Electrical Systems and Electronic Controls
This domain covers measurement practice, charging and starting circuits, and network diagnostics.
Core rules
- Ohm's law is the working tool: V = I × R. Resistance tests on a live circuit mislead; use voltage-drop testing under load - under about 0.1 V per connection and 0.5 V on a ground path is acceptable.
- Battery: about 12.6 V at rest after surface charge is removed; a load or conductance test decides replacement. Charging voltage at the battery should be roughly 13.5–14.8 V at mid rpm.
- Charging system: three stator phases should have equal resistance (often 0.2–0.5 Ω) and no continuity to ground; a failed regulator-rectifier usually shows low charging voltage, a boiled battery or a blown main fuse.
- Parasitic draw: meter in series at the negative post with everything off; typically below 50 mA.
- Electronic controls: 5 V reference sensors (TPS, MAP), hall-effect crank sensors, CAN bus with about 60 Ω termination across the pair. Read fault codes before unplugging anything, and clear codes only after the repair is verified.
Worked example
A bike cranks slowly when hot and the dash dims. Reasoning: slow cranking with a known-good, fully charged battery points at the starter circuit, so voltage-drop the positive cable from battery to starter and the ground from battery to engine. A 0.9 V drop on the ground strap means a corroded engine ground - cleaning it restores cranking without replacing the starter.
Traps that cost marks
- Condemning a battery without a load test.
- Measuring resistance with the circuit powered.
- Charging a lithium battery with a lead-acid charger profile.