SOLAS Heavy Vehicle Mechanic key terms

The sequence of intake, compression, power and exhaust strokes that a piston completes in two crankshaft revolutions. Most heavy vehicle diesel engines run on this cycle, so understanding valve and piston timing is fundamental.

Ignition of fuel caused by the high temperature of air compressed in the cylinder, with no spark plug involved. Diesel engines in trucks and buses rely on this principle, which is why compression pressure and injector condition matter so much.

An exhaust-driven compressor that forces extra air into the engine cylinders, increasing power for a given engine size. Lubrication and cooling of its bearings are critical, and a failed turbo can contaminate the intake with oil.

A valve that bypasses exhaust gas around the turbine once a set boost pressure is reached, protecting the engine from overboost. A stuck wastegate causes low power or overboost faults and should be checked during turbo diagnosis.

A heat exchanger that cools compressed air after the turbocharger before it enters the engine. Cooler air is denser, so it improves combustion and lowers thermal stress on pistons and valves.

The pressure above atmospheric that the turbocharger delivers to the intake manifold. Mechanics measure it to diagnose turbo, wastegate and intake leaks, and to confirm the engine is producing expected power.

A precision component that atomises diesel fuel and delivers it into the combustion chamber at the correct time and quantity. Worn or clogged injectors cause rough running, smoke and high fuel consumption.

A fuel system in which one high-pressure pump feeds a shared rail that supplies all injectors. Injection pressure and timing are controlled electronically per injector, allowing multiple injection events per cycle.

The computer that reads sensor inputs and controls actuators such as injectors, EGR valves and turbo vanes. Fault codes are stored in it, and diagnosis usually starts by reading these codes and live data.

A stored alphanumeric code that identifies a circuit or performance fault detected by the engine control unit. It narrows the search but does not by itself prove which component has failed.

A system that routes a measured portion of exhaust gas back into the intake to lower combustion temperature and reduce nitrogen oxide emissions. Carbon blockage of the valve and cooler is a common heavy vehicle fault.

A trap in the exhaust that collects soot particles and burns them off during regeneration. Excessive short-run operation can clog it, causing high back pressure, derate and warning lamps.

The process of raising exhaust temperature to burn accumulated soot from a particulate filter. It may occur automatically while driving or be commanded by a technician using a diagnostic tool.

A urea solution injected into the exhaust to reduce nitrogen oxides in selective catalytic reduction systems. Low level, poor quality or a faulty dosing system can trigger power derate.

The small gap between the rocker or cam follower and the valve stem, set to allow for thermal expansion. Incorrect clearance causes noise, burnt valves or lost compression, so it is checked at service intervals.

The casting that closes the top of the cylinders and houses valves, injectors and often the camshaft. It must seal combustion gases and coolant, so flatness and correct torque are essential.

The sealing layer between cylinder head and engine block that contains combustion pressure, oil and coolant. Failure shows as coolant loss, white smoke or bubbles in the radiator.

A tool that applies a measured twisting force so fasteners are tightened to specification. Correct torque and tightening sequence prevent distortion and joint failure on heads, wheels and suspension components.

The specified order in which multiple fasteners are tightened, usually in stages from the centre outwards. Following it distributes clamping load evenly and avoids gasket leaks or warped parts.

A braking system that uses compressed air to apply service brakes, commonly found on trucks and buses. It includes a compressor, reservoirs, valves, chambers and slack adjusters that must be inspected regularly.

A cylinder that converts air pressure into mechanical force to apply the brakes. It contains a diaphragm and return spring, and a ruptured diaphragm causes weak braking or air loss.

The linkage that transfers brake chamber pushrod travel to the brake shoe and adjusts for lining wear. Incorrect adjustment causes poor braking or dragging brakes and is a frequent inspection item.

A device that removes moisture and oil from compressed air before it reaches the brake reservoirs. A failed dryer lets water into the system, causing corrosion, frozen valves and brake faults.

A parking brake that uses a strong spring to apply the brakes when air pressure is released. It fails safe, so the vehicle cannot be moved safely without sufficient air pressure.

Electronics that momentarily release and reapply brake pressure to prevent wheel lock-up during hard braking. Wheel speed sensors and their wiring are common fault points on heavy vehicles.

A sensor that reports wheel rotation to the antilock braking system. A dirty or damaged sensor causes ABS warning lamps and disables antilock function while normal braking remains.

A friction coupling that connects and disconnects engine power from the transmission so the vehicle can start and change gear. Wear, slipping or dragging are diagnosed by pedal feel and road test.

A gearbox in which the driver selects gear ratios through a shift mechanism. It uses shafts, gears and synchronisers, and diagnosis often involves noise, jumping out of gear or hard shifting.

The components that carry power from the transmission to the wheels, including driveshafts, universal joints, differential and axles. Vibration and noise under load often point to worn joints or bearings.

A gear assembly that lets driven wheels turn at different speeds while transmitting power. It also provides the final gear reduction, and incorrect lubricant or setup causes whine and premature wear.

A joint that allows a driveshaft to transmit power through changing angles. Worn needle bearings cause vibration and clunking, and the joint must be greased or replaced as specified.

The ratio between driveshaft speed and wheel speed set by the differential gearing. It affects pulling power and cruising fuel economy, and must match the vehicle's intended duty.

A rubber bellows filled with air that supports vehicle load and provides ride height control. Leaks cause sagging, uneven stance and compressor overload, and bags must be replaced in axle pairs.

A hydraulic damper that controls spring oscillation and keeps tyres in contact with the road. Leaking fluid or worn valving causes bouncing, poor handling and uneven tyre wear.

Adjustment of camber, caster and toe so wheels point correctly relative to the vehicle and road. Incorrect alignment causes rapid or uneven tyre wear and steering pull.

A code showing the maximum load a tyre can carry at its rated inflation pressure. Fitting a tyre below the axle requirement is unsafe and may be illegal for the vehicle's plated weights.

The air pressure inside a tyre, specified cold by the manufacturer for the load and duty. Wrong pressure causes overheating, uneven wear, poor fuel economy and reduced grip.

A test instrument that measures voltage, resistance and current. Technicians use it to check circuits, sensors and charging systems before replacing parts.

A store of electrical energy that supplies starting current and supports electrical loads. Heavy vehicles use large batteries, and testing includes state of charge, load test and terminal condition.

A belt-driven generator that charges the battery and powers electrical systems while the engine runs. Low output shows as warning lamps, flat batteries and dim lights, often from worn brushes or a slipping belt.

Clothing and gear worn to reduce injury risk, such as safety boots, gloves, eye protection and hearing protection. It is the last line of defence and does not replace guards or safe work procedures.

A procedure that isolates energy sources and attaches locks and tags before maintenance. It prevents accidental start-up or release of stored energy such as air, hydraulic pressure or springs.

A structured review of a task to identify hazards and decide how to control them before work starts. Controls follow a hierarchy from elimination to protective equipment, and findings are recorded.

The manufacturer's document giving specifications, procedures and torque values for a vehicle or component. Following it ensures repairs meet design intent and helps avoid damage or unsafe work.