Automotive: Heavy Vehicle Systems, Diagnosis and Repair
Domain 1 - Automotive. This section tests the heavy-vehicle workshop knowledge behind safe, compliant repairs: diesel engine operation and fuel systems, air brakes, driveline and suspension, electrical and hydraulic fault-finding, and the inspection consequences of each repair. Questions often give a symptom and ask for the most likely cause, the correct test sequence, or the safety/legal result.
Engine and fuel system fundamentals
- Diesel is compression ignition: air is inducted, compressed to a high pressure and temperature, and fuel is injected near top dead centre. Classic diesel intake air is not throttled.
- Common-rail systems depend on rail pressure and injector performance. Low rail pressure can cause a no-start or derate; air in the fuel can cause rough running and white smoke.
- Turbocharger faults include boost leaks, restricted intake or exhaust, and stuck VGT actuators. Charge-air cooler leaks cause low boost and black smoke.
Brakes, steering and suspension
- Heavy air systems use a compressor, dryer, governor and reservoirs. Drain tanks daily; know that low pressure triggers a warning and eventually spring-brake application.
- ABS/EBS wheel-speed sensors and modulator valves can set a warning lamp. ABS being disabled does not mean the base service brakes are gone.
- Check kingpins, drag links, wheel-bearing play and tyre wear patterns before condemning a steering complaint.
Diagnostics
Verify the complaint, read codes and live data, then test power, ground and mechanical basics. A fault code is a starting point, not a diagnosis.
Worked example
A 6x4 tipper is down on power, blows black smoke and logs an underboost code. Reasoning: black smoke suggests over-fuelling or insufficient air. First check the air filter and charge-air hoses for restriction or splits, then compare boost-sensor live data with key-on/engine-off, then inspect VGT actuator travel and exhaust restriction. A leaking charge-air hose or stuck VGT explains both low boost and smoke; replacing the turbo first is guesswork.
Traps that cost marks
Treating a code as the cause; ignoring air in fuel; using the wrong coolant or oil; over-torquing torque-to-yield fasteners; assuming an ABS lamp means total brake failure; and forgetting that a repair may affect CoF or WoF compliance.