When the car is being driven, electricity is generated by either a dynamo (early cars) or an alternator (later models) driven by the crank pulley belt, and is used to provide energy to create a spark at the spark plugs, to operate the electrical equipment such as lights and wipers, and to keep the battery fully charged.
The battery provides a source of electricity to operate the starter motor, and to make up the shortfall in generated electricity when the car is on the move and demand exceeds the available supply.
The battery can provide a great deal of power, despite being 'only' 6 volt (early cars) or 12 volt, because it can provide a high amperage, or current. To help visualise just how much power a battery can generate, remember that it is possible — don't try this except to move the car to safety in an emergency — to move the car by placing it in gear and turning the engine over using the starter motor. In achieving this feat, the battery is supplying not only the energy needed to turn over the engine but also to physically move the car! One terminal of the battery is connected to earth; that is, to the bodyshell/chassis of the car, This allows every electrical device on the car to also be earthed, so that just one feed wire is needed to complete a circuit to the battery and provide power for the device concerned. Most electrical faults — incidentally — are caused by poor earthing, so always check the earth connection first. If a wire which is live (connected to the live terminal of the battery) touches any part Of the car which is earthed, current will flow through the wire to earth.
When the battery is connected to a circuit which has very little resistance (i.e., when it is shorted to earth through a wire) current passes through the wire at the battery's maximum amperage. Not even the meaty wire which connects the battery to the starter motor is capable of passing this current, and as a result the wire which shorts gets hot very quickly, burns off its insulation and an electrical fire ensues.
The ability of a wire to carry electricity safely is determined chiefly by its cross sectional area, and so wires which have to carry more current (starter motor feed. high-tension (spark plug) wires and to a much lesser extent the headlights) are thicker in section than wires which have only a small current to carry. The smaller the diameter of a wire, the more easily it catches fire if too high a current is passed through it.
To prevent wires carrying dangerously high currents through shorting to earth or some other electrical fault, mast circuits of cars are fitted with fuses. A fuse is a
short piece of un-insulated wire of a thickness design to melt if the current passing through it exceeds a ' certain level. In the interests of safety, the lights and
ignition are not normally fused though Beetle lighting systems usually are. Whenever a fuse blows, it is not a fault in itself but a symptom of a problem'elsewhere in its circuit. Most people simply replace blown fuses — sometimes with fuses of higher rating — but this-is a dangerous practice, because it allows too high a current to pass through the wiring which the fuse is intended to protect, and that wire could overheat and start an electrical fire because too highly rated a fuse has been fitted.
If a fuse blows, always trace the fault before fitting a new fuse. Never replace the fuse with one of a higher
. rating or worse with bits of tin foil — unless you enjoy being inside a burning car.
For the same reason, never add extra electrical devices to a fused circuit, Either the demand will outstrip the fuse's safety level causing it to blow or, if a higher rated fuse is foolishly fitted, it can cause a fire in the existing wiring if a fault develops. What is an electrical fire like? The author can answer this one — a minor incident, thankfully — from first-hand experience. Within a few seconds of a wire shorting to earth on a non-fused circuit, the insulation melts and begins to give off choking fumes which can fill the interior of a Beetle to the extent that you literally cannot see your hand in front of your face within maybe thirty seconds. If there are any combustible materials in the vicinity of the fire, these too will ignite from the heat and within a minute or two of the short to earth occurring the car could be full of flames in addition to smoke, which would by now have rendered any occupants of the car unconscious. If the fire includes brake fluid or fuel lines, the car will be a burnt-out shell in minutes.
What this is all leading to is the advice that, unless you are a qualified and experienced electrician, it is best to leave electrical problems and the fitting of extra electrical devices to an auto-electrician.
Restoration and electrics
Car wires are carried tightly bound in the loom. In order that you can, with the aid of a wiring diagram, trace which wires go where, the wires are colour coded. Many elderly cars have at some time been fitted with extra lengths of wire, either to replace damaged lengths or to power an extra device. Sometimes, the wires are of an appropriate colour and rating, but often they are not.
A damaged loom (frequently caused by welding too Close to the loom and melting off the insulation, often caused by shorting to earth which also burns off the insulation) is best replaced — even though looms are far from cheap to buy and anything but easy to install. Damaged individual lengths of wire outside the loom may be replaced with others of the same colour and diameter, except in cases where the damage is melted insulation and the wire runs into the loom, because the wire (and adjacent wires) in the loom could also have melted insulation.
Wire damage includes anything which bares the copper core or anything which reduces its effective internal cross sectional area, such as being pinched so that some of the strands of wire break but others survive. Check all visible wiring in the car for damage, and also check that spade and bullet connectors are insulated — it is not unknown for an un-insulated connector to drop off its terminal and start a fire.