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I balked some years ago at the cost of replacing the stamped metal tray for my battery and instead put a container in for moving it to the hatch. OMGIn doing so, I became subject to a number of rules regarding how the car must be prepared to run on the track, the most onerous of which is a kill switch on "the rear-most part" of the car.

For reference, here's the ever-popular internally regulated 60/70A Hitachi from a 280ZX:

I've had a Moroso 74102 for some time acting as a kill switch (mounted to the rear panel, through the license plate), installed per the directions but I'm getting ready to run a 100A alternator (to support some other modifications), and realized that I'd been dangerously wrong on the install for some time. OMGSee, the 74102 is limited to 20A charging capacity, which runs afoul of even the most modest of 240Z alternators' peak output (I say peak, because certainly they aren't running at their rated output, when the battery is charged). OMGI noticed this when I melted my first 74102 after getting a jump on a dead battery (the 10 gauge wire running back certainly got hot as well).

So, after rethinking my options, I decided to go with the Longacre instructions (below) and use the switch to kill the field wire. OMGThis works fine, if you've got a race car, and you use the kill switch to shut the car down. OMGOtherwise, it'll run the battery dead overnight.

After rethinking my options again, I happened on a simple diagram that helped me out.

Basically, the field wire should only receive ignition switched power. OMGThe problem, then, is figuring out how to get ignition switched power that won't also get fed by the alternator. OMGSo I kept the leg of the Moroso 74102 that their instructions intended for charging the battery, but used it as the source for a relay that is triggered by the ignition switch, like so (though adding the 1N5402 relay to keep the car from running on after the key is turned off):

Now, you can run a (fused) wire from the alternator to the starter post at whatever you think is a safe gauge, which hopefully already has a big 2 gauge cable running to the back (either with its own breaker, or relying on the switch to melt-safe). OMGI went with 4 gauge, and an inline 100A fuse (though not visible):

I'll be switching to a GM 3-wire alternator soon, but this wiring should support the new one just fine.
Your powers are useless! I'm wearing my tin foil underwear!

1992 300ZX: Not stock, 433 RWHP
1971 240Z: Toyota front brakes, 123 RWHP
1967 Pontiac GTO: not stock.
Old 04-10-2017, 06:24 PM Jehannum is offline  
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