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Use of Relays within the Vehicles Electric's


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Hi All just started sorting out my mountain of spaghetti electrical wiring :blink: and wondered if anyone could tell me what typical items use/need relay's within their circuits? I hope someone any assist. thanks

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Relays are designed for switching higher currents that you wouldn't want to put through a switch. Things like the headlights on defenders are switched with just a big switch, but they benefit greatly from being fitted with relays. I assume the series vehicles would be the same.

Other things you would want to fit relays for are things like driving lights or an air compressor.

Generally speaking, I'd be wanting relays for anything over about 10A? There are more electrically experienced people on here who can give you a better idea though.

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What's the vehicle? What are you trying to achieve?

Are you re-wiring using a new loom?

For standard vehicles the only relays that I can think of were a relay in the horn circuit for S2's or 2a's fitted with the optional twin horns (Wow) and some on S3's and military lightweights to get the brake test light to come on when the key was turned. If its a standard vehicle with standard components it doesn't need any.

Lights may benefit from one - but remember the standard setup doesn't have any fuses so won't fail by reason of a fuse blowing. When I've fitted relays in the light circuit I've used one for each filament and fused each one. (Fused relay bases are easy to get and use) The reason for using them is the dipswitch mounted in the centre of the steering wheel on early S1's and the sheer length of wiring on a S3 together with the steering column mounted wand control.

Basically if you're running any device that's going to use a heavy current its worth using a relay - especially if its sharing the ignition switch and its wiring.

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If a switch is up to the current it is carrying/switching, there' no REAL need to relay it, yes you will save your switch contacts a little, but you will just transfer the burning to the relay, so 6 of one, half dozen the other.

Relays are used to allow low current switches to switch larger loads, like heated screens, compressors as mentioned above etc, and can benefit light output on standard vehicles.

There are other reasons to do it, like you can have your relay situated close to a large cable, and the switch a long way away, this means you aren't pulling fat cables up through the dash to a switch, which can be tricky, but also expensive, as thicker cable contains much more copper, and as we know metals prices have soared in recent years!

Other benefit is less voltage drop as your cable run length will be shorter, meaning you get more power to where you want -this is shown in the example of relaying the headlights on a Defender.... cable run goes from alternator/starter down the wing, through a relay and to the headlamp. Previously it would have run all the way up to the stalk on the steering column, from the fuse box, through the bulkhead, and then down each wing to the headlamps.... and probably with thinner wire as well, making the situation worse again.

If you want just standard functionality in your loom, I would just buy a new loom, so much easier for trouble-shooting at a later date. They work, and have worked for many years in millions of LR, so if it ain't broke....

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Oh, and as Jeremy says, if switching things on with ignition live, that is another very good reason to have a relay, otherwise you are likely to be overloading the ignition switch very quickly.

Same goes for driving lamps, don't just wire them in parallel with your main beam :rtfm:

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Same reasoning for relays used to protect known vulnerable components that are less convenient and more costly to replace than relays.

example - you can use relays to save wear and tear damage to 90/110 column switch which are known to fail. The light switch is now only switching a few hundred milliamps rather than the high current start up surge of a cold halogen filament. If you do this it is worth fusing the coil circuit at a much lower value than standard as you are protecting the switching device not the cable.

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As a mafter of course on series rewires I fit relays to the dip and main beams. I think it's too much to expect 30 year old switches to cope with the power of 55 watt bulbs, which will probably be upgraded to 100 watt crystals after a nights driving.

G.

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