Diagnose and Troubleshoot Engine and Electrical System Faults
Diagnose and Troubleshoot Engine and Electrical Faults
This domain tests logical fault-finding. Marks go to the technician who tests in sequence and proves the fault, not the one who replaces parts.
Core rules
- Confirm the complaint, then do a visual check: fuel level, kill switch, side-stand switch, clutch switch, loose battery terminals, blown fuse, disconnected hose.
- An engine needs four things: correct fuel/air mixture, good spark at the right time, adequate compression, and free exhaust flow. Test in that order - it is fast and cheap.
- Compression test: engine warm, throttle wide open, plug removed, crank until the gauge stops rising. A reading well under spec that rises when oil is added points to rings; no rise points to valves or the head gasket.
- Spark test: hold the plug body against clean metal and crank for a strong blue spark. No spark: check plug, cap, coil resistance, pickup coil, CDI, and grounds.
- Battery: about 12.6 V at rest and 13.5–14.5 V running. Low charging voltage points to the regulator/rectifier or stator.
- Smoke: blue = burning oil; black = rich mixture; white = coolant or condensation.
Worked example
A 125 cc motorcycle cranks but will not start. Fuel reaches the carburetor (the drain screw flows), spark is strong and blue, but compression is 90 psi against a 150 psi specification. Adding a teaspoon of oil raises it to 130 psi. Reasoning: fuel and spark are proven good; the jump after oil confirms worn rings and cylinder, so the top end is disassembled instead of the ignition being replaced.
Traps
Replacing CDI, coil, and plug at once without testing; overlooking the kill switch and side-stand switch; checking compression with the throttle closed; ignoring a bad ground strap that mimics a dead battery.