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Why use a relay when a switch will do?


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It's an article on www.polevolt.co.uk that I was told was a well worth read and explains relays.

Only problem is that I cannot open it. :angry:

I'm having to put relays in the Series IIA / Range Rover Hybrid where there weren't any before.

For lights, wiper motor, etc. just the basics.

The old Series wiring diagram did not have any yet I wire up the lights etc. as is, the switches start getting a bit hot.

Or maybe it's just the wiring is incomplete and as soon as everything is connected up I might not need any. :unsure:

Dose anyone here know what I'm talking about? 'cause I certainly don't! :blink:

How dose a relay work? where dose 30,85,86,87 go?

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OK, you're starting from basics by the sound of it, so:

The more powerful an electrical device, the more amps (current) it draws. To handle more amps you need bigger wires otherwise they get hot and melt. The same is true of switches - they are designed to handle a certain power, beyond that they get hot and melt = not what you want.

Now, you could just buy a bigger switch, but at some point that gets too big and clunky / expensive, plus you are running really big wires all over the palce to keep things going.

So, a relay is a switch that is turned on or off by electricity - a small electro-magnet moves a contact inside. This is useful because the magnet uses a very small amount of electricity (usually less than 1 amp) to move the big clunky contact that can handle a BIG current (usually 30-40 amps) which is more than any reasonably sized switch.

This means you can use a small and cheap switch on your dashboard, with skinny wires as all it has to do is supply 1 amp to the relay coil. Then you just need a big fat wire from the battery to the relay, and from the relay to the load (your fog lights or whatever). Because you don't need to touch the relay, it can be somewhere convenient, which means you can run the fat wires more directly - saving space, and saving wire.

There are several standard types of relays, they are all on VWP's website:


The diagrams show them in their "off" state, the 85-86 contacts are the magnetic coil, and 30, 87, 87a and 87b contacts are the switch part.

If you're new to all this electrical gubbins can I humbly suggest you put FUSES in every circuit, as close to the battery as possible. Land Rover don't do a very good job as standard and the glass fuses are cack.

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A big thanks to FridgeFreezer, his explaination of relays was spot on.

I asked many and never got such a straight answer.

Western's diagrams will be of good use.

A lot of the old Land Rover switches are faulty, weak leaves suffering from water contamination and verde gris so I had to buy new ones, I decided on rotary for the lights and indicator stalk switches for the wipers and signals.

I can get back to my truck now with a better idea as to what I'm doing.

Once again thanks guys!

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