Electrical Systems
Electrical fundamentals
Automotive electrical diagnosis rests on Ohm's law (V = I × R), series and parallel circuits, and voltage-drop testing under load. The battery supplies cranking current; the starter draws hundreds of amps; the alternator recharges at roughly 13.5-14.5 V. Fuses, relays, grounds, and connectors are common failure points. A parasitic draw test measures current with modules asleep, and a charging-system test checks output and AC ripple.
Traps: an ohmmeter reading on a powered circuit is meaningless and can damage the meter. Resistance tests on high-current cables often pass while the cable fails under load; use a voltage-drop test instead. Surface charge after charging can mislead a battery test; remove it with a load. Do not assume a new battery is charged or correctly sized. A loose ground can cause multiple unrelated symptoms.
Rule of thumb: total cranking voltage drop should be 0.5 V or less, with each connection or cable contributing about 0.1 V or less. A battery at 12.6 V is fully charged; about 12.2 V is roughly 50 percent state of charge.
Worked example: A slow crank with bright headlamps. Connect a voltmeter from battery positive to the starter positive stud and crank; then from the engine block to battery negative. A reading above 0.5 V on either side identifies excessive resistance in that cable or connection. Clean and repair as needed, then retest.