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Power Supply Design - Help Required

Practical Motorsport Wiring - Club Level

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Hi All,

Just finished the EFI Wiring Fundamentals and Practical Motorsport Wiring courses - really enjoyed them! I'm now looking to build a loom for my rally car.

Bit if background of what I want to plan to do to help context:

Replace entire loom, ecu etc with standalone management and fully re-do loom. My plan was to make two separate looms - one for the engine side, digi dash ecu etc and a seperate one for the lights, indicators, horn etc. The car has to run an Autolec switch due to regs hence it being included in the diagram attached.

So I've just completed my first draft of what I think will be the power supply wiring and was hoping someone could take a look at it and verify that it's correct, or correct me!!

If theres anything anyone would change please comment as I'm bound to have got something wrong - but wanted to rectify and learn before proceeding. (Note I am awaiting a new ICV to measure resistance due to my existing one being broken)

Thanks in advance,


Attached Files

Forgot to add, will be using no key -

Start procedure will be - Autolec on -> Ign On -> Start Button :)

Most of your diagrams look ok, except...

You have the fuel pump and fan on the whole time. It is more normal for the ECU to switch the relays for pump and fan. This is so you can switch the fan only when needed according to temps, and save a whole lot of current draw. Also if the fuel pump is switched by the ECU, the pump will automatically switch off when the motor is not running.

Your start relay is unusual, not sure what is happening there. I don't know why a start relay needs to be switched by the ECU. (My car doesn't even have a start relay).

Hey Jonny,

Thankyou for taking the time to reply I appreciate it.

I've redrawn the diagram with the errors made corrected. I accidentally put the ECU to control the starter solenoid as got the relay terminals mixed up in my head. The ECU now controls the FAN relay and Fuel Pump relay as it should I believe. I would be most grateful if you could verify my revised design as it's nice to get a second opinion before I proceed.

Attached Files

I have to assume that your "autolec" is a FIA type battery kill switch.

There is a problem with the wire going to your Z and 2 terminals. Inside the switch the Z terminals go to an on/off switch, and this is used to interrupt the power to one of your devices (any device you choose). The way it is in your diagram it does not cut power to the ignition. Either delete the wire or make that switch something.

Other than that wire, everything else looks good.

Yeah it is one of the 6 pin FIA red key ones.

If I understand what you're saying correctly:

'Z' destination should be Pin 30 on the Main Power relay.


'2' destination should be joined to the wire on the ignition switch (switched live to Pin 86 on Main Power Relay)

Once again,

Thanks for your help

Here Is a simple diagram how to wire your battery disconnect switch. And I can suggest you to take power to the relays directly from battery feed, not via battery switch, as sum. of all currents (like ignition coils, Fans, pumps, and starter) can exceed current that can handle second circuit of battery switch.

Attached Files

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