Fuses

A very good question. It is amazing when you look at the number of fuses in a modern car, but apart from corrosion in the fuse holder the three seemed to work reliably.

As a crude rule of thumb a fuse should be twice the expected maximum current. The wiring from the factory was fused as follows:

A1-A2 35 Amp - Protects all the circuits that are live with the ignition switch off, includes headlamps, horn etc. The headlamp alone account for around 10 amps and maybe twice that or more when turning on from cold so the 35 Amp fuse is required.

A3-A4 35 Amp - Protects all the circuits that are only live when the ignition is turned that are not protected by fuse A5-A6 includes the wipers, voltage control box (dynamo cars), ignition, starter solenoid, electric fuel pump (Mk1), heater blower, heated rear window etc plus anthing else that may have been fitted such as radios. Again some of these items such as the wipers exhibit high surge currents on turn on so the 35 Amp may be a little generous but 15 Amp could be a little on the low side for reliable operation.

A5-A6 35 Amp - Protects the indicators, side lights and brake lights, number plate lights etc. The 35 Amp rating appears generous but you must also anything else that has been connected - some of these components only operate with the ignition on.

Too high a fuse rating can lead to burned out wiring if a fault develops. I recall a neighbour's Triumph had a short where the sidelight live feed rubbed against the body because of a missing grommet and the wiring acted as the fuse with lots of smoke and a horrible repair. It could have been much worse. On the other hand, too low a fuse rating may lead to failure and the associated dangers if the lights or indicators fail. Given that option I would go for the original fuse ratings and carefully check the wiring especially where it can be abraded or is corroded. You are working with the limitation of just 3 fuses and their current rating is probably close to or exceeds that of some of the old wiring to the individual components and connectors.

It is good practice to use a separate fuse for any accessories that are added to the car and in the case of high current items such a spotlights they should be wired using a relay to avoid overloading the ignition switch and improve their efficiency.

David

There are a number of UK cars which have had the fusebox replaced. The plastic becomes brittle with age and they are hard to find. That gives you the opportunity to sub-divide circuits further and add things more easily. Against is the "look" of the thing.

Otherwise, clean and shiny terminal clips and nicely pinched up tight is as good as you can get.

David's comments regarding relays is a sound one - high and low beam and horn is worth doing at the minimum. You can even buy relays with integral fuses, which simplifies wiring further.

Chris.