediately means a dead short to earth; a fuse that blows after a delay means an intermittent overload.

Volt drop testing is more reliable than resistance testing for high-current circuits because it tests under load.

Measure resistance to earth with the component disconnected; a reading near 0 ohm on a supply wire indicates a short to earth.

Parasitic drain is measured with the meter in series at the battery negative after all modules sleep; normal drain is typically under 50 mA.

Intermittent faults are found by wiggle-testing harnesses and connectors while monitoring the circuit with a meter or scope.

Read fault codes before clearing them; freeze-frame data and live sensor values are needed to confirm the cause, not just the code.

A scan tool shows sensor values as the ECU sees them; a sensor reading plausible but wrong (for example a coolant sensor stuck at 40 degrees C) still needs a resistance check at the sensor.

After any repair, verify with a road test or a full-load test of the affected circuit, and confirm no new fault codes are stored.