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Can someone check my engine wiring harness?

EFI Wiring Fundamentals

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I’ve attached a basic hand drawn engine wiring harness diagram. It’s a V8 and has 2 x wasted spark coils.

Can you check the way the relays are wired up and the fuses are correct for the current draw I’ve calculated.(in attachment also)

Thanks in advance.

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I've never seen relay triggers wired that way. Does that work?

Connecting your relay coils in series like you did will not work. Battery voltage will be divided (or drop) by the number of relays connected to the circuit (as long as they have the same resistive load) so each relay coils will get approx 4v right now.

You need to provide power and ground to each relay coils separately (known as in parallel).

As Frank said, and if "series" and "parallel" doesn't make sense, there are some basic guides on-line, such as https://www.youtube.com/watch?v=sbocWMTmPK8

Substitute the relay solenoids for the bulbs, and it should be made clear.

Thanks everyone.

This is my first loom, so all the help coming my way is much appreciated.

A bit more studying and I’ll get back to the drawing board.


Here’s my redrawn V8 engine loom. I’ve put the relays in parallel all controlled by the main relay. I hope this one is better? Any help, advise or ideas is much appreciated.


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That's the way I would do it for a street car. For a race car, the Master Switch would go where you ignition switch is, and I have a separate ignition switch that triggered the Ignition Relay. That signal would also go to the ECU as a "Run/Stop" switch input.

Hi David,

yes, it’s a street car.

I would like to make one more modification to the diagram. The main ECU +12v power comes from the power/battery side of the main relay. Due to the cable only carrying 1amp, can I move this cable to the ignition switch side of the relay instead of earth? And just leave that side of the relay to turn the other relays on.(See updated diagram attached)

Would this make any difference? It would clean up the wiring a little with one less splice on the power/battery side of the relay.

Thanks again

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I would leave the permanent 12V feed to the ECU on a fused circuit directly from the battery or kill switch, if used, with it being 'turned on' with the ignition switch - depends on the wiring of the ECU being used, though.

Primarily because the ECU can then control things that one may like to continue running for a short time after the ignition is turned off, such as coolant pumps, cooling fans, if turbo-charged a delay to keep cooling oil (and water if water cooled) may be a good idea to extend the bearing life and reduce possible coking of the oil, etc.

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