Electrical and Electronic Systems
Core concepts
Know 12 V and 24 V systems, batteries, alternators, starters, relays, fuses, and grounds. Understand series vs parallel battery connections and how voltage and capacity change. Learn circuit diagnosis with a multimeter: voltage drop testing, resistance, and current draw. Electronic content covers controllers, CAN bus (J1939), sensors (hall-effect, potentiometer, pressure, temperature), and fault codes. Know the difference between a chassis ground fault and an open circuit.
Traps
Questions often confuse voltage drop with resistance or assume a battery reading 12.6 V is good under load. A corroded ground can cause multiple unrelated faults. Series batteries add voltage, parallel add capacity. Do not assume a CAN fault means a bad controller; check termination resistance and wiring first.
Rule of thumb
Voltage drop across a good connection under load should be small (typically under 0.5 V for high-current paths). Load-test batteries rather than trusting open-circuit voltage alone. CAN bus termination is commonly about 60 ohms across the pair when healthy.
Worked example
A starter cranks slowly. Measure battery voltage at rest (12.6 V), then during crank at the battery (should stay above about 9.6 V) and at the starter terminal. If battery stays high but starter terminal drops, the voltage drop is in cables or ground; repair the high-resistance connection rather than replacing the starter.