Weekend Workshop

How to restore your classic’s alternator to as-new condition

Check and repair your alternator.

Alternators call for no routine maintenance and are generally ignored until they stop working or start making funny noises. The common solution is to throw the whole unit away and fit a new or reconditioned replacement. Many faults, however, can be fixed at home for little or no money. New parts – if needed – are available for many classic alternators and they aren’t generally expensive.

Before removing a ‘faulty’ alternator, clean and test its earth (even if it earths directly through its mountings) and its other electrical connections. Look for dirty, loose or corroded connectors, blown fuses, etc.

The alternator should supply 13.3V to 14.4V within a couple of minutes of the engine starting. The most common failure is of the built-in regulator. Carbon brushes may wear out after long service, too. You can often cheaply replace the whole brush/regulator unit. At the same time, spin the rotor and dress the copper slip rings with fine wet-and-dry paper. You should be able to do this without dismantling the unit. Blow out dust before reassembly.

Worn bearings may squeal, rumble or seize, making the fan belt shriek or break. Rarer defects include burned or broken windings and/or diodes and severe corrosion of internal electrical parts.

Wiring variations

The generating (stator) coils have three wires connected to the rectifier diodes. If there’s a fourth wire, it’s a central junction in the windings that goes to the no-charge warning light. We’ll refer to this as the ‘neutral wire’ later on. Our demonstration alternator is a common six-diode design. It takes current from the ignition circuit to the rotor, via the regulator. Some later ‘self-exciting’ alternators have three extra (smaller) diodes and have no such external charging circuit – just a built-in regulator. They don’t have a ‘neutral’ wire, as such.