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Wiring for LS swap. Battery, alternator and start up.

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Hi guys!

trying to wrap my head around the wiring of my ls swap. The car is a empty shell and I’m trying to simplify everything as much as possible.

I have two sections of the wiring I’m trying to get my head around. I’m very new to wiring.

For the starter and alternator loom, I’m confused on what size wire I need to run. Will it be ok to run from the battery to a isolator then from that to a post where I’ll split to the starter and Alternator which will all be ran in 0 gauge. Is this too big?

The next part I’m confused out is the power up and start of the engine. I have a switch and push button start I want to use. From my understanding from the course and other forums. The Haltech terminated harness (what I’m using) for ls1 has a 12v feed wire which needs to be ran to a switch. I’ll run power from the battery to the switch which will then Power the harness and ecu ect. But also run the output of that switch to the push button start then from what I can see they have incorporated a wire within the harness labelled starter that goes from the cabin to engine bay. So I will run the output of that push button to the starter solenoid? I’m just wondering if I need any type of relay within this setup and also very lost on what size wires needed to be used along with what needs to be fused. From what I can see Haltech recommends a 75a circuit breaker.

Any help would be largely appreciated. I just don’t want to blow the ecu up.

Comments, rather than recommendations, as I don't know enough about your full set-up.

I'm not sure you can "blow the ECU up", it seems to be very well designed for pretty much anything you can do to it, electrically.

For the battery cable to the starter, are you using a rear mounted battery? Are you using the OEM starter motor? The minimum gauge of the cable is going to depend on the cable length and current draw during cranking as you're going to want to control the voltage drop across the starter - this includes the ground/earth path as well. For me, I'd run a size or two up on that, but I have a 'thing' about voltage drop and it can make a noticeable difference when cold cranking an engine, when the battery is a bit discharged and 'weak', or if heat soak is a problem (header proximity can be an issue) for hot cranking. The heavier gauge can also make a significant improvement on cranking speed, which may aid starting. That said, I haven't been in a situation where weight is critical and would figure, anyway, the little extra mass is less likely to cause problems that can cause pit delays, or even retirements.

It does depend on your specific installation, some will use a post in the engine compartment, like a through-bulkhead fitting, and run the starter and ECU power from that point, others will run it to the starter and run the ECU from there. it may also make for a handy alt' connection point.

With the alternator, it's primary output current is to support the electrical devices on the vehicle when the engine is running with the battery charging being secondary to that. Depending on what you're powering, and it's output, this can be a comparable current to the starter, so don't scrimp there, either. Oh, some OEM fitments use isolators to protect the alt'r against vibration - double check it doesn't need to be grounded to the engine (I've had an OEM ground wire get hot enough to melt the insulation).