Caring for your engine’s alternator

We barely give a thought to the spark plug. All it does is supply a gap over which a spark jumps. But when you consider its working environment, the plug certainly deserves respect. The nose of the insulator surrounding the central electrode must operate between 350 and 700 degrees C. Too low a temperature will result in carbon build-up and, as carbon is a good conductor, it is not wanted there.

Conversely, if the temperature is too high, pre-ignition is likely. We can get an idea how a plug is running by examining it immediately after a full power run. If the insulator nose is clean and white, it is running too hot and, if black, too cool. The ideal colour is light brown. Use only the plugs recommended for your vehicle.

Charging systems have changed. At one time we had a magneto and dynamo. But, as electrical demands grew, the system was inadequate. The battery handles starting, lights, signals and ignition, and eventually it all became too much for the battery/dynamo setup and alternators appeared.

Unlike the dynamo, even at idle the alternator is charging the battery which is certainly a boon when stuck in traffic for any length of time. The electronic regulator plays a part too because it controls output better than the dynamo and allows the use of maintenance-free batteries. The alternator is not as heavy as a dynamo, has a longer service life and requires less maintenance.

Alternators require an occasional check for dirt on the slip rings and brush wear. When a fault appears, examine cables for splits and breakage and all connections. Check drive belt for signs of slippage. Check the charge warning lamp; if this blows the alternator may not start charging. With battery on charge, look for excessively gassing cells or terminal corrosion.

For basic testing you will need a moving-coil voltmeter. Add a hydrometer, plus a high and low-range ohmmeter and a DC moving-coil ammeter .

Beware; a thermostatically controlled fan can start spinning even though the ignition is off. Before connecting/disconnecting main alternator leads disconnect the battery earth cable - and remember to switch the ignition off before connecting/disconnecting any vehicle cable.

Connect a 0-20V voltmeter across the battery terminals then switch on all loads except the windscreen wipers because wipers may scratch a dry screen. Run the engine at 3000rpm for three minutes. Provided the battery is OK, it should show 13.5V after a minute or two. If it doesn’t, suspect the alternator.

Rotor slip ring brushes can stick in their holders, so clean them and ensure free movement. Minimum brush length varies from system to system, so check the handbook. Corroded or loose battery connections or a poor earth can cause diode failure, as can even momentary connection of the battery the wrong way round. Note that disconnecting the alternator or battery while the engine is running will also cause failure.

The parts which may fail in an alternator are the power and field diode pack, rotor and stator windings, brushes and bearings. When an alternator is more than a few years old, I believe, rather than messing around with individual parts, it is better and to get a reconditioned unit with a guarantee.