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NAVTTC Auto Mechanic practice questions

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Question 1Engine systems diagnosis and repair

A four-stroke diesel engine completes one full power cycle in how many crankshaft revolutions?

  1. Half a revolution
  2. Two revolutions Correct answer.
  3. One revolution
  4. Four revolutions

Explanation

In a four-stroke cycle the piston travels through intake, compression, power and exhaust, which requires two complete turns of the crankshaft (720 degrees). Each stroke corresponds to half a revolution, so four strokes equal two revolutions.

Question 2Engine systems diagnosis and repair

In a compression-ignition engine, combustion is initiated by which condition?

  1. Pre-ignition of the air-fuel mixture in the intake manifold
  2. A glow plug remaining hot throughout engine operation
  3. A spark plug firing just before top dead centre
  4. The high temperature of highly compressed air igniting injected fuel Correct answer.

Explanation

A diesel engine draws in air only and compresses it to a high ratio, raising its temperature above the fuel's ignition point. Fuel injected near the end of compression self-ignites on contact with this hot air, so no external ignition source is needed.

Question 3Engine systems diagnosis and repair

A technician finds the engine oil level at the lower mark on the dipstick after the engine has been switched off for ten minutes. What is the correct immediate action?

  1. Run the engine at high speed to circulate oil before checking again
  2. Drain all the oil and refill with a different viscosity grade
  3. Continue operating and recheck only after the next service interval
  4. Top up with the specified grade until the level reaches the upper mark Correct answer.

Explanation

The dipstick marks define the safe operating range, and a level at the lower mark means the engine is at the minimum acceptable quantity. The correct response is to add oil of the manufacturer-specified grade until the level sits at the upper mark, then recheck for leaks.

Question 4Engine systems diagnosis and repair

Which component drives the camshaft on a pushrod overhead-valve engine?

  1. The oil pump driveshaft
  2. The distributor shaft
  3. The flywheel ring gear
  4. Timing chain or belt from the crankshaft Correct answer.

Explanation

The camshaft must rotate at exactly half crankshaft speed, so it is driven by a timing chain, belt or gear train connected to the crankshaft. This fixed drive relationship keeps valve events synchronised with piston position.

Question 5Engine systems diagnosis and repair

Blue-grey smoke that persists from the exhaust after the engine is fully warm most strongly indicates which fault?

  1. Excess fuel delivery from the injection pump
  2. Oil entering the combustion chamber Correct answer.
  3. A restricted air filter element
  4. Coolant leaking into the cylinder

Explanation

Oil burning in the cylinder produces the characteristic blue-grey haze, typically from worn valve guides, valve stem seals or piston rings. The colour distinguishes it from black smoke (rich mixture) and white smoke (coolant or unburnt fuel).

Question 6Engine systems diagnosis and repair

What is the primary purpose of the thermostat in an engine cooling system?

  1. To detect coolant temperature for the gauge
  2. To pressurise the cooling system and raise the boiling point
  3. To circulate coolant through the radiator
  4. To regulate coolant flow and maintain a stable operating temperature Correct answer.

Explanation

The thermostat is a temperature-sensitive valve that stays closed while the engine warms and opens to admit coolant to the radiator once operating temperature is reached. This regulation keeps the engine within its designed temperature band for efficiency and emissions.

Question 7Engine systems diagnosis and repair

During a cylinder compression test on a warm petrol engine, one cylinder reads significantly lower than the others. Adding a few drops of oil raises that reading. What does this indicate?

  1. Worn piston rings or cylinder bore Correct answer.
  2. Incorrect valve clearance
  3. A burnt exhaust valve
  4. A leaking head gasket

Explanation

Oil temporarily seals the gap between rings and bore, so a compression rise after oiling points to ring or bore wear. If the fault were in the valves or head gasket, the oil would not seal the leak and the reading would remain low.

Question 8Engine systems diagnosis and repair

Which component converts the reciprocating motion of the piston into rotary motion of the crankshaft?

  1. Connecting rod Correct answer.
  2. Rocker arm
  3. Camshaft
  4. Flywheel

Explanation

The connecting rod links the piston pin to the crankpin, so as the piston moves up and down the rod swings and forces the crankshaft to rotate. This is the mechanism that converts reciprocating motion into rotation.

Question 9Engine systems diagnosis and repair

An engine has a persistent misfire on one cylinder and the spark plug from that cylinder is wet with fuel. Which fault is most likely?

  1. An over-advanced ignition timing setting
  2. A lean fuel mixture across all cylinders
  3. A faulty ignition coil or plug lead on that cylinder Correct answer.
  4. A restricted exhaust system

Explanation

A wet plug shows fuel is being delivered but not ignited, which points to a loss of spark on that cylinder. Testing the coil, lead and plug on the affected cylinder will confirm the ignition fault.

Question 10Engine systems diagnosis and repair

A timing belt on an interference engine has snapped while the engine was running. What is the most likely consequence?

  1. The engine will simply stop and restart once the belt is replaced
  2. The crankshaft will bend but the valves remain intact
  3. Only the water pump will be damaged
  4. Valves and pistons may have collided, causing bent valves Correct answer.

Explanation

In an interference design the valve and piston paths overlap, so loss of camshaft drive lets pistons strike open valves. This typically bends valves and can damage pistons and cylinder heads, so the head must be inspected before restarting.

Question 11Engine systems diagnosis and repair

A vehicle overheats in traffic but returns to normal temperature at highway speed. Which fault best explains this pattern?

  1. A faulty coolant temperature sensor
  2. A stuck-open thermostat
  3. A weak or inoperative electric cooling fan Correct answer.
  4. A leaking radiator cap

Explanation

At highway speed ram air through the radiator provides cooling, but in traffic the system depends on the fan. Overheating only at low speed therefore points to a fan that is not pulling enough air.

Question 12Engine systems diagnosis and repair

During a cylinder leakage test, air escapes continuously from the radiator filler neck with the cap removed. What does this indicate?

  1. A blocked crankcase breather
  2. A leaking head gasket or cracked cylinder head Correct answer.
  3. Worn piston rings
  4. A leaking intake valve

Explanation

Air escaping into the cooling system pressurises the coolant passages, which points to a breach between the combustion chamber and a coolant jacket. That breach is typically a failed head gasket or a cracked head.

Question 13Engine systems diagnosis and repair

An engine oil pressure warning lamp illuminates at idle and extinguishes as engine speed rises. Which cause is most consistent with this behaviour?

  1. A faulty oil pressure switch that is stuck closed at all speeds
  2. Oil pressure that is marginal at idle because of a worn pump or excessive bearing clearance Correct answer.
  3. An overfilled sump
  4. A blocked oil filter bypass valve

Explanation

Oil pressure rises with pump speed, so a system that is only just adequate at idle will trigger the warning at low rpm and clear as speed increases. This pattern suggests worn pump internals or increased bearing clearance.

Question 14Engine systems diagnosis and repair

What is the correct sequence for adjusting valve clearances on an overhead-valve engine with adjustable rockers?

  1. Adjust the exhaust valves only, since intake clearances are self-adjusting
  2. Set each cylinder to top dead centre on its compression stroke and adjust that cylinder's valves to specification Correct answer.
  3. Adjust all valves with the engine running at idle speed
  4. Set the engine to top dead centre on number one exhaust stroke and adjust every valve

Explanation

Valve clearance must be set when the valve is fully closed on the base circle of the cam. Bringing each cylinder to top dead centre on its compression stroke guarantees both valves are closed so the feeler gauge reading is accurate.

Question 15Engine systems diagnosis and repair

A diesel engine produces dense black smoke and lacks power under acceleration. Which cause is most likely?

  1. Low coolant level
  2. A leaking injector delivering too little fuel
  3. Restricted air intake or a faulty turbocharger delivering insufficient boost Correct answer.
  4. A stuck-open thermostat

Explanation

Black smoke is unburnt carbon from an over-rich air-fuel ratio, so insufficient air for the fuel delivered is the usual cause. A blocked air filter, collapsed intake hose or failed turbocharger all reduce charge air and produce this symptom.

Question 16Engine systems diagnosis and repair

Which measurement is used to check a cylinder bore for taper and out-of-round?

  1. Crankshaft end float measured with a dial indicator
  2. Bore diameter measured with an inside micrometer at several heights and in two perpendicular directions Correct answer.
  3. Piston ring end gap measured with a feeler gauge
  4. Valve stem diameter measured with a micrometer

Explanation

Taper is detected by comparing bore diameters at the top, middle and bottom, while out-of-round is found by measuring at right angles to each other at the same height. Both require an accurate internal measuring instrument such as a bore gauge or inside micrometer.

Question 17Engine systems diagnosis and repair

A petrol engine idles roughly and the vacuum gauge needle fluctuates rapidly at idle. Which fault should be investigated first?

  1. A worn clutch release bearing
  2. A faulty radiator fan relay
  3. A clogged fuel filter
  4. A vacuum leak or a leaking intake valve Correct answer.

Explanation

A fluctuating vacuum reading at idle indicates an unstable intake condition, commonly a vacuum leak or a valve that is not sealing. Both disturb manifold vacuum in a rhythmic way that shows on the gauge.

Question 18Engine systems diagnosis and repair

When installing a new timing belt on an engine with alignment marks, which step is essential before releasing the tensioner?

  1. Confirm the crankshaft and camshaft marks are aligned exactly with their reference points Correct answer.
  2. Coat the belt with engine oil to reduce friction
  3. Loosen the camshaft sprocket bolts to allow the belt to seat
  4. Rotate the crankshaft two turns before fitting the belt

Explanation

The belt must be fitted with the crank and cam marks matched to the engine reference points so valve timing is correct. Only after alignment and correct tensioning should the engine be rotated by hand to recheck the marks.

Question 19Engine systems diagnosis and repair

Which condition causes detonation (knock) in a spark-ignition engine?

  1. Low oil pressure at idle
  2. End gas igniting spontaneously ahead of the flame front under high pressure and temperature Correct answer.
  3. Excessive valve clearance on the intake valve
  4. The spark plug firing too late after top dead centre

Explanation

Detonation occurs when the unburnt end gas reaches its auto-ignition temperature and burns explosively before the normal flame front arrives. High compression, hot spots, over-advanced timing or low-octane fuel all promote this condition.

Question 20Engine systems diagnosis and repair

A technician is diagnosing a no-start condition on a petrol engine. There is good spark and the engine cranks normally, but the spark plugs remain dry. Which fault is most likely?

  1. The fuel pump is not delivering fuel to the injectors Correct answer.
  2. Ignition timing is retarded by ten degrees
  3. The exhaust system is restricted
  4. The battery has a low state of charge

Explanation

Dry plugs after cranking show that fuel is not reaching the cylinders, so the fault lies in the fuel delivery path such as the pump, relay or injector circuit. Spark is already confirmed, which rules out the ignition side.

Question 21Engine systems diagnosis and repair

An engine has been reassembled with new pistons and rings but the cylinder bores were not honed. What is the most likely result during running-in?

  1. The valve clearances will close up rapidly
  2. Compression will be higher than specification
  3. The rings will not seat properly, causing high oil consumption and blow-by Correct answer.
  4. The crankshaft will seize within minutes

Explanation

Honing creates the crosshatch pattern that retains oil and allows rings to bed into the bore. Without it the rings cannot seat, so combustion gases escape past them and oil is not wiped from the bore.

Question 22Engine systems diagnosis and repair

A diesel engine with a common-rail fuel system cranks but will not start, and a scan tool shows rail pressure far below the commanded value during cranking. Which fault is most consistent?

  1. Excessive injector leak-off or a failing high-pressure pump Correct answer.
  2. A worn camshaft lobe on the exhaust valve
  3. An over-advanced injection timing setting
  4. A faulty coolant temperature sensor

Explanation

The engine control unit will not command injection until rail pressure reaches a minimum threshold during cranking. Excessive leak-off from worn injectors or a weak high-pressure pump prevents that pressure building, so the engine cannot start.

Question 23Engine systems diagnosis and repair

A cylinder leakage test on a warm engine shows air escaping from the oil filler cap. Which conclusion is best supported?

  1. Air is passing an exhaust valve into the exhaust
  2. The head gasket is leaking into the coolant jacket
  3. Air is passing an intake valve into the intake port
  4. Air is passing the piston rings into the crankcase Correct answer.

Explanation

The crankcase is separated from the combustion chamber only by the piston and rings, so leakage heard at the oil filler indicates ring or bore sealing failure. Other leak paths produce noise at the intake, exhaust or radiator instead.

Question 24Engine systems diagnosis and repair

An engine shows low oil pressure at operating temperature, and the oil has a strong petrol smell and a thin consistency. Which fault best explains the pressure loss?

  1. A failed oil temperature sensor
  2. A stuck-closed oil pressure relief valve
  3. Fuel dilution of the engine oil from a leaking injector or rich running Correct answer.
  4. A clogged oil pickup strainer

Explanation

Fuel diluting the oil lowers its viscosity, so the pump cannot maintain pressure at operating temperature and the oil smells of petrol. The underlying cause is excess fuel entering the sump, such as a leaking injector or persistent rich running.

Question 25Engine systems diagnosis and repair

A turbocharged diesel engine develops a whistling noise and low boost, and the fault is traced to a split in the charge-air hose. Which effect explains the low boost reading?

  1. Exhaust gas temperature falls below the turbine's operating range
  2. Pressurised air escapes before reaching the intake, so manifold pressure cannot build Correct answer.
  3. The wastegate is forced permanently open by the leak
  4. The turbocharger shaft speed drops because the compressor is unloaded

Explanation

A split charge-air hose vents compressed air to atmosphere between the compressor and the intake manifold. The manifold therefore never receives the full boost the turbocharger produces, so measured boost and power fall.

Question 26Engine systems diagnosis and repair

Which component in a four-stroke engine opens the intake and exhaust valves at the correct time relative to crankshaft position?

  1. The oil pump
  2. The water pump
  3. The flywheel
  4. The camshaft Correct answer.

Explanation

The camshaft carries lobes that press on lifters, pushrods or buckets to open the valves, and it is driven at half crankshaft speed so each valve opens once per two crank revolutions. The relationship between piston position and valve events is therefore set by the camshaft. Other listed components do not control valve timing.

Question 27Engine systems diagnosis and repair

On a four-stroke spark-ignition engine, during which stroke is the intake valve open and the piston moving downward?

  1. The exhaust stroke
  2. The power stroke
  3. The compression stroke
  4. The intake stroke Correct answer.

Explanation

The intake stroke draws the air-fuel mixture into the cylinder as the piston travels from top dead centre to bottom dead centre with the intake valve open. Compression and power strokes occur with both valves closed, and the exhaust stroke has the exhaust valve open.

Question 28Engine systems diagnosis and repair

Which property of engine oil is mainly indicated by its viscosity grade such as 15W-40?

  1. Its resistance to foaming under aeration
  2. Its flash point
  3. Its total alkalinity reserve
  4. Its resistance to flow at different temperatures Correct answer.

Explanation

The viscosity grade describes how thick the oil is and how it behaves across temperature, with the W number reflecting cold-flow behaviour and the second number the grade at operating temperature. Foaming, alkalinity and flash point are separate characteristics not described by the grade.

Question 29Engine systems diagnosis and repair

A technician removes the radiator cap from a cold engine and finds the coolant level low, with no external leaks visible. Which additional check is most useful to confirm an internal coolant loss?

  1. Measuring the battery electrolyte density
  2. Checking the coolant for combustion gases or oil contamination Correct answer.
  3. Checking tyre pressures on the driven axle
  4. Measuring the alternator charging voltage

Explanation

Coolant that disappears without an external leak is often being consumed internally, for example through a failed head gasket or cracked head. Testing the coolant for combustion gases or inspecting it for oil emulsion confirms whether exhaust gas or engine oil is entering the cooling system. The other checks do not involve the cooling circuit.

Question 30Engine systems diagnosis and repair

During a wet compression test, a small amount of engine oil is added to a low-reading cylinder and the compression rises noticeably. What does this result indicate?

  1. The camshaft timing is retarded
  2. The head gasket has failed between cylinders
  3. The piston rings or cylinder bore are worn Correct answer.
  4. The intake valve is not sealing

Explanation

Oil temporarily seals the gap between the rings and the bore, so compression improves only when the sealing loss is at the piston rings or bore. If the leakage were at a valve or head gasket, the added oil would not raise the reading appreciably. This makes the wet test a standard way to separate ring faults from valve faults.

Question 31Engine systems diagnosis and repair

An engine has a loud rhythmic tapping from the top of the engine that increases with engine speed and is worst when cold. Which component should be inspected first?

  1. The clutch release bearing
  2. The starter motor pinion
  3. The crankshaft main bearings
  4. The hydraulic lash adjusters or valve clearance Correct answer.

Explanation

A rhythmic top-end tap that follows engine speed is typical of excessive valve clearance or a collapsed hydraulic lash adjuster, especially when cold before oil pressure builds. Main bearing knock is usually a deeper sound from the lower block, and the starter and clutch bearings are not driven by engine speed in the same way.

Question 32Engine systems diagnosis and repair

A vehicle with a turbocharged petrol engine has a noticeable drop in power and the boost gauge shows low boost, but there is no abnormal noise. Which check is most appropriate first?

  1. Inspecting the charge-air hoses and intercooler for leaks or restrictions Correct answer.
  2. Replacing the engine oil and filter
  3. Measuring the turbocharger shaft end float by dismantling the unit
  4. Checking the exhaust tailpipe for soot

Explanation

Low boost without noise commonly comes from a leaking or collapsed charge-air hose, a loose clamp or a blocked intercooler, which are quick and cheap to inspect. Dismantling the turbocharger is a major step that should follow simple checks. Oil changes and tailpipe inspection do not verify boost integrity.

Question 33Engine systems diagnosis and repair

A diesel engine is hard to start when cold but starts readily when warm, and the glow plug warning lamp stays on longer than normal. Which fault best fits this pattern?

  1. One or more glow plugs not heating Correct answer.
  2. A weak battery earth strap only
  3. A restricted air filter
  4. A clogged fuel return line

Explanation

Cold starting in an indirect-injection diesel relies on glow plugs to heat the combustion chamber; if one or more fail, cold cranking becomes difficult while a warm engine starts easily. The extended warning lamp time points to the glow plug circuit being monitored as faulty. The other faults would affect starting or running regardless of temperature.

Question 34Engine systems diagnosis and repair

When checking and adjusting the ignition timing on an engine with a distributor, which condition should be met before the setting is trusted?

  1. The engine should be at normal operating temperature with any timing connector or vacuum advance disconnected as specified Correct answer.
  2. The air conditioning should be switched on
  3. The transmission should be in gear with the wheels chocked
  4. The engine should be cold and idling at fast idle

Explanation

Ignition timing is specified at a defined engine speed and temperature, usually warm idle, with vacuum or electronic advance disabled so the base setting is measured. Checking cold or with accessories loading the engine changes idle speed and advance, giving a false reading. The other conditions do not establish the specified test state.

Question 35Engine systems diagnosis and repair

During a cylinder leakage test, air is heard escaping from the exhaust pipe. Which component is most likely leaking?

  1. The piston rings
  2. The exhaust valve Correct answer.
  3. The intake valve
  4. The head gasket between two cylinders

Explanation

With both valves closed on the tested cylinder, air escaping into the exhaust port travels out of the exhaust pipe, which points to an exhaust valve that is not sealing. Intake leakage would be heard at the throttle body, and ring leakage would be heard at the oil filler or dipstick tube.

Question 36Engine systems diagnosis and repair

An engine consumes excessive oil but shows no external leaks, and the exhaust emits blue-grey smoke that is worst after idling and then accelerating. Which cause is most consistent?

  1. An over-tight accessory drive belt
  2. A faulty thermostat
  3. Worn valve guides or valve stem seals Correct answer.
  4. A weak fuel pump

Explanation

Oil drawn past worn valve guides or hardened stem seals collects in the ports during idle and is burned when the throttle opens, producing a puff of blue-grey smoke after idling. External leaks are absent because the oil is consumed inside the engine. Fuel, cooling and belt faults do not produce oil smoke.

Question 37Engine systems diagnosis and repair

A timing chain engine has a rattle at the front of the engine on start-up that fades after a few seconds, and the noise has gradually worsened over months. Which component is the most likely cause?

  1. The water pump bearing
  2. The timing chain tensioner or worn chain guides Correct answer.
  3. The crankshaft rear seal
  4. The exhaust manifold gasket

Explanation

A worn chain, guide or weak tensioner allows the chain to slap at start-up before oil pressure fills the tensioner, and the noise worsens as wear increases. It is heard at the timing cover. Water pump bearing noise is continuous and often accompanied by coolant loss, while seal and gasket faults produce leaks or exhaust ticks rather than a chain rattle.

Question 38Engine systems diagnosis and repair

A scan tool on an engine with a misfire shows normal fuel trims but a misfire counter that increases only on one cylinder, and swapping the ignition coil from that cylinder to another moves the misfire to the other cylinder. What conclusion is best supported?

  1. The injector in the original cylinder is faulty
  2. The engine has low compression on all cylinders
  3. The spark plug in the original cylinder is faulty
  4. The ignition coil is faulty Correct answer.

Explanation

When a fault follows a component that is moved to another position, that component is the cause. Because the misfire moved with the coil, the coil is at fault, not the plug or injector left in the original cylinder. Low compression on all cylinders would not produce a single-cylinder misfire that moves with a coil.

Question 39Engine systems diagnosis and repair

An engine is being reassembled after a rebore, and the specified piston-to-bore clearance must be verified. Which method gives the correct clearance figure?

  1. Comparing the old and new pistons by eye
  2. Measuring the piston crown height with a steel rule
  3. Measuring the ring end gap with a feeler gauge and adding it to the bore diameter
  4. Measuring the bore with a telescopic gauge and the piston skirt with a micrometer, then subtracting Correct answer.

Explanation

Piston-to-bore clearance is the difference between the finished bore diameter, measured with a bore gauge or telescopic gauge and micrometer, and the piston skirt diameter measured at the specified point with a micrometer. Ring end gap is a separate ring fitment check and does not give the clearance. Visual comparison and crown height measurement cannot provide this figure.

Question 40Engine systems diagnosis and repair

A diesel engine with electronic unit injectors runs roughly and emits grey-white smoke at idle, and a cylinder cut-out test shows one cylinder contributes little power. Which diagnostic step best isolates an injector fault from a compression fault on that cylinder?

  1. Checking the radiator cap pressure rating
  2. Increasing the idle speed setting
  3. Replacing all injectors as a set
  4. Running a compression test and comparing the suspect cylinder with the others Correct answer.

Explanation

A compression test shows whether the low contribution is due to mechanical sealing loss; if compression is even, the fault is more likely fuel delivery from that injector or its wiring. Replacing all injectors is expensive and unjustified before this basic test. Idle speed and radiator cap checks do not isolate the cylinder fault.

Question 41Transmission, drivetrain and clutch systems

In a manual transmission, what is the function of the synchronizer rings (baulk rings)?

  1. To multiply engine torque before it reaches the gearbox
  2. To lock the differential when one wheel loses traction
  3. To equalize the rotational speed of the gear and the output shaft before engagement Correct answer.
  4. To hold the clutch release bearing against the pressure plate

Explanation

Synchronizer rings use friction cones to bring the gear and the shaft sleeve to matching speed before the dog teeth mesh, preventing grinding during a shift. This speed equalization is what allows clash-free engagement of gears that are otherwise rotating at different rates.

Question 42Transmission, drivetrain and clutch systems

A single-plate clutch in a light vehicle begins to slip under acceleration while the pedal is fully released. Which condition is the most likely cause?

  1. Seized pilot bearing in the crankshaft
  2. Air trapped in the hydraulic release line
  3. Excessive clutch pedal free play
  4. Worn or contaminated friction disc facing Correct answer.

Explanation

When the pedal is fully released the clamping force should hold the disc firmly; slipping then points to reduced friction at the disc, typically from worn lining or oil contamination. The other listed faults would tend to prevent full engagement or cause noise rather than slip with the pedal up.

Question 43Transmission, drivetrain and clutch systems

What is the primary purpose of the clutch in a manual-transmission vehicle?

  1. To keep the engine speed constant regardless of load
  2. To reverse the direction of the output shaft
  3. To progressively connect and disconnect engine torque from the gearbox Correct answer.
  4. To reduce the final drive ratio at high speed

Explanation

The clutch allows smooth take-up of drive from rest and interrupts torque flow so gears can be selected. It does this through friction between the disc and the flywheel and pressure plate surfaces.

Question 44Transmission, drivetrain and clutch systems

In a rear-wheel-drive vehicle, which component transmits torque from the transmission output to the differential while allowing suspension movement?

  1. Drive shaft with universal joints Correct answer.
  2. Flywheel ring gear
  3. Half shaft with constant-velocity joints
  4. Transaxle input shaft

Explanation

The drive shaft (propeller shaft) carries torque along the vehicle and uses universal joints to accommodate changes in angle as the suspension moves. Half shafts are the final side shafts from the differential to the wheels.

Question 45Transmission, drivetrain and clutch systems

A vehicle with a worn constant-velocity joint on the outer end of a front half shaft is most likely to produce which symptom?

  1. A vibration felt only when the vehicle is stationary
  2. A rhythmic clicking noise that worsens during turns Correct answer.
  3. A steady whine that increases with road speed only in a straight line
  4. A grinding noise that appears only when the clutch pedal is pressed

Explanation

Outer CV joints are stressed most when the wheels are turned, so a worn joint clicks or knocks rhythmically during cornering. Straight-line speed whine is more typical of differential or bearing wear.

Question 46Transmission, drivetrain and clutch systems

What does the term 'final drive ratio' describe in a vehicle drivetrain?

  1. The ratio of wheel diameter to tyre width
  2. The ratio of engine speed to gearbox input speed
  3. The ratio of the crown wheel teeth to the pinion teeth Correct answer.
  4. The ratio of clutch disc diameter to flywheel diameter

Explanation

The final drive ratio is set by the crown wheel and pinion in the differential and determines overall torque multiplication and speed at the wheels. It is multiplied by the gearbox ratio to give the total drivetrain ratio.

Question 47Transmission, drivetrain and clutch systems

A technician finds that a manual gearbox jumps out of top gear when the accelerator is released. Which cause is most consistent with this symptom?

  1. Worn engagement teeth and detent spring on the top-gear selector Correct answer.
  2. Worn outer CV joint boot
  3. A leaking clutch slave cylinder
  4. Low oil level in the gearbox

Explanation

Jumping out of gear under overrun load usually indicates worn dog teeth, a weak detent spring, or excessive shaft end float that lets the sleeve slide back. The overrun condition reverses the torque direction and pushes the worn parts apart.

Question 48Transmission, drivetrain and clutch systems

In a hydraulically operated clutch, what is the effect of a small leak in the release (slave) cylinder seal?

  1. The gearbox will select two gears at once
  2. The clutch will slip continuously with the pedal released
  3. The differential will lock during cornering
  4. Pedal travel increases and the clutch may fail to disengage fully Correct answer.

Explanation

A leaking slave seal loses hydraulic pressure, so part of the pedal stroke is wasted and full release travel may not be achieved. The driver notices a spongy or low pedal and difficulty disengaging gears.

Question 49Transmission, drivetrain and clutch systems

What is the main advantage of a limited-slip differential over a conventional open differential?

  1. It limits speed difference between the two driven wheels to improve traction Correct answer.
  2. It disconnects drive to the wheels during braking
  3. It allows the vehicle to steer without a steering box
  4. It eliminates the need for a final drive ratio

Explanation

A limited-slip differential transfers some torque to the wheel with better grip when one wheel starts to spin, improving traction and stability. An open differential tends to send torque to the wheel with least resistance.

Question 50Transmission, drivetrain and clutch systems

Which statement correctly describes the operation of a torque converter in an automatic transmission?

  1. It multiplies engine torque at low speed through fluid coupling between impeller and turbine Correct answer.
  2. It changes the final drive ratio automatically
  3. It uses a friction disc pressed by a diaphragm spring
  4. It locks the engine to the gearbox mechanically at all times

Explanation

A torque converter uses fluid driven by the impeller to turn the turbine, and the stator redirects flow to multiply torque at low vehicle speeds. At higher speeds the flow angles change and the unit approaches a simple fluid coupling.