With the engine running and a substantial electrical load applied - measuring both voltage and AC ripple, not just voltage at idle.

Measure the voltage at the battery terminals with the engine running and confirm it rises above the resting battery voltage.

Then apply load: headlights on main beam, blower on maximum, heated screen, and confirm the voltage holds rather than collapsing.

Measure AC voltage on the same terminals; a healthy alternator produces almost none, and a significant AC reading indicates a failed diode.

A current clamp on the alternator output shows what it is actually delivering against its rating.

On a managed charging system, compare against the vehicle's own commanded values with a scan tool, because a low voltage may be intentional.

It lets alternating current onto the vehicle's supply, producing electrical noise, flickering lights and unpredictable electronic behaviour - and it hides from a simple voltage check.

The alternator generates three-phase alternating current, which the diode pack rectifies to direct current.

A failed diode leaves part of that AC waveform on the supply, which is what a meter's AC range detects as ripple.

Symptoms include lights that pulse at engine speed, radio interference and, on modern vehicles, apparently random fault codes from modules upset by the noise.

Charging voltage can still look acceptable, which is why ripple must be measured separately.

A diode can also fail short, allowing a parasitic drain through the alternator that flattens the battery overnight.

Because at idle an alternator delivers far less than its rated output - a vehicle idling with heavy loads can be in continuous deficit.

Rated output is measured at a speed well above idle, and output falls steeply as speed drops.

A utility vehicle idling with beacons, work lights, a compressor and a heater can draw more than the alternator produces at that speed.

The shortfall comes from the battery, so it discharges while the engine runs, which surprises people.

The answers are a higher-output or high-idle-output alternator, a raised idle speed, or managing the loads.

This is a specification problem rather than a fault, and it is worth calculating the actual load before adding equipment to a vehicle.

A bearing whine that rises with engine speed, a rhythmic growl from a failing diode or winding, and belt-related squeal that is not the alternator at all.

Bearing wear produces a whine or rumble that tracks engine speed and is often loudest when the electrical load is high.

A growl that changes when loads are switched on suggests an internal electrical fault rather than a bearing.

A squeal on start-up or under load is usually the belt slipping, not the alternator - though a seizing alternator bearing causes exactly that squeal.

A rattle at idle that disappears as revs rise can be a failed overrunning pulley, which is frequently blamed on the tensioner.

Spin the pulley with the belt removed: roughness or noise confirms the bearing, and it should turn freely with the overrun in one direction only if fitted with a decoupler.

A one-way clutch in the alternator pulley that decouples the rotor's inertia from the crankshaft's speed fluctuations.

A diesel engine's crankshaft speed fluctuates measurably within each revolution, and the alternator rotor has significant inertia.

Rigidly coupled, that mismatch loads the belt and tensioner heavily and creates noise and wear.

An overrunning pulley lets the rotor continue at its own speed during deceleration phases, smoothing the drive.

When it fails it can seize, giving the noise and belt wear it was fitted to prevent, or slip, giving low charging output.

Test by turning the pulley by hand with the belt off: it should drive in one direction and freewheel smoothly in the other.

Remanufactured exchange units are the normal route; individual repairs make sense for brushes, a regulator or a pulley on an otherwise sound unit.

Brush packs and integrated regulators are frequently available separately and are straightforward to replace.

A worn brush set is a common failure with a distinctive symptom - intermittent charging that improves when the engine is revved.

Diode packs and bearings are also replaceable, though the labour and the special tools start to favour an exchange unit.

Remanufactured alternators come tested, with a warranty and a core charge, and for a fleet the predictability is usually worth more than the saving.

Whichever route, fit a new belt and check the tensioner and pulley - a new alternator on a worn belt does not fix a charging complaint.

By calculating the continuous load at the engine speed the vehicle actually runs at - not by comparing peak ratings.

List every load the vehicle carries and its current draw, separating continuous loads from intermittent ones.

Compare the continuous total against the alternator's output at the relevant engine speed, which for a working vehicle is often idle.

Include the base vehicle load, which on a modern vehicle is substantial before anything is added.

Leave margin for battery recharging; an alternator exactly matched to the load never recovers the battery.

Where the deficit is only at idle, a high-idle-output alternator or an automatic fast-idle system is frequently a better answer than a larger unit.