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Old 05-11-2017, 12:08 AM   #1
kvjb
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Main harness power

Here is a quick question for the electrical gurus. I'm putting an aftermarket harness in my '51 and I'm wondering if it makes any difference if I run the main harness power lead from the alternator post instead of directly from the battery? the alternator post is closer and provides a more straightforward connection to my fusebox. I will have a 6 gauge wire from alternator to the starter and heavy gauge wire back to the battery. I plan to run the fusebox power wire through a fusible link either way. Any insight is appreciated.
Thanks
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Old 05-11-2017, 12:21 AM   #2
mongocanfly
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Re: Main harness power

Why not run your harness power from the fuseable link?..who's harness are you using? Do their instructions not tell you how to wire it up?
I wouldn't run it from the alternator but that's just me..
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Old 05-11-2017, 12:57 AM   #3
kvjb
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Re: Main harness power

Thanks for the input. It's a generic 8 fuse hotrod harness and the instructions say to wire it to battery +. The way I understand it, the battery, starter and alternator are connected on the same circuit so it shouldn't really matter at what point you draw power from. I'm just wondering if my understanding is flawed.
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Old 05-11-2017, 02:44 AM   #4
VetteVet
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Re: Main harness power

Quote:
Originally Posted by kvjb View Post
Thanks for the input. It's a generic 8 fuse hotrod harness and the instructions say to wire it to battery +. The way I understand it, the battery, starter and alternator are connected on the same circuit so it shouldn't really matter at what point you draw power from. I'm just wondering if my understanding is flawed.
The thing to remember is that the alternator is the main power source and the battery is the back up. It's amazing how many people, even professionals think just the opposite.

You are correct in thinking that the alternator and battery leads are connected on the same circuit, but you can't just wire the alternator directly to the battery and then run the battery power to the fuse panel. Unless you used a large wire from the battery to the fuse panel and you'd have to use a large fusible link. It doesn't require a very large wire to charge the battery and it wasn't designed to run the electrics on the truck for very long.

A much better way to do it is to run the alternator to the starter terminal with a fusible link on the alternator wire if you want to, The factory does on the 74 and up vehicles. Then you can run a 6 gauge wire from there to the main harness junction, probably the fuse panel, and feed all the circuits off of that.


You won't need to run a wire back to the battery since the alternator will charge it through the positive cable and the battery can feed the truck circuits through the same cable. Pretty simple and easy to do.

You will still have to run the wiring from the circuit switches to the loads and the alternator wiring, but that should be included in the harness you bought.

This is how the factory did the wiring, and they used the alternator output as the main power source to the main junction and then ran a wire from the battery to the same main junction.

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The main junction on these was just a four wire soldered junction and one wire carried all the current from there to the fuse panel and the ignition and headlight switches. It wouldn't work too well on today's multi-circuit vehicles.
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Old 05-11-2017, 03:57 AM   #5
kvjb
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Re: Main harness power

Vettevet,
that was a very informative post. Thanks for taking the time to lay it out
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Old 05-11-2017, 08:13 AM   #6
gmachinz
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Re: Main harness power

Also, Id suggest mounting your junction block in the middle per se of your intended power runs. In other words, dont install the junction 20 feet away from the alternator-stay as close as possible and run a large power supply to it-from there route all of your +12V runs. What kind of alternator are you using?
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