Basic Mechanical Knowledge, Troubleshooting, and Minor Repairs
What the exam tests
This domain tests whether you understand machine systems well enough to diagnose a fault, decide what you can repair in the field, and know when to stop and refer.
Core rules and concepts
- Diesel basics: combustion needs air, compression, fuel and timing. Symptoms map to systems: black smoke suggests over-fuelling or restricted air, white smoke at start suggests cold cylinders or injector trouble, and blue smoke suggests oil burning.
- Aftertreatment: DPF, DEF and SCR systems need the correct DEF, clean fuel and a suitable duty cycle. A regen that keeps derating the machine is a fault, not an operator problem; record the codes and report it.
- Hydraulics: pump, control valve, cylinder and relief. Slow or weak functions point to low oil level, clogged filters, a relief valve stuck open, internal leakage, or a worn pump. Contamination is the main killer, so cap lines, wipe fittings and use the specified cleanliness level.
- Pressure: relieve stored pressure before opening any line. Lower the attachment, shut down, cycle the controls and lock out.
- Electrical: 12 or 24 V systems. Check battery terminals, fuses and relays before condemning a component; a fuse that blows repeatedly means a short to ground.
- Wear parts: track tension, teeth, cutting edges, pins, hoses and belts are routine replacements an operator can make within their authority.
- Troubleshooting logic: verify the symptom, check the simplest cause (level, filter, fuse, connection), test, then repair or refer with a clear description and any fault codes.
Worked example
A machine hydraulics get slow as it warms up. Reasoning: cold oil is thick, so a worn pump or internal leakage shows up hot. Check oil level and temperature, inspect the suction strainer and return filter for restriction, then check relief pressure with a gauge. If pressure is correct but flow falls off when hot, the pump is suspect; report with the test readings rather than replacing parts by guess.
Traps that cost marks
- Opening a hydraulic line without releasing pressure.
- Substituting engine oil or generic fluid for the specified hydraulic oil.
- Clearing a fault code without recording it.
- Over-tightening filters or fittings and cracking a housing.
- Continuing to run a machine in derate instead of reporting it.