First: confirm it is actually the alternator
The classic symptoms — battery warning light, dimming headlights at idle, a battery that keeps going flat — can also come from a failing battery, a corroded earth strap, a slipping belt, or a wiring fault. Replacing a working alternator because of a bad earth is a common and expensive mistake.
Before buying anything, have the charging system checked properly: battery condition and state of charge, voltage at the battery with the engine running and under electrical load, belt condition and tension, and the main charging cable and earth connections.
Visual inspection
- Housing and mounting ears. Look for cracks, particularly around bolt holes. A cracked mounting ear is not repairable in any way you would want to rely on.
- Rear cover and regulator area. Melted plastic, discoloured metal or a burnt smell indicate the unit has overheated.
- Connector and terminals. Corroded or heat-damaged terminals, spread pins or a damaged plug housing are all reasons to walk away.
- Cooling vents. Heavy debris packing in the vents suggests hard service and poor cooling.
- Signs of previous repair. Mismatched fasteners, sealant, paint overspray on the body. Not automatically disqualifying, but you want to know.
Pulley and bearing check
Spin the pulley by hand. It should turn smoothly and quietly, with no roughness you can feel through your fingertips and no side-to-side play in the shaft. Rumble, grinding or notchy resistance means the bearings are worn.
If the unit has a free-wheel or overrunning pulley — common on more recent vehicles — it should drive in one direction and free-wheel in the other, without slipping or locking in both. A seized overrunning pulley causes belt noise and premature belt wear.
Check the pulley face itself for wear ridges, and count the grooves. Groove count and diameter must match your application.
Bench testing
This is where an alternator earns its place among used parts you can buy with reasonable confidence. On a proper bench tester, the unit is driven at a controlled speed and its output measured. A good test tells you:
- Whether the unit charges at all
- Whether output is stable across the speed range rather than only at high revs
- Whether the regulator holds voltage within the correct range
- Whether there is excessive ripple, which points to a failing diode
- Whether output collapses under load
A unit that charges but only weakly, or whose voltage wanders, has a fault developing even if it would technically start the car.
It confirms the alternator charges correctly today. It cannot predict remaining brush life, bearing life or how the unit will behave after another two years in an engine bay. This is exactly the gap a replacement warranty is meant to cover — and why we would rather explain the limit than claim a used part is “guaranteed”.
Confirm the specification
Beyond condition, check that the unit is right for your car:
- Output rating. Printed on the label, typically as something like
12V 90A. Fitting a lower-rated unit to a car with a higher electrical load leads to chronic undercharging. - Pulley type, diameter and groove count.
- Connector type and pin count. Two-pin, three-pin and LIN-controlled smart-charging alternators are not interchangeable.
- Mounting pattern and bracket geometry.
Before you fit it
The single most common cause of a replacement alternator failing early is that the underlying fault was never fixed. Fitting a good alternator to a car with a failed battery, a damaged charging cable or a poor earth will damage the new unit, often quickly — and that is not a warranty claim anywhere.
- Test and, if necessary, replace the battery first
- Check and clean the main charging cable and all earth connections
- Inspect the belt and tensioner, and replace them if they are worn
- After fitting, check charging voltage at the battery with the engine running and under load
What we do before an alternator leaves us
Visual inspection of housing, mounting ears, connector and rear cover; pulley and bearing assessment by hand; free-wheel pulley function where fitted; bench testing for output and regulator behaviour; and photographs of the actual unit including the part number label, sent to you before dispatch.