Electrics

Lucas, in their wisdom, really did a number when they designed the Tiger's electrical system. At least it's easy to change the fuses - there are only two! Most modern automobile manufacturers employ fuses combined with a variety of relays, resistor links etc. to protect the car's wiring. How did Sunbeam provide protection for the electrical system using only two? They didn't!

Fuses

The purpose of any fuse is to provide a weak link in an electrical circuit which burns open before the wiring does in case of an overload or short. The size of the fuse is expressed by the amperage rating and indicates the maximum current which can be carried before it opens. If the rated value is exceeded, the fuse link opens, stopping the current flow. High-current rated fuses may not protect circuits with small diameter wires since the wires may burn before the fuse can blow. Conversely, a low-current rated fuse in a large diameter wire circuit may blow even though the circuit was never in danger. Unprotected circuits have no fuses and thus will burn at the weakest point if the current load exceeds the capacity.

Your Tiger, like MGs and other mid-60s British jewels, has a potentially serious problem in these unprotected circuits. Which ones? Most! A glance at the wiring diagram in the Shop Manual shows that the following circuits are unprotected:

The only fuses in the entire electrical system consist of one wired directly to the battery (to protect the horns and map light) and another wired through the ignition switch (to protect the directional lights, back-up light, instruments, windscreen wiper and heater blower motor).

Fusing

Before discussing the circuits to protect, a few words about fusing are in order. The standard automotive fuse is of the metal ribbon type. The current rating of the fuse is established by the diameter or thickness of the metal ribbon. The large 35-amphere fuse has a pretty big ribbon, whereas the one- or two-amp fuses have a fine wire. Although this type of fuse is widely used and adequate for most applications, there are two disadvantages which may limit its use. Mechanical damage can cause the fuse to fail prematurely due to vibration (for this reason, critical circuits are protected instead with fusible-link wire). Fusible link wire looks like conventional rubber-coated auto wire, but is made of special wires which burn out if the rated current is exceeded. The wire is resistant to mechanical damage and can be spliced and soldered in the usual way. It is available in several wire gauge sizes and is found in most auto electric shops.

The second disadvantage of the conventional metal ribbon fuse is that it opens immediately, and in circuits which draw high peak current (e.g. cooling fans) may blow in normal use. This problem is solved by using a special type of slow-blow ribbon fuse.

Fusing corrections

There are lots of ways to correct the deficiencies in the Tiger's electrical system. My approach requires minimal effort, consisting of adding a Lucas fuse block, nine in-line standard ribbon fuse holders, and three fusible link wires. Slow-blow fuses can be substituted in other circuits as you see fit.

In my attack on the Tiger's wiring deficiencies I decided to place in-line fuse holders on each exterior light (except directional and brake which are already fused). Fuse holders are available from most auto parts stores, but beware of holders used on car radios as the plastic may not withstand the heat in the engine compartment.

With the help of the Tiger wiring diagram it should be easy to locate the appropriate wires. The wiring for the headlights and front side lights runs along the hood under the inner fender. I placed three in-line fuse holders on each side, staggered to keep the bundle smaller. Attach snap-in plug type connectors to each holder and remove the wires from the existing snap connectors and insert the holder. You will need an extra female to complete the fusing. Check each circuit as it is reconnected.

The tail lights on the rear feeder can be fused in much the same way, using the connector at the left side of the dash. Fusing for the fuel level sender can be omitted. The fuel pump needs fusing in a different manner (see below). The indicators and brake lights are already fused.

The final step is the addition of another Lucas fuse block (available from most Tiger parts dealers). The extra block allows you to separate the circuits so a short in, say, the temperature gauge doesn't wipe out the tachometer. You might also consider buying two and replacing the existing block, which may be corroded after years of service.

In the stock set-up, the fuse with the brown (in) and purple (out) wires is powered directly from the battery and is hot all of the time. The other fuse (green and white) is powered from the ignition switch. Locate the new, extra fuse box beside the existing box and re-arrange the load so that one becomes battery-powered and the other switched. From the input sides (brown and white) make a short jumper cable to supply power to the other set, thus making more circuitry independently fused (see figure below).
Diagram of wiring

Thus far, we have doubled the number of fuses in the main circuits and installed individual fuses on each exterior light circuit. For additional protection in the headlight / tail light circuit I installed a fusible link in the main power feed into the dash mounted light switch, using the appropriate spade connectors. The size of the fusible link wire must be the same as the wire size of the circuit in order to provide proper protection. In addition to protecting the entire exterior light circuit, this fuse link also protects the instrument lights. (You can't use a conventional glass/metal ribbon fuse here because of the mechanical damage problem outlined above).

Also install a fusible link wire to protect the ignition feed wire, which runs from the ignition switch through the tachometer and ballast resistor to the coil. Terminal fittings may need juggling to keep the installation neat. As the stock electric fuel pump has no fuse protection, install a fusible link in the harness at the rear feeder connection at the left side of the dash in the white wire.

With all these extra fuses, labelling each one is a good idea! And make sure each circuit is tested as it is installed.