HOW TO SURVIVE AT NIGHT IN AN AUSTIN SEVEN

By Nik Plevan

(Written in Oct 1993) - reproduced from the club magazine - prices will no longer be current !!!

"Turn on your headlights, please" said the NOT tester.

"They are on" I replied.

"Hmmm. . ." he said, "I’ve seen better lights on a moped. I suppose you’ll no be taking it out in the dark".

The Scots have a dry sense of humour.

"Ah... Actually, I’m driving non-stop from John OGroats to Lands End in a couple of weeks"

I said, reaching behind my seat for a bottle of Halt.

"Is that a bottle of Highland Park you have there? Aye. perhaps they’re not as bad as I thought. . ." he re-considered, thumbing his chin and eyeing-up the whisky. "Och aye, that'll do fine!"

Sound familiar? ‘Nell, perhaps things are not quite as bad as that, but it cannot be denied that

Austin Seven 6 volt electric's just aren’t up to today’s traffic conditions. In fact, the humble

Austin Seven dynamo has never really been up to the job.

Let’s look at the facts. At night, in the rain, the battery/dynamo combination of a Box saloon has to supply I 20 Watts of power:

Ignition coil (engine running)    2 Amps              =12 Watts
Side lights                              2 Amps 2 x 6W  = 12 Watts
Rear lights                             2 Amps 2 x 6W  = 12 Watts
Head lights                         12 Amps 2 x 36W = 72 Watts
Wiper motor                         2 Amps              = 12 Watts
TOTAL                                 20 Amps            120 Watts

Now, even if the dynamo could supply 120 watts (which it can’t), at that current the ignition panel switch assembly would melt and the dynamo would burn out because of the excessive heat generated by such a high current (heat generated in a wire is proportional to the square of the current passing through it). So, it would seem that there is nothing for it but to have your dynamo re-wound for 12 volt operation, or to fit a VW beetle unit, or some other expensive and/or unsightly conversion. But there is another solution. The laws of physics tell us that power is equal to voltage times current (i.e. Watts = Volts x Amps) so that, for a given  wattage, if we double  the  voltage    we can halve the current. In our case, that means that if we could run the electrical system on 12 volts, we could reduce the current required to deliver 120 watts of power to 10 amps.

Consequently, the dynamo would operate much more efficiently and its working temperature would be reduced. Sounds simple? In practice it is that simple, although the electrical theory is much more complex and outside the scope of this article. So, how can we convert an Austin Seven dynamo designed to run at 6 volts to 12 volts? It turns out that this is equally simple. The Seven dynamo uses what is knows as "third brush control" to restrict the voltage it delivers to the battery to about 6 volts. In other words, without the third brush, the dynamo’s output voltage would rise and keep rising until being limited by the speed of rotation of the armature up to a maximum voltage which is above 12 volts and therefore sufficient to charge a 12 volt battery. This happy state of affairs can easily be achieved by merely removing the third brush altogether and re-arranging the internal connections of the dynamo. Having arranged things so that the dynamo operates on 12 volts, we have to find a way to control this voltage (it must not rise too much or the battery may be damaged) and also replace the original cut-out because this only works at 6 volts.

Luckily, modern electronics are available which take care of both of these operations in a single "black box" which has only three wires: two are connected to the dynamo and the third goes to the battery via the ignition switch. What could be simpler than that. In addition, the box is only 2" x 2" x I and can easily be either mounted out of sight or inside the cover of the old Lucas cut-out unit which is no longer needed. These "black boxes" are supplied by Mick Kirkland of the Northern branch of the Seven Workshop at £39.99 (+ p & p) and come with full fitting instructions which include how to convert the dynamo to 12 volt operation.

Two types are available: negative earth for Chummies, Boxes, and pre-1936 Rubies, or positive earth for post 1935 Rubies. Apart from working at 12 volts, these units can also be switched to 6 volt operation and are therefore an ideal replacement for burnt-out or damaged 6 volt cut-outs. Now we have 12 volts, but your lights will still not be very good because the reflectors on Austin Seven head lights are not a very good design and usually require expensive re-silvering. A much cheaper and better solution is to fit modern reflectors into your original shells. I have replaced the reflectors in the CAV Chummy head lights on my 1926 Gordon England "Brooklands" replica with Yamaha (horror of horrors!) sealed-beam motorcycle units for about £12 each (available from your local motorcycle shop). These have no side light bulbs because the GE has separate side lights mounted on the wings, but a Lambretta unit with side light bulbs is available for pre-1929 Chummies. For 1929 Chummies and later cars, Mick Kirkland supplies the right size of modern reflectors (either with or without side light bulbs) for about £22-£24.

Fitting is simplicity itself. You remove the modern glass lenses from the sealed-beam units and replace them with the glass lenses from your original head lamps which you fix in place with silicone sealant (bath sealant!). When this has set, you either use spring clips to fix the modified sealed-beam units into the rims or, if this proves difficult, you can "glue" them in using silicone sealant again. Make sure that you keep the 12 o’clock position of the sealed-beam unit coincides with the 12 o’clock position of the rim when locked in place as this is critical for "dip / main beam" to work properly. Believe me, it’s simple,it works, and the silicone sealant keeps out the dirt and wet. So, if you invest in a 12 volt battery, a 12 volt coil, a 12 volt wiper motor (you could fit ballast resistors for both the coil and the wiper motor and use your original 6 volt units see your local Lucas agent) new reflectors and bulbs, and the £39.99 "black box" from Mick Kirkland, you’ll be able to run your Austin Seven on 12 volts.