Obvious place to try first would be Austin owners club. Also what model Austin would probably be helpful, or did they only make one model in 1930?

This company rebuilds 3 brush dynamos and cut-out boxes to use their own electronic regulator.

They seem to know what they are talking about.

https://www.dynamoregulatorconversions.com/about-me.php

On 6 July 2026 at 12:26 Robert Butler Said:

The original post specifies an Austin 12

According to the 1930 illustrated Austin catalogue I have There were several Austin 12 models.  There was the Austin 12-4 which was a model that originated in the 20s, sometimes known as the Heavy 12-4.  Then there was the new model the Austin Light 12-4 and, on the same chassis was the Light 12-6.

I once owned a Light 12-6 “Harley” Saloon De Lux but that was in the early 1970s and I’m afraid that I can’t remember many details except that it did have the Summer/Winter setting on the dash.

Russell

The dynamo will almost certainly be a Lucas.

The car probably did not have a Regulator, only a cut out, to disconnect the battery from the charging circuit at low speeds when the dynamo voltage would be less than 12 volts.

The output of the third brush dynamo is set by positioning the third brush relative to the other two.

The output will be limited, probably not much more than 10 – 12 Amps.

The charge rate is controlled by two resistors, swtched by the “Off”, Side”, “Head” switch

They might well be positioned in a holder on the engine side of the bulkhead (To allow the heat generated to disperse)

These will control the voltage applied to dynamo field, through the third brush and hence its output.

In the “Off” postion, both resistors are in series with the field, minimising charge rate.

(It only has to cope with load of the ignition system)

In the “Side” postion, the higher value resistor will be shorted out, increasing the output to accomodate the current load of the Number Plate, Side and Tail lights and probably the instrument panel lights.

Probably 12 volt, 6 watt lamps; say 30 watts, possibly 36 with the panel lights.

In the “Head position, both resistors will be shorted out for maximum output from the dynamo.

In addition to the 30 wats load of the Side/Tail position, it should be able to cope with two 36 watt headlamp bulbs (When on main beam, only one on Dip)

My prewar Singer 9, like many other cars, used this system, and properly set up, it works well.

HTH

Howard

10 to 12 amps thats pushing it!

Where are you located John? If near I could bring over a rheostat and do some tests. I’ expect a value between 0.1 ohm and 5 ohms.
I checked my contemporary references and while they talk about the resistance none mention a value. Even if they did, very few garages, never mind owners would have had the ability to measure such low resistance. They would just change the unit.
If you aren’t near I could send you some fixed values to try.

Robert.

If you can track down a copy, Automobile Electrical Repairs by E. Alber (E. & F.N. Spon, London, 1938) is very imformative, and was well-written by somebody who really knew his stuff.  I haven’t looked to see if it covers this exact question.

George

Good result!

Some British motorcycles of the 60s and 70s had a similar charge control, the alternator had 6 coils arranged radially with a rotating permanent magnet. With the headlight on all 6 were in circuit, with it off only 2 in circuit. Later on a power zenner diode was put across the output to limit the voltage. Crude but not a lot to go wrong, no brushes to wear at least. I can’t imagine it did the battery any good

On 9 July 2026 at 12:40 John MC Said:

Slightly off topic, would the OP consider converting to 12 volts?   A friend did this with an Austin light 6.  If originality is important read no further!

If you mean an Austin Light 12-6 that was originally fitted with a 12 V system.  I think the only 6 volt ones were the ten and the seven.  Converting an Austin 12-6 to use an alternator may well have made it more reliable although I never had any problems with the original electrics.

Russell

The early electrical systems did not provide voltage regulation as this was largely provided by the lead acid battery. When this reached the fully charged state any excess current would be “boiled off” by the battery producing hydrogen and oxygen. So the ammeter shows whether the generation/load balance is correct. However a high ammeter current indication can be produced if a discharged battery is being recharged or the battery is being “boiled”. So the chauffeur would normally have a container of distilled water to refill the battery when need, which might be several time in a journey. This regular charge/discharge cycling meant batteries had fairly short lives and if a cell shorted the lights were dim and the generator was overloaded and failed if the chauffeur wasn’t on top of his game.

Aren’t you glad that electronics now manages everything.

Clive S