Weekend Workshop

Make sure your dynamo or older alternator charges as it should

How to service your voltage regulator at home.

If your classic car has a dynamo or an older type of alternator, it’ll have an electro-mechanical voltage regulator. This turns on and off the current that’s supplied to the generator’s field coils, and thus regulates its output. They’re elegantly simple, but they can go wrong over time. Faults commonly lead to too low a charging voltage and, ultimately, a flat battery.

Regulators rarely stop working altogether, but they can lose their calibration due to wear or a physical shock. Also, the contacts may become dirty, tarnished or suffer metal transfer from one contact to the next, which is perfectly normal. A little TLC will very often get a tired regulator functioning like new. You’ll need nothing more than a voltmeter, wet-and-dry paper and a few small tools. The only real skill required is that of leaving the more mysterious bits of the regulator (resistors, etc) well alone.

Some regulators have a second function built-in. For dynamos, a separate set of contacts – the reverse-current cutout – opens to stop the battery discharging through the dynamo’s generating coils when its output is low. The no-charge warning light is also operated by these contacts. If a second contact set is fitted to an alternator regulator, it will solely operate the warning light – the diodes inside the alternator prevent reverse current flow.