Firstly that 6 volt system

 

A 6 volt system and an unreliable low charging rate are one of the weaker points of the original A7 design. To be generous the system was probably par for the course in its day and up to powering the standard set of lights. Changes in legislation since the fifties and sixties have required twin dipping lights, twin tail lights, greater wattage bulbs etc placing a greater loading on the friable original and often tired components. We also seem to have a hard time with 6 volt batteries, it isn’t easy to get one to fit into the original battery box size under the floor—Ruby ones have loads of room however. They seem to dislike standing even more than 12 volt ones, I suppose one dodgy cell out of six is less noticeable than one out of three. One word of advice, make sure the battery is clamped into position, just think what would happen if, heaven forbid, you turn the car over, you don’t want it moving and shorting. Also fit a cover to stop you dropping a spanner across the terminals. A battery cut out switch is a good move.

 

So much has been written about the Sevens electric system that I am not going to try and give a full guide – there’s so much elsewhere—see the links to other sites. Most Austin Seven manuals go into a lot of detail about getting the charging working. A good fault finding guide was printed in the 1979C & D A7CA magazines a few years back and the Technical Articles collated by the Pre War Austin Seven club has an excellent coverage.

 

If you do get it working, the balance between half and full charge doesn’t always match what you need. One way round this is to set the full charge with the third brush at the most you dare bearing in mind the advice about low charging rates and the reputation of easy burn out. Removing the band over the brushes gets some cooling round the dynamo and gives a bit more margin against overheating. Only use the bare minimum wattage bulbs bearing in mind their safety role and configure the switching so the front sidelights are off when the headlights are on. Set the full charge so that it just balances the ‘night’ load, ie all lights on. Now go to half charge and adjust the half charge resistance to give two or three amps charging for ‘day’ driving. Old electric fire elements make good resistors, the longer they are the greater the resistance. Remove the original and loosely wire in a long replacement. Trim down the length until the desired charging rate is achieved, something like 3 to 5 Ohms are needed. Then wire the resistor into the correct location or neatly somewhere unobtrusive. Remember, the resistor is live, it will have to be electrically insulated. It will also warm up with the current flowing through it, an air flow over it will help.

 

My ultimate advice is to keep everything absolutely spot on and don’t try and take too much from the charging circuit.

12v electrics & the dreaded dynamo

A VW Bosch dynamo and regulator

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· Our modern world revolves around electronics, everything that happens in a modern car is controlled by electronics. They’ve now caught up with our type of car and can do a very respectable job regulating the voltage. One advantage of electronics is there are no moving parts and no contacts that can wear/burn and need setting up. All the switching and setting is inbuilt into the electronics, it can regulate to that perfect voltage—enough to give a charge but not too much to gas off the battery. A disadvantage is that if visible it is obviously ‘out of period’, however it is so small, something between the size of a box of matches and a fag packet, that it can be hidden under the dash. It is possible to fit one in the large terminal box on some of the C35A dynamos and I’ve even heard that they can be fitted in the dynamo amongst the brush gear. Another disadvantage is that they can’t have the polarity changed like the traditional set up, if you’ve got a positive earth regulator you have to change it to a negative earth one if you want to change polarity. These are also available in 6v and 12v so there’s further options there.

 

· For those who like making everything themselves there’s some circuit diagrams of electronic regulators. I got the positive earth one from a motorcycling magazine, apparently old British bikes also had small Lucas three brush dynamos, 12v and electronics is popular with them as well for the same reasons. Barry Riseley and Ron Hayhurst have kindly redrawn the circuit to convert it to negative earth. There’s another negative earth circuit again taken from a bike site, this one is more complex but apparently mirrors a modern alternator regulator and has the potential for adjusting the voltage.

 

· One thing worth mentioning is that the only time we’ve had problems with an electronic regulator was when we ran a car with a virtually flat battery, I don’t think it could cope with the continual high charging and went phutt/kapput. We always charge a battery well up before running now. I believe it’s possible to limit the charging rate in the electronics, I don’t know if the design for a bike has a limit in to suit a small bike battery. Either way, a charging rate fuse seems a good idea. If you do a lot of night driving and worry about the regulator carry a spare.

12 volts – why go that way?

 

Ask anyone who’s had charging problems! The A7 system can be a pain and anything like normal motoring mileage needs reliability, not many places offer a 6 volt recharge when you need it and carrying a spare battery is not only a nuisance but potentially unsafe. One of the big bonus’s of 12 volts is the starting. You don’t need to change the starter motor, when it is fed 12 volts it spins the engine very fast and starting that reluctant engine becomes easier. In addition most modern electrical components are 12 volts so if you want to use such things as electronic rev counters, fuel pumps, water pumps, electronic ignitions or even a full blown stereo system, 12 volts is the way to go.

 

Technically it’s also a good move. For the same wattage equipment, bulbs etc, doubling the voltage halves the amps needed to be generated in the dynamo. My O level physics has stuck with me, the heat generated is proportional to the square of the amps flowing. Halving the amps potentially reduces the heat by a factor of four.

 

To change, you obviously need a 12 volt battery. There are number of small ones available, most small modern cars use this size. Mounting it could be a problem so think of that in advance, if you haven’t got a battery box fitted yet get one that fits the battery size. I believe there is a 12 volt size for lawn mowers that even fits into a standard 6 volt battery box.. Negative earth is the norm with modern installations and most modern equipment is wired this way. Rotate the battery in the box to give the best lead locations and fasten it down. Now look at the other electrical components, you will need a 12 volt coil, charging system, light bulbs, trafficators etc. Things that don’t need changing are the starter motor, it just runs faster, the distributor and the wiring. Small motors like the wiper will probably need changing, they seem OK running faster on 12 volts but do get hot so it’s best to change them. Another item I’m not sure about is the petrol tank sender and gauge– any advice out there? Most Lucas components were available in 6v and 12v and look the same. You can find replacements at Autojumbles or new through suppliers.

Modifying the dynamo

To make these work on a 12v system you have to move from three brush to two brush. Simply remove the third brush—that’s the one that isn’t 180 degrees from another, it’s usually a very thin brush. Remove the brush holder and spring etc from the end plate. This will leave a lead from the end of the field coils that normally connects to the third brush, depending on which circuit you’re using it has to be connected to either the ‘live’ brush terminal or the ‘earth’ one. Some circuits regulate the voltage by controlling the field at the ‘live’ or ‘top’ end, others do it at the ‘earth’ or ‘bottom’ end. Check your circuit out before making this connection in the dynamo.