Rebuild your classic’s alternator

Matt building an alternator

by practical-classics |
Published on

Weekend workshop: save £200 in four hours

We explain how to test and rebuild your alternator for as little as £20

Matt building an alternator



Using a Lucas 15 ACR as a guinea pig, we'll reveal all you need to know to diagnose and rectify faults in a typical alternator. There are many designs, but the basics are the same.

You’ll need


0-75A ammeter

Relay and push switch

Charged car battery


Crocodile clips

Two- or three-legged puller

Soldering iron

Flux and solder

Will take (HR) 4

Will cost (£) 20

Money saved (£) 200


1 The basic principle

When a conductor (the wire) is moved through a magnetic field, a current flows in one direction. Reversing the wire’s direction reverses the flow. The output is increased by having a stronger magnet, using several wires or moving them faster.

2 The rotor

An alternator works by rotating a magnet – the rotor. Its magnetic field moves through static conductors. It comprises of a piece of wire wound around an iron core. The ends are connected to circular contacts, called slip-rings.

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