Electrical/Electronic Systems
Fundamentals
Start with Ohm's law (E = I × R) and the two families of measurement: resistance (component isolated, power off) and voltage drop (circuit loaded and operating). Voltage drop is the technician's best tool because it tests the circuit under real current. Know battery ratings (CCA, reserve capacity), series versus parallel connections, the cranking circuit, the charging circuit (alternator, rectifier bridge, regulator), relays, fuses, and multiplexed networks such as J1939 CAN.
Rules of thumb
- State of charge: about 12.6 V at rest is fully charged; roughly 12.2 V is about half.
- Cranking: battery voltage should stay near or above 9.6 V on a 12 V system; charging output typically 13.8–14.5 V.
- Voltage drop: keep total circuit drop around 0.5 V or less on control circuits; a cranking circuit is commonly allowed about 0.5 V per side (insulated and ground).
- Parasitic draw: after modules sleep, usually well under 50 mA.
Traps
Questions offer a resistance reading of a ground strap as proof it is good - resistance checks miss a single strand carrying the load. They also confuse series (voltages add, capacity stays the same) with parallel (capacity adds, voltage stays the same), and they blame an alternator for a no-charge condition actually caused by a bad sense wire or belt.
Worked example
A starter cranks slowly. Measured drop from battery positive to starter terminal is 0.1 V and battery negative to starter case is 0.65 V. The ground side exceeds the 0.5 V allowance, so clean and repair the engine-to-frame ground path rather than replacing the starter.