NABTEB Automobile Electrical Works common mistakes
NABTEB Automobile Electrical Works · Understand the traps and how to avoid them.
Applying Ohm's law with total circuit resistance but individual branch voltage, or adding resistances in parallel by simple summation when working out current, voltage drop and power in series-parallel circuits.
Candidates memorise V = IR and P = VI but practise almost only on single-resistor examples, so they treat a parallel network as one resistance and forget that branch currents add while voltage is common across parallel branches.
What to do instead
Before calculating, redraw the network and label each node; reduce parallel groups with 1/RT = 1/R1 + 1/R2, add series parts, then work back to find branch currents and voltage drops. Check the answer by confirming that power dissipated in each resistor sums to the power supplied by the source.
Testing a battery by open-circuit voltage alone and declaring it serviceable, without a load test or hydrometer/specific-gravity check on a lead-acid unit.
A surface charge makes a weak or sulphated battery read about 12.6 V, and candidates have often been taught only the voltmeter method, so they skip the high-rate discharge test that reveals a collapsed cell.
What to do instead
Apply a load tester or crank the engine briefly with the voltmeter connected; a good 12 V battery should hold above roughly 9.6 V at 10.5 V cut-off under load at normal temperature. On serviceable lead-acid batteries, confirm with a hydrometer (about 1.260–1.280 fully charged, cells within 0.025 of each other) and correct readings for temperature.
Diagnosing a no-crank condition by replacing the starter motor first, without checking battery terminals, earth straps, the solenoid control circuit and the ignition/starter switch feed.
Candidates associate 'nothing happens when I turn the key' directly with the starter, and they skip the voltage-drop tests that would show a corroded terminal or bad earth is eating the current.
What to do instead
Follow a logical sequence: battery state of charge, terminal tightness and corrosion, earth strap continuity, control-circuit voltage at the solenoid terminal with the key in START, then the main cable and solenoid contacts. Use a voltage-drop test (crank, measure from battery positive to starter terminal and from starter body to battery negative; more than about 0.5 V indicates excessive resistance).
Fitting a ballast resistor or coil of the wrong type, or bypassing the ballast permanently, then blaming the points or condenser when the engine misfires or the points burn.
Candidates do not realise that a ballast-type ignition system runs the coil on reduced voltage in normal running and full voltage only during cranking, so they fit a 12 V coil in place of a 6–9 V ballast coil or leave the bypass wire connected.
What to do instead
Identify the system first: measure voltage at the coil positive with the ignition on (about 6–9 V indicates a ballast in circuit) and check whether an extra wire from the starter solenoid feeds the coil. Match coil primary resistance to the system, and verify dwell and point gap with a dwell meter or feeler gauge before condemning components.
Diagnosing lighting and indicator faults by swapping bulbs at random instead of tracing the circuit, and misreading a fast-flash indicator as a bulb failure when the flasher unit or earth is at fault.
Bulb replacement is quick and familiar, so candidates stop there; they also forget that many lighting faults are earth-side faults, and that a poor earth lets current back-feed through the other filament and produce dim or cross-lighting symptoms.
What to do instead
Test systematically with a multimeter: supply voltage at the lamp holder, continuity of the earth return to the body, and resistance across the bulb. For indicators, check all lamps and the flasher unit rating; a fast flash usually means a lamp out or wrong wattage, a slow or no flash points to the flasher or supply. Confirm earths by measuring voltage between the lamp earth pin and battery negative with the lamp on (should be near zero).
Choosing a fuse or cable gauge by what fits the holder or by copying the old one, without calculating current demand and voltage drop for the cable length.
Candidates learn fuse colour codes but not the relationship between load current, cable cross-sectional area, length and permissible voltage drop, so accessories are wired with undersized cable or over-fused circuits.
What to do instead
Calculate load current from P/V, select cable so voltage drop stays within about 0.5 V for lighting and 0.2–0.3 V for sensitive electronics over the run, and fuse to protect the cable, not the load. Use the fuse rating just above the load current and below the cable's current-carrying capacity, and state the calculation in the answer.
Wiring auxiliary equipment such as horns, wipers, radios and electric fans straight to an existing circuit without a relay or separate fused supply, and ignoring suppression of inductive spikes.
Candidates treat accessories as simple loads and do not consider that motors and coils draw high inrush current and generate back-EMF, so they overload the existing switch and wiring and cause recurring failures.
What to do instead
Feed high-current accessories through a relay controlled by the switch, with a dedicated fused supply taken from the battery or main bus and a cable rated for the inrush. Earth the relay coil and accessory properly, and fit suppression (diode across a relay coil, capacitor across a motor) where the manufacturer specifies it.
Using a test lamp or a cheap meter on a live electronic circuit, or measuring resistance on a circuit that still has power applied, and reporting 'no fault found' without recording the test conditions.
Candidates are used to test lamps on older vehicles and do not appreciate that a low-impedance lamp can damage electronic modules, or they forget to isolate the circuit before a continuity test, so readings are meaningless.
What to do instead
Use a high-impedance digital multimeter for electronic circuits; isolate and prove dead before resistance or continuity tests; use a test lamp only where the circuit can tolerate it. Record battery voltage, key position, temperature and meter range with each reading, and compare against the specification rather than a guess.
Working on live circuits with rings, watches and loose clothing, or using the wrong tool for the job (mole grips on terminal nuts, a screwdriver as a pry bar), and skipping the battery disconnect step.
Candidates rush to demonstrate a repair under exam time pressure and underestimate arc, short-circuit and acid risks; they also have not been taught the sequence for safe isolation and the correct tool for each fastener.
What to do instead
State the safety sequence explicitly: disconnect the earth (negative) terminal first and reconnect it last, remove metal jewellery, wear eye protection when working near batteries, and insulate spanners. Select the correct size spanner or socket, use a torque wrench where specified, and support the vehicle securely before working underneath.
Misreading automotive wiring diagrams: confusing terminal numbers and colour codes, treating a line crossing as a connection, and failing to identify the earth path or the component shown in the 'rest' position.
Candidates learn symbols in isolation but not the conventions of terminal designation (for example 30, 15, 31, 85, 86, 87), DIN colour abbreviations, or the rule that a dot indicates a joint while a plain crossing does not.
What to do instead
Practise reading a complete circuit from supply through fuse, switch and load to earth, writing out the path in words. Memorise the common terminal numbers and colour abbreviations, check whether switches are drawn in the unoperated position, and mark junctions with a dot before answering questions about continuity between points.