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alternator charging wire

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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if my battery is in the boot is it ok to use the starter cable to send alt power back to battery

Yes, very typical in the cars I do.

does the cable between starter and alternator need a fuse?

Stock toyota (90's) had an alternator fuse, but i cant see a need?

I can't see how an alternator could produce more current than the output wire is sized for. I never use a fuse in that circuit.

My factory harness had a fusible-link near the alternator. But in my custom wiring, I did consider, but ultimately did not include anything like that.

@David Ferguson: If there's an intermittent short anywhere along the main alternator wire. Then I'd expect the real problem is most likely to be sparking that could start a fire, rather than excess current melting the wire.

But I'd also be concerned about overloading and melting the windings on the alternator if there is a dead short.

However, any short on that wire would probably stop the motor, since no current would get to any devices. So it's kind of a self-correcting problem :) As long as the motor/alternator stops before anything melts.

i plan to run another cable to power all other circuits, only starter and alt will be using the stareter cable

Depending on the alt', some will use a reference voltage wire that should be routed to the battery, to ensure it's getting the correct charge voltage (can't really see it being an issue, anyway, if the alternator is that type as the cable gauge is going to be rather large.

Some alt's benefit from, or even require, an earth from their body to engine/chassis ground - if it has insulated/anti-vibration mounts it WILL almost certainly need this.

Because I'm a bit anal about seeing at night, I'd suggest running the headlight relays, if used, directly from the alt'/starter terminal to maximise the voltage and hence light output.

I can see why a fuse 'could' be used on the alt'output, but seems to be more of a problem in the making. A correctly fitted output wire shouldn't be at risk of shorting , and OCing a running alt' can cause internal voltage spikes that destroy the diodes and/or voltage regulator.

David, my 10 cents' worth of thoughts.

There can be times when even the OEM alt' causes problems with the OEM wiring. because modern vehicles have rather a lot of electrical equipment that draws on the battery (for example, stop start driving in winter with lights, heater fans, maybe heater seats and exterior mirrors, sound systems, etc, all drawing current) and the characteristics of alt'rs where they have quite low idle output, manufacturers will usually fit 'oversized' alt's so they supply the current required at idle/off idle to keep the battery charged in town driving and expect the voltage regulation to limit the current at the higher rpm where the potential output is higher. With a discharged battery added to the mix, when the engine speed is increased there could be close to the alt' maximum current potential going through the wire, especially if the alt' uses battery voltage reference, as often used to counter excess voltage drop through under-sized wiring. If further current drawing equipment is fitted, like noisier ICE, it can be even worse.

One of my vehicles used plastic insulators in the alt' mounts, for vibration insulation, and a separate ground wire - I double up mine with a spare as the single OEM one had got hot enough to melt the insulation! The output was fortunately a heavier gauge.

This wiring issue may be worsened if a "bigger" alternator is fitted to counter high draw problems without upgrading the cabling.

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