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Wiring relays to battery question

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Do we have any examples of how the battery terminal connects to the bank of relays.

I've been through the course and seen there is the holder for the relays and fuseboxes. How are these wired up?

If we are sending one wire from the battery to this holder of relays would we size it based on the current draw for all the circuits together or can we use a generic size?

My OEM car uses a main battery fuse a big 80a would it be worth incorporating something like this in the power supply design or is it unnecessary?

In terms of a road car would main be the first step of ignition barrel and enable be step 2 with 3 being crank? Is there any sort of diagram which would show how this is step up as it would greatly help my understanding to visualise it. As with ECU back feeding the main relay powers the coil on the enable relay. So I'm guessing we could switch relay coil to ground off the ignition barrel?

Sorry for the meaty question just trying to get my head around these concepts.


If you're going to use the battery as the source for main power feed to fuse box/relay bank, then you size the wire for the potential maximum current draw, and then size the fuse to protect that wire.

Most ECUs provide a ground to control the relay (though some have selectable positive trigger too), and you would use a switched source, such as the one powering the switched wire on your ECU to provide power to the relay coils. For ECU outputs that are positively switched, you can simply ground the relay coil, there is no "backfeeding" when it comes to ground in this scenario. You only need to worry about backfeeding when using a ground controlled output.

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