Cert III Auto Electrical cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. Use these to size conductors, fuses and predict voltage drop across a load.

Series circuit: total resistance adds (R_T = R1 + R2 + ...), current is identical everywhere, and voltage divides in proportion to resistance.

Parallel circuit: 1/R_T = 1/R1 + 1/R2 + ...; voltage is equal across branches and current divides inversely with branch resistance.

12 V system voltage drop rule: total drop on the positive and earth return path should not exceed about 0.5 V at full load; measure under load, not open circuit.

Battery capacity: reserve capacity and cold cranking amps (CCA) describe different duties; a battery may crank but still fail a load test if capacity has fallen.

Isolation before work: disconnect the battery earth (negative) lead first and reconnect it last to reduce short-circuit and arc risk when using tools.

Insulation resistance testing uses a megohmmeter at a specified test voltage; a low reading to earth indicates insulation breakdown, not a normal load.

Voltage measurement requires the meter in parallel with the component; current measurement requires the meter in series, so never place an ammeter across a supply.

Alternator output is regulated by controlling rotor field current; a low or open field circuit causes low or zero charge output even with a good belt.

Charging voltage check: with the engine running and loads off, expect roughly 13.5–14.5 V at the battery for a 12 V lead-acid system; readings outside this warrant diagnosis.

Starter current draw: a healthy starter draws a high inrush current that falls as the engine cranks; abnormally high draw with slow cranking suggests a seized or shorted starter or excessive mechanical resistance.

Voltage drop on the starter circuit is tested cranking or with a load applied; more than about 0.5 V on a cable or connection indicates excessive resistance.

Relay terminals: coil pins are the low-current control side, contact pins carry the load current; a relay can click yet still have burnt contacts and no output.

Lighting faults: check globe, fuse, switch, relay and earth in that order of likelihood; a shared earth fault can affect multiple lamps on the same circuit.

Controller area network (CAN) buses need terminating resistors at each end; a bus fault can disable multiple modules that share the network.

Accessory circuits must be fused at the supply end of the conductor so a short anywhere downstream is protected.