Used alternator: how to buy one that works
To buy a used alternator that works first time, match the manufacturer part number stamped on your old unit, confirm the amp rating is the same, and check that the pulley type and the wiring plug match your car. An alternator is one of the safest used parts to buy because it either charges or it does not, and a genuine unit from a known donor car costs far less than a new or reconditioned one. The only real risk is ordering a version built for a different engine or equipment level, and this guide shows you how to avoid that.
How do I know my alternator has actually failed?
Before you buy anything, make sure the alternator is the problem. The classic signs are a battery warning light that stays on with the engine running, headlights that dim at idle and brighten when you rev, a whining or grinding noise from the front of the engine, or a battery that keeps going flat even though it is fairly new. The quick test is a multimeter across the battery terminals: around 12.6 volts with the engine off and roughly 13.8 to 14.4 volts with it running points to a healthy charging system. If the running voltage is barely above the resting voltage, the alternator or its regulator is at fault. Also check the auxiliary belt and tensioner first, because a slipping belt gives the same symptoms for a fraction of the cost.
How do I match the right alternator to my car?
The same model often left the factory with several different alternators depending on the engine, the gearbox and the equipment fitted. The surest match is the manufacturer part number stamped or printed on the casing of your old unit, for example CV6T10300FA on a Ford, 03L903023 on a VW group diesel or 13222931 on a Vauxhall Corsa D. Match that number to the number in our listing title and the guesswork is gone. Alternators also carry the maker's own number from Bosch, Valeo, Denso or Mitsubishi, which works as a second check. No number to hand? Send us your registration and we will identify the correct unit for your exact engine.
Does the amp rating matter?
Yes. Cars with more electrical load, such as air conditioning, heated front screens, a diesel engine with glow plugs or a large infotainment system, were fitted with higher output alternators, and the rating in amps is usually printed on the casing. Fitting an alternator rated lower than the original can leave the battery undercharged on short winter journeys, with a flat battery and dim lights as the result. Fitting a higher rated one is not usually harmful but may not mount or plug in the same way. The part number pins the rating down for you, which is another reason to match it rather than buying on model name alone.
What should I check on the pulley and the plug?
Two physical details catch people out. The pulley must be the same type and have the same number of belt grooves as your old one. Many modern cars use a one way clutch or overrunning alternator decoupler (OAD) pulley rather than a plain solid pulley, and swapping types causes belt noise and premature wear. The wiring plug must match your loom exactly; the same alternator family can come with two, three or four pin plugs depending on the car's charging management. Hold your old unit next to the listing photos and compare both.
| Check | What to compare | Why it matters |
|---|---|---|
| Pulley type | Solid, or one way clutch (OAD) | Wrong type causes belt noise and tensioner wear |
| Belt grooves | Count the ribs, usually 5 to 7 | Belt must sit fully in the pulley |
| Plug | Pin count and connector shape | Loom will not connect to the wrong plug |
| Mounting ears | Position and bolt holes | Must line up with the bracket |
Does my car have a smart alternator?
Most cars built since around 2010 with stop start or a battery management system use a smart alternator, where the engine control unit varies the charging voltage depending on load and battery state. These look like a conventional alternator but carry a signal wire in the plug (often labelled LIN or COM). You must replace a smart alternator with another smart unit of the same part number; a conventional one will not be controlled properly and will trigger warnings. Matching the part number handles this automatically, and if you have an AGM or EFB battery on a stop start car, fit the same type of battery when you change it, otherwise the system will overcharge it.
What should I check on a used alternator before fitting?
Look at the casing for heavy corrosion or cracks, especially around the mounting ears, which take all the belt tension. Spin the pulley by hand: it should turn smoothly with no roughness or grinding from the bearings, and on an OAD pulley it should free wheel one way and drive the other. Check the plug terminals are clean and the main output stud is intact. Our listing photos show the casing, the pulley and the label, and we inspect every alternator before listing. When you fit it, use a new belt if yours is glazed or cracked, and check the tensioner at the same time.
What we usually have in stock for alternators
Stock changes daily as cars arrive and parts sell, so the electrical and electronics collection is the place to see what is live now. Alternators we have listed recently include:
- Vauxhall Corsa D 1.0 petrol alternator, 13222931
- Ford Fiesta Mk7 1.2 petrol alternator, AV6N10300HA
- Ford Focus 1.0 EcoBoost alternator, CV6T10300FA
- Ford Grand C-Max 1.6 diesel alternator, AV6N10300GE
- Peugeot 308 1.6 BlueHDi alternator, 9678058680
- Peugeot 208 1.2 petrol alternator, 9806007480
- Audi A4 2.0 TDI alternator, 03G903016G
- Audi A6 2.0 TDI alternator, 04L903017C
- BMW 118d E81 E87 alternator, 12317802261
- Mercedes B180 W246 1.8 diesel alternator, 32082150
We also list alternator wiring looms, belts and tensioners for many of the same cars. If your unit is not showing, send us your registration and we will check the yard. Our guides to buying a used turbocharger and checking a used engine follow the same approach for the bigger jobs.
Buy with confidence
Every alternator we sell comes off a known donor car at our Doncaster yard, is inspected before listing and is photographed so you see the actual unit, not a stock image. Tracked UK delivery is included on every order and is free on most parts; panels, engines and gearboxes carry a charge shown on the product page. Order before 2pm on a working day for same day dispatch. If a part is not right, you have 30 day returns. Send us your registration and the part you need and we will confirm fitment before you buy. Parts worth your trust.
Frequently asked questions
How do I match the right used alternator?
Match the manufacturer part number stamped on the casing of your old unit to the number in the listing, because one model often had several alternators depending on engine and equipment. The Bosch, Valeo or Denso number on the label works as a second check. If you have no number, send us your registration and we will identify the correct unit.
Does the amp rating of an alternator matter?
Yes. Cars with more electrical load were fitted with higher output alternators, and one rated too low can leave the battery undercharged, especially on short winter journeys. The amp rating is usually printed on the casing and is fixed by the part number, so matching the number keeps the rating right.
What about the pulley and wiring plug?
The pulley must be the same type, solid or one way clutch, with the same number of belt grooves, and the plug must match your loom pin for pin. A unit that is electrically right but has the wrong pulley or plug will not fit cleanly or will cause belt noise. Compare both against your old alternator before ordering.
What is a smart alternator and does it change what I buy?
A smart alternator is controlled by the engine ECU and carries a signal wire in its plug. Most stop start cars have one. You must replace it with a smart unit of the same part number, not a conventional alternator, or the charging will not be managed and warnings will appear. Matching the part number covers this.
What should I check on a used alternator?
Check for a clean casing with no cracks around the mounting ears, a pulley that spins smoothly without bearing noise, clean plug terminals and an intact output stud. We inspect and photograph every alternator before listing. Fit a new belt if yours is glazed or cracked and check the tensioner while you are there.