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Old 05-10-2017, 09:14 PM   #1
Speedracer39
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Alternator wiring question

I have a new to me 72 c10 the previous owner installed a big block. I went to start it today after sitting for a few days and the battery was dead after charging it I removed the neg cable to check with a test light for a drain. This showed a drain I one by one removed and checked all fuses still a drain . I removed the red wire from alternator--- bingo this is it-- I'm not sure if he wired it correctly when they installed the alt that is internally voltage regulated. Here are some pics. I'll check tomorrow to see if it is charging just wanted to see what you thought of the pics. The only wires used from factory plug is the red-- the white/blue is untouched as well as all the wires at the old external regulator. Any ideas?
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Old 05-11-2017, 12:16 AM   #2
VetteVet
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Re: Alternator wiring question

It appears that your truck has been converted to an internally regulated alternator. Or at least they tried to convert it. I suspect that your drain is the resistor connected between the two red wires on terminals 1 and 2 on the alternator.

Normally the alternator is wired by extending the brown wire at the external regulator to the no. 1 terminal on the alternator, and then looping the no. 2 terminal over to the alternator output wire on the back of the alternator.

The best way to do the wiring is to extend the brown wire from the regulator to no.1 and extending the red wire from the regulator to the no. 2 terminal instead of looping the no. 2 back to the alternator output wire.

This does a better job of letting the regulator read any voltage drop down stream from the alternator and will keep the voltage up at the accessories for best performance.

Here's a picture of exactly what I'm describing.


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The small blue and white wires can be used to extend the brown and red wires if you jump the white wire with the brown one at the regulator and the blue one with the red one, but you'll still have to change the plug at the alternator.

I still don't know the purpose of the resistor in the alternator plug. I'd pull that out and recheck the battery drain.
Post back with the results.
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Old 05-11-2017, 08:15 AM   #3
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Re: Alternator wiring question

Yeah-that resistor is indeed odd.
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Old 05-11-2017, 10:33 AM   #4
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Re: Alternator wiring question

I think the non red wire in the two wire plug is normally connected either through the Gen/Alt light or a under dash resistor for a gauge panel. Either way, it provides the initial current to energize the alternator. Normally a no connect when the ignition is off. By replacing that wire with the resistor, it will likely draw current even with the ignition off.

BTW - In the picts, your main charging wire looks to be undersized. Especially if that alt is a higher current output unit. Might check if fusible links are installed.
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Old 05-11-2017, 10:45 AM   #5
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Re: Alternator wiring question

Thanks for all the good advice, I'm hoping to get a chance in the next few days to dig into it. I'll post later what I find.
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Old 05-11-2017, 07:24 PM   #6
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Re: Alternator wiring question

I have a question for VetteVet ----this is your reply. It appears that your truck has been converted to an internally regulated alternator. Or at least they tried to convert it. I suspect that your drain is the resistor connected between the two red wires on terminals 1 and 2 on the alternator.

Normally the alternator is wired by extending the brown wire at the external regulator to the no. 1 terminal on the alternator, and then looping the no. 2 terminal over to the alternator output wire on the back of the alternator.

The best way to do the wiring is to extend the brown wire from the regulator to no.1 and extending the red wire from the regulator to the no. 2 terminal instead of looping the no. 2 back to the alternator output wire.

This does a better job of letting the regulator read any voltage drop down stream from the alternator and will keep the voltage up at the accessories for best performance.
From what I'm reading the only connections used on the alt is 1,2 nothing wired into the battery post?
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Old 05-11-2017, 09:05 PM   #7
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Re: Alternator wiring question

Except for the one-wire alternator and the 94 and newer CS alternators that only use an exciting wire and an output wire, that is correct.
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Old 05-12-2017, 04:29 PM   #8
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Quote:
Originally Posted by VetteVet View Post
Except for the one-wire alternator and the 94 and newer CS alternators that only use an exciting wire and an output wire, that is correct.
I hope you don't hate me after this but I do have another question I was looking at the wiring diagram and was wondering exactly were does the red wire coming from the back of alt go to it is marked b on the alt
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Last edited by Speedracer39; 05-12-2017 at 08:27 PM.
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Old 05-12-2017, 06:31 PM   #9
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Re: Alternator wiring question

That #2 red wire is the voltage sense wire. It allows the alt to allow for voltage drop on the main(bat) wire between the alt and the junction block. Push 40 or 50 amps through a wire, you can get enough voltage drop to adversely impact battery charging.

Voltage drop is why it is good to increase the bat wire size if you install a higher cap alt.

The way the PO wired yours, it will still work, you just loose the voltage drop adjustment.

Keep in mind a lot of the early alternators only put out 30~40 amps. The average 12SI is rated at 78amps.
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Old 05-12-2017, 11:03 PM   #10
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Re: Alternator wiring question

Quote:
Originally Posted by Speedracer39 View Post
I hope you don't hate me after this but I do have another question I was looking at the wiring diagram and was wondering exactly were does the red wire coming from the back of alt go to it is marked b on the alt
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This is my harness on my 67 showing the junction in real time.


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The red wires are just soldered together by the factory to make the junction.
My junction does not have the two small black wires for the ammeter
because I have the charging light in the dash.
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74 corvette convertible
1965 Harley sportster
1995 Harley wide glide

Growing old is hell, but it beats the alternative.
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