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Amperage Draw On Common Components

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I am rewiring my ECU and Chassis Harness. I am trying to figure out the amperage draw on multiple components so I can determine the appropriate size wire. I have created a document to keep me organized and I will attach. I think I have determined a rough estimate of voltages and amperage. but I have some gaps. i wonder if someone can help me to cover some of the missing pieces.


Sensors (Cam, Crank, ECT...) = 5v @ 0Amp

Injectors Power from Relay - 12v @ 4-6Amp

Injector ECU LSS (The Low Side of the Injector, triggered by the ECU) - 12v @ 0Amps (all the power would be drawn from the injector right?)

Idle Speed Controller Power - 12v @ 4Amps

Idle Speed Controller ECU LSS - 12v @ 0 Amps

Igniter Trigger from ECU - 12v @ 0 Amps (negligible - pretty sure this is another LSS)

Igniter Power From Relay - 12v @ 10-20 Amps (Read this somewhere, but i feel this is high)

Coil Pack Power From Relay - 12v @ 2A

CoilPack Power From Igniter - 12v @ 2A

I know I am off somewhere, if someone could help that would be great.

Attached is what I have worked on there are 5 tabs (Connectors,Pricing,Tools,Wiring,For Sale) Look at the Wiring Tab.

Attached Files

The current on both sides of the injectors will be the same.

Do you mean general ignition power relay or just for the coils?

Depending on what you have coils could pull 7A each.

Thanks for the post Slides

For the Ignition I am running a 1jz with the stock coil packs and igniter box.

the wiring goes like so

from the ECU -> igniter box -> coil packs

Main EFI Relay -> Coil Packs

So I am assuming the Relay -> Coil Packs is the majority of the power

ECU -> Igniter -> Coil Packs (is a trigger so low Amp?)

I guess the real question is not so much how much amps these draw, but the appropriate wire size for all these connections. currently the gauge wire looks to be around 16 or 18.

I will be using Raychem Spec 55 wire. which I believe can handle a bit more current than standard wire which i have been running.

G'day Drew.

As Michael mentioned, the current entering any device will be the same as what is leaving it, so the power supply wire to a device (Injector, Coil, etc) will be the same size as the wire on the other side, heading back to the ECU in the case of injectors / idle speed control, or the igniter in the case of the coils.

Your spreadsheet is bloody awesome work, keep up that level of attention to detail and you're going to have a killer wiring harness!

Your current draw estimates seem to be a bit on the high side though. For example, each injector is going to draw closer to 1A than 4A. Pretty sure the stock JZ injectors are high impedance? Even if they were low impedance, Toyota would have a ballast resistor in the circuit to bring the current draw back down.

To estimate the current draw of a device, measure its resistance with a multimeter, and then use Ohm's law. I = V/R. Assume 13.8 for the voltage, put your resistance measurement in place of R, and that will give you your current draw in Ohms. Your injectors will have a resistance of somewhere around 12 Ohms, giving you 13.8V / 12Ohms = 1.15A.

Its not quite this simple though, as most of the devices you're powering will be pulse width modulated, meaning the averaged current draw over an engine cycle is actually lower than this. Have a read of this thread to see what I mean:

For some rules of thumb, I typically supply injectors and wire all sensors using 22AWG wire. Coils are supplied with 18AWG, fuel pump and fan with 14AWG. These need to be double checked, but are a good starting point.

@zac.perston. thanks for another great response to my question. I am trying to get as many questions on this harness answered before i start any work or buy any parts/tools/supplies.

I am currently running FIC 750cc Low impedence injectors with a supra mk3 resistor box. so i believe the ohm would be between 2-4. I'm not at the shop so I can't check the resistance. You mention that the resistor box is like a ballast? I'm trying to understand how this works and I use the analogy of electrical current like water a lot. I mean it's in the name the resistor box is resisting the current between the injectors and the ECU so it doesn't fry the circuits. so does that in effect also reduce the amperage on the entire circuit? So can I effectively say that the current on each injector will still be ~1.15A?

Yes, you're on the money there. Resistor box = Ballast box, same thing.

If you measure the resistance of one of the resistors in your resistor box, and then add in the resistance of your injector, you have your R value to work out your peak current. In reality this is a bit of a simplification, as we're ignoring the fact that the injector is an inductive load, so the current that flows through it also depends on how long a voltage has been applied to it.... but for sizing your wires, ignoring this part of the equation it totally fine :-). Plus, you know, calculus.