NABTEB Motor Vehicle Mechanics common mistakes
NABTEB Motor Vehicle Mechanics · Understand the traps and how to avoid them.
When asked to describe setting tappet/valve clearances, the candidate rotates the engine and adjusts whichever valves happen to be loose on that cylinder, without confirming that the cylinder is at TDC on the compression stroke (both valves closed, pushrods free, rockers loose).
It happens because candidates memorise the feeler-gauge size but not the cylinder-by-cylinder sequence, so any position where a rocker feels slack is treated as 'correct'. On overlap, one valve is partly open and the reading is meaningless.
What to do instead
Learn and write out the sequence first: bring No.1 to TDC on compression (rotor pointing to No.1 plug lead, both rockers free), set the listed valves, turn the crank one full revolution, then set the remainder. Always state whether the clearance is specified hot or cold, and quote the figure with units.
In circuit questions, the candidate draws or describes the voltmeter connected in series with the load and the ammeter connected across (in parallel with) the component being tested.
The two instruments look interchangeable on paper and candidates learn 'meter goes in the circuit' as a single rule without the reason for each connection.
What to do instead
Anchor the rule to the measurement: a voltmeter measures potential difference, so it goes in parallel across the component with high resistance; an ammeter measures current, so it goes in series with the circuit with very low resistance. Add a one-line sketch and check polarity (+ to +) before writing the answer.
For hydraulic brake bleeding, the candidate states that bleeding starts at the wheel closest to the master cylinder and does not mention keeping the reservoir topped up or closing the nipple before the pedal is released.
The procedure is memorised as a list of steps with no understanding of why air is pushed away from the master cylinder, and the reservoir/nipple details are dropped because they seem minor.
What to do instead
Write the sequence with its reason: start at the wheel furthest from the master cylinder (longest pipe run), have an assistant hold the pedal down, open the nipple, close it before the pedal returns, and check/top the reservoir after every few strokes to avoid drawing air in. Note the exception for split or diagonally piped systems, which follow the manufacturer's order.
Describing clutch operation, the candidate writes that the clutch is 'engaged' when the pedal is pressed down and 'disengaged' when the pedal is released, or muddles which side of the diaphragm spring the release bearing acts on.
Everyday language ('press the clutch to engage a gear') is carried straight into the technical answer, so the mechanical state and the driver's action get reversed.
What to do instead
Draw the two-state power-flow sketch before answering: pedal released = pressure plate clamps the disc to the flywheel = drive transmitted (engaged); pedal pressed = release bearing pushes the diaphragm fingers, spring pivots, plate lifts = no drive (disengaged). Label the release bearing, pressure plate, friction disc and flywheel on the sketch.
On battery charging, jump-starting or working near the fuel system, the candidate answers with generalities such as 'be careful' or 'wear overalls' instead of naming the specific hazard, the gas or fluid involved, and the control measure.
Safety questions are treated as filler marks and answered from habit; candidates have not linked each workshop activity to its particular hazard (hydrogen gas, sparks, electrolyte, fuel vapour, high-pressure lines).
What to do instead
Answer safety questions in a three-part pattern: hazard (e.g. hydrogen given off during charging is explosive), source/condition (sparks, smoking, loose terminal), control (charge in a ventilated area, connect the negative lead last and disconnect it first, wear goggles and acid-resistant gloves, remove rings and watches). Use one hazard-control sentence per mark available.
The candidate uses a torque wrench as a breaker bar to loosen a seized nut or to tighten a bolt past the click, and, when reading a vernier caliper or micrometer, records only the main-scale reading and ignores the vernier/thimble coincidence line.
Torque wrenches look like strong bars and the 'click' is treated as a target to beat; on instruments, the small graduations are rushed because the main scale already gives a plausible-looking number.
What to do instead
State the rule: torque wrenches are for final tightening only, set to the specified figure and used with a steady pull until the click/slip, then released to zero after use. For instruments, write the three-part reading every time: main scale + vernier/thimble coincidence x least count, quote the least count, and repeat the measurement at least twice at different points.
Wheel alignment terms are swapped: camber is defined as the forward or backward tilt of the kingpin, caster as the inward or outward lean of the road wheel, and toe-in is confused with the turning angle or with camber wear patterns.
The four angles are learned as a list of words rather than as tilts seen from a stated viewing direction, so the definitions collapse into each other under exam pressure.
What to do instead
Fix each angle to a viewing plane: camber = wheel lean in or out seen from the front; caster = steering axis/kingpin tilt fore or aft seen from the side; toe = difference in distance between the front and rear of the two wheels seen from above; steering axis inclination = tilt in the front view. Attach the typical fault to each (uneven shoulder wear for camber, pull for caster, feathered tread for toe).
Carburettor and ignition answers attach the wrong circuit or component to the wrong operating range - for example saying the idle/pilot jet supplies mixture at high engine speed, or that the main jet controls idling - and the candidate states that a compression-ignition engine uses a spark plug to start combustion.
The circuits are memorised as a set of named parts with no map of when each one is in operation; the diesel/petrol distinction is assumed rather than reasoned from 'compression ignition'.
What to do instead
Build a speed-range table for the carburettor (idle/pilot circuit at closed throttle, progression and main circuit at part throttle, power/enrichment and accelerator pump at wide throttle, choke for cold start) and learn the ignition difference as cause-and-effect: petrol = spark ignition of a compressed air-fuel mixture; diesel = fuel injected into highly compressed hot air, igniting by heat of compression, with no spark plug (glow plugs assist cold starting only).
Preventive maintenance answers are given as vague advice ('check the oil regularly', 'service the brakes when needed') with no interval, no unit and no acceptance criterion, and scheduled items are listed in the wrong order for a full service.
Candidates know the checks from routine workshop experience but have never written them as a specification, so they assume the examiner will fill in the intervals.
What to do instead
Write every maintenance item as three fields: interval (in km and/or months, e.g. engine oil and filter every 5 000 km or 3 months), action (drain, renew, torque sump plug to specified figure), and acceptance criterion (dipstick level between MIN and MAX with the vehicle level and engine off; tyre tread not below the legal minimum; brake fluid above the MIN line and not discoloured). Order the service from safety-critical to cosmetic.
Given a fault symptom, the candidate names a likely failed component straight away and writes it as the diagnosis, without a test sequence, without a measured result and without confirming or eliminating the other causes.
Workshop habit is to swap the most common part first; the exam rewards a traceable reasoning chain, and candidates do not realise that the reasoning, not the guessed part, is what is being marked.
What to do instead
Answer in a fixed four-column format: symptom - possible causes (list at least three, cheapest/most probable first) - test to distinguish them (name the instrument and the expected reading) - conclusion and rectification. Include at least one measured value with units and state what result would rule each cause out or in.