Fusing error

Hi!

You might have noticed I keep on coming back and I hope I am not a bother, the car is finally at a wiring stage. Which begins with lots of research, documentation and questions.

I stumbled upon a little issue, that might not be an issue in reality, but sure seems to be in theory.

Attached a sketch of a main relay (ignition coil relay) diagram.

The relay is 30A which should suffice for coils that according to the internet draw UP TO 7A each. I don't believe I will ever get that high with the goal of 400ish HP and a reasonable RPM ceiling.

If need be, I can uprate the Relay itself to 40-50A without changing anything, the wiring will be big enough.

Main relay was supposed to feed power for all engine requirements. Injectors, coils, solenoids.

Someone along the line suggested I implement a separate Relay for ECU power (though the manual doesn't call for it, only a 3A fuse). They wanted me to hang Injectors onto that ECU Relay as well. It had to do with calibration of dead times if I am not mistaken. A 5A fuse should support 6 injectors and the ECU (instead of 3A).

That still leaves VSS, WBO, Mac and VVTi power requirements on the Main relay. BUT! Coils need thicker wires, a capable relay and therefore a bigger fuse (30A, like the relay?)

Those other loads however require barely any current in comparison. Also use thinner wires.

Surely I can't protect 0.5mm2 wires with the same 30A fuse, right?

How is this normally wired? I would appreciate a diagram if you have one. I was thinking, going from MREL 87 to the splice, then to Coils and the other branch back to the fuse box into a 3-5A fuse. Then finally to the solenoids.

The goal is to simplify. This doesn't seem simple.

Fuse separate circuits after the relay is also a good practice, especially if there are high (relatively) and low current circuits being powered off the same relay.

I've powered the injectors separate from the ECU and haven't experienced any issues, in theory the entire system voltage should fluctuate about the same, and with engines being imperfect machines, a little variance between what the ECU sees and what the injectors see for voltage isn't going to cause drastic changes in fuel delivery, and I'm sure you'll find that even in a perfectly wired system, there will be some level of voltage difference between the injectors and what the ECU reads.

It's a common practice to use a relay and circuit breaker (fusable link or otherwise) with a large capacity to supply power to other sub-circuits. Especially if using an ECU with limited connectivity options - in some cases even a supplimentary PDM may not be sufficient.

However, as has been pointed out, lower current rated wiring and devices won't be protected by the main fuse, so each sub-circuit would normally have an additional circuit breaker to protect that circuit.

It is going to depend on what level of re-wiring, complexity and budget you are working around, but you may find it easier to use the vehicle's original fuse/relay box(es) for some of the circuits, with the additional benefit it's already wired for that (those).

If not, you can use the high current circuit to feed a bank of fused relays close to the point the devices they're powering.

Oh, with the coils, they aren't all drawing full current all the time, it will depend on their dwell' duration. Same with injectors, but that is going to vary with the duty cycle and, to be 'safe', you should plan around a 100% loading for all.

As I was reading your answers, it hit me, I forgot to say this is a groud up wiring job. Everything done anew by me, including fuse box.

The other thing that hit me is the fact that my fuse box won't allow me to bring a new wire in, fuse it, then out again. It's one of those power distribution type fuse boxes, where a thick cable comes in then splits to all fuses.

I might have to dedicate a fuse for solenoids and injectors directly, without a relay. Do I expect any problems this way? Any back feeding or anything?

Mind you, it's all after the master switch that turns everything off at thee same time since it's coming from the same fuse box. ECU, injectors, Solenoids, IGN coils, Fuel pumps. The fact that ECU even controls Mrel and fuel pumps in my design is simply to satisfy Ecumaster manual requirements.