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UK parts supply and recommendation for relay

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I have some tidying up to do, I've added two more gauges to my car and need to achieve two things:

1) Gauges light up with car lights

2) AFR gauge is on/off from a switch (it's noisy!)

So I thought easiest job is to take the feed for my head unit that is ignition on/off and lights dimmer on/off, and use those to control two relays. See diagram attached.

I bought some relays from Amazon, and they're massive and sound loud, seems a rubbish.

Can anyone recommend something I can buy in the UK please? Can I use MOSFETs? It sounds like my cheap relays will use a lot of power to stay switched, not much compared to what my alternator chucks out of course but still I'd like to do a nice job. I can fit these up behind the dash, and guess I'll use relays with in built fuses.

Many thanks

Attached Files

Your local car parts store (can't recall what you call them in the UK - motor factors?) should have what you need. Failing that, there are numerous other sources, such as Demon Tweeks, RS and Farndon (been a while, so may have that wrong) with the latter two being rather more expensive.

You may wish to reconsider the wiring diagram, darned if I can see how it is to work?

Thanks Gord.

Yeah motor factors, they're all a bit weak though. I was hoping someone might be able to recommend a proper online motorsport electrical place, and a brand to look for. Who would a rally team buy parts from? I see Bussmann is a brand to look for, but we don't have them here. RS are a good shout though and I'll check out Farnell thanks.

I've found https://www.autoelectricsupplies.co.uk/ as a basic place (not very motorsport orientated) which will do what I need for now. I'm going to try their micro relays and see how I get on.

As for the diagram, it might just be my poor schematic skills. Basically the car head unit iso connector has a "dimmable" feed that tells the headunit to go darker when the lights are on, this will trigger a relay that feeds the bulbs in my gauges. The iso connector switched on will trigger a relay that will provide the power to my afr gauge, and I'll have a switch inline to turn it off as it's annoyingly loud. I've already bodge it up in the car and it works great, just the components I used are nasty, hence my post.

If anyone can recommend an automotive electrical supplier with all the fancy components I'd appreciate it.


Relays are a very common item, I don't think there's a parts shop down here that doesn't carry them. If you haven't done so, you can also google "Auto electricians", or similar, in your area or, if you have an on-line or hard copy, check out the yellow pages - assuming they're still a thing there.

Is there any reason you aren't simply taking the gauges' and head unit's lights' power from the park, or main beam, terminal of the headlight switch? If you want to be able to leave the park lights on and avoid having the gauges illuminated and using power, the latter option would do that.

If you're concerned about the current through the switch, you can use a single, fused, relay switched from the HL switch terminal or convert to LEDs - that actually opens up colour choices, too, for example red to reduce the affect on your night vision.

With a single supply for all the gauge' lighting, it's also easier to incorporate a dimming rheostat, if desired.




On top of those youve got RS components, farnells, Tweeks and most local factors will be able to order from Durite.

You are correct, the standard motor factors and generic automotive wiring supply places don't stock this stuff here. Drives you mad. I cant see the video because i dont have the course, but i presume you are looking for an ISO micro relay. Put that into google and you'll have better luck finding them. You can get them from RS components, Farnell, Mouser, little fuse.. etc... check the data sheets, make sure you get one with the right configuration and 12v because they can be a minefield those websites, if you dont know very specifically the thing your looking for.

This is what my local auto' parts store carries - https://www.repco.co.nz/en/globes-batteries-electrical/electrical-accessories/flashers-relays/c/1862918004?q=%3Arelevance&text=# - for "relays", with the type you're looking for being the "mini" type.

As you can see there're quite a few variants, but my preference for headlight wiring (most common application) is to use a permanently attached and wired socket, with a fused relay. Most sockets can be interlocked for a neat appearance, or there are relay socket blocks available in the aftermarket for a series of relays, some with integrated fuses. With the fused relay/socket, it's very quick and easy to check and replace if required - especially if using an electronic circuit breaker 'fuse'.

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