Relays, what, why, where......

Relays are switches triggered by a seperate wire and designed to handle high current loadings such as you might need when turning on uprated lights or amplifiers, unless so designed, the contacts of traditional switch may suffer from burn-out or other damage caused by arcing across the contacts as they close, this arcing can result in a reduction in the effective contact area of the switch.

If you take a look under the bonnet of any modern motor you will be greeted by a fusebox or relay box bristling with relays, listen for clicks as you turn stuff on and off.

If you have an early Mini you will be the pround owner of a car likely to be sporting only one relay, the one that connects the battery to the starter motor when you turn the ignition key to start. Everything else that is turned off or on is controlled by simple contact switches........

Now if your mini is pretty much standard, the ignition switch and headlight switch could last for years and years BUT since every time you operate it a wee spark is created as the contacts touch, you might well experience the following:

• Car stops at random, everything else is good, turn engine off and it then starts fine, ditto,
• The headlights fail but are ok when you turn the switch off then on again they are back to normal.

.....Mini switches were just never designed to last for 45+ years, and certainly never designed to take the increased current loading generated by uprated lights (either, say 80/100 H4s bulbs or a bank of 50w spotlights), uprated ignition coils and big electric fuel pumps also place an increased load on switches...... what is the answer?

If you can source a relay with a built in fuse, the wiring is easier....... but still put an inline fuse in the circuit close to where you are taking the power from and make sure all runs of wire through your vehicle are fastened down (for safety and in the UK in order to pass the MOT if visible).

Relaying those big spotlights: identify the high beam power feed from the steering column and route to the trigger terminal of the relay, route a suitably fused power feed from the solenoid live side to one side of the relay switch, the existing lights are connected to the other side as are the spotlights

Relaying ignition fed items (uprated fuel pumps, ignition modules etc: isolate the ignition feed from the steering column and route to the trigger terminal of the relay, route a suitably fused power feed from the solenoid live side to one side of the relay switch, the ignition switched items are connected to the other side

Remember that on a bog-standard mini, everything that only operates when the ignition switch is turned to I, II or III will draw its current through the ignition switch!

• My own approach with this was to use the main ignition switched side to trigger a relay feeding what the switch would otherwise have fed (coil, pump etc)

Remember that on a bog-standard mini, everything that only operates when the headlight switch is turned will draw its current through the headlight switch! lay

• My own approach with this was to use the headlight (not sidelight) switched side to trigger a relay feeding what the switch would otherwise have fed (headlamps and high beam on circuit)