Electrical and Electronic Systems
Electrical and Electronic Systems tests your ability to diagnose 12- and 24-volt circuits on machines that increasingly run networks rather than simple wires. Expect Ohm's law and power calculations, series and parallel circuits, battery and charging-system testing, starter circuits, relays and fuses, voltage-drop testing, sensor types, CAN bus fundamentals, controller area network faults, and diagnostic trouble code strategy.
Core circuit rules
- Ohm's law: volts equal amps times ohms; power in watts equals volts times amps. In series, current is common and voltage divides; in parallel, voltage is common and current divides.
- A healthy flooded battery rests near 12.6 volts (2.1 volts per cell) and should hold above roughly 9.6 volts while cranking at room temperature. Charging voltage is typically 13.8 to 14.4 volts.
- Test with a load, not just a voltmeter. A surface charge can show 12.6 volts on a battery that cannot crank.
- Voltage drop is the decisive test. With the circuit operating, total drop on the cranking circuit should stay under about 0.5 volt, and each connection under about 0.1 volt. Measure from the battery post to the starter stud, and from the starter case back to the battery negative.
- Relays use coil terminals 85 and 86 and contact terminals 30, 87, and 87a. A relay can click and still have burned contacts; test the contact circuit under load.
- Never increase fuse rating to stop a fuse blowing; the fuse is sized to protect the wiring.
Networks and sensors
- CAN bus is a twisted differential pair, typically biased near 2.5 volts, with a 120-ohm terminating resistor at each end. With the key off, resistance across CAN high and CAN low should read about 60 ohms.
- Two-wire sensors are usually variable resistance; three-wire sensors use a reference, signal, and return. Use duty cycle and frequency functions on a meter for PWM and digital sensors.
Worked example
A starter cranks slowly and the battery tests good. Measure voltage drop from the battery negative post to the starter housing during cranking: you read 0.9 volt. That exceeds the 0.1 to 0.2 volt target for a connection, so a corroded ground strap or dirty engine ground is dropping power that should reach the starter. Clean and retest before condemning the starter, because the reasoning is that the motor never saw full voltage.
Traps that cost marks
- Measuring resistance on a live circuit or with the meter in the wrong function.
- Assuming a battery is good because it shows 12.6 volts open circuit.
- Chasing a code to a component before checking power, ground, and connectors.
- Ignoring that an intermittent CAN fault may be a missing or corroded terminating resistor or a chafed harness.