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Old 12-30-2017, 12:31 AM   #1
putnam00
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alternator excite wire

ok Im using the belt drive from a 1995 silverado on in my 71 Chevy. is the excite wire that connects to the alternator ran through the computer in 95? I lost the new alternator and I have a friend that said he lost like three hooking the excite wire to a keyed 12 volt source, and thought it was maybe ran through the computer.?
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Old 12-30-2017, 04:48 AM   #2
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Re: alternator excite wire

My 88 to 98 schematics show the brown wire going to the dash plug and the 99 goes to the PCM or VCM which excites the brown wire.

I found this on the internet which I believe is true. He states that the brown wire runs to the charge light in the dash and gets it's exciter voltage through the power in the light and it needs the lights resistance to work.
All the exciters in all the GM trucks use a resistance wire to excite the alternator
and the earlier ones came from the key switch on the acc terminal.

The brown wire has to have some resistance to keep from frying the diodes in the alternator which may have been the reason your buddy fried three of them when he ran the brown wire to a key on source. Just run your brown wire to the charge light in the dash or through a resistor in series with the key on voltage source and it should work fine. Use any resistor more than 35 ohms and 1 watt up to 300 ohms. Painless wiring recommends an 85 ohm 1 watt resistor.

I assume you have the 71 gauge cluster or the idiot light cluster but either one has the resistance wire off the key switch but the gauge cluster doesn't have the idiot charging light and even if it did it is only 10 ohms so you still need to add a resistor in series with the brown wire to the alternator. The CS series alternators need more than 10 ohms to protect the diodes.

here is his post.

Every 1988 - 1995 C/K truck I've seen with a V8 has had the same alternator; one wire goes to the battery to charge it, and there is a plug with one wire in it that clips into an actual connector on the alternator, NOT spade terminals like older model alternators. Most of the information I've found on Chevy alternators lists a one-wire or three-wire alternator; the one-wire is self-exciting and only has the wire going to the battery, and the three-wire has two blade terminals in addition to the battery wire; one terminal is used for the battery light/gauge, and one terminal is used to excite the regulator.

This two-wire model our trucks use (which I'm pretty sure applies to at least everything '88-'95, since the plug is part of the wiring harness)has the battery wire, and one wire going to the the little plastic plug that clips onto the alternator. They are internally regulated. After some reading I did find the answer I was looking for, so I'm going to share it here for anyone else who might need to know.

The wire that goes into the regulator plug comes from the battery warning light (not necessarily the gauge). When the key is turned on, the warning lamp receives 12v switched power from the battery, and the ground from the bulb is connected to the one wire running to the alternator. The bulb adds a load to the circuit, so its not an exact 12v at the actual regulator plug; I measured on my actual truck, and with the key on you should get 10v. Voltage from the battery light actually excites the regulator.

So, when the key is off, the light doesn't receive any power and stays off, and the regulator doesn't excite. When the key is on, voltage runs through the battery lamp, illuminating it, and grounds through the regulator, exciting it. Once the engine cranks, +12 volts is still applied to the battery light, but the action of the regulator feeds back +12 volts to the other side of the lamp when the regulator is activated and the alternator is in charge mode. With 12+ applied to both sides of the lamp, no current flows and it doesn't illuminate. Thats why if the regulator fails or the alternator doesn't produce a charge, the light will illuminate; the balancing voltage won't be sent back to the bulb to keep it from illuminating.

For my retrofit application, I'll have to wire up that one wire to the GEN light in the dash; I thought I had it that way, but over the course of 35 years its possible the bulb is history. So an interesting note; if the regulators in these alternators aren't being excited, it could possibly be caused by the battery warning lamp bulb going out in the cluster. For anyone who stumbles across this thread in a search, just watch for the battery light to come on when you turn the key on without starting.




Here is a diagram showing how the exciter wire is wired in the early trucks.
It is modified to show how it wires to the internal regulated alternators.

Name:  internal regulated alternator.jpg
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This one shows the resistor wire and the light and it goes to an SI style alternator but yours is a CS with just the brown wire so you won't need the S terminal wire which is the voltage sensing wire. Your alternator has the voltage sensing internally.

I don't normally read the newer truck forums but I look at the electrical forum and the 67 to 72 forum almost every day. i will check this one to see how you made out.
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Old 12-30-2017, 12:14 PM   #3
putnam00
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Re: alternator excite wire

Ok I will add a resistor to mine with a quick search Iíve found a 100K Ohm 1W Flameproof Resistor. Will this work. My 71 had the generator gauge, i retro fit a gauge from an 80ís chevy it has a resistor on the back of the gauage I could run a wire from it to the excite plug on my alternator, or buy the resistor and just add it in series to the wire thatís already there.
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Old 12-30-2017, 02:10 PM   #4
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Re: alternator excite wire

Quote:
Originally Posted by putnam00 View Post
Ok I will add a resistor to mine with a quick search Iíve found a 100K Ohm 1W Flameproof Resistor. Will this work. My 71 had the generator gauge, i retro fit a gauge from an 80ís chevy it has a resistor on the back of the gauage I could run a wire from it to the excite plug on my alternator, or buy the resistor and just add it in series to the wire thatís already there.
do the resistor in series from a key hot source but don't use the ignition coil wire, pink.
use the ignition unfused terminal on the fuse panel and you don't need a fuse in the exciter wire. Like I said the wire from the key switch ACC terminal is the original resistance wire but it is only ten ohms so you need the extra resistor, yours should be fine.

Measure the brown wire that connects to the alternator for 12 volts with the key on. If you get 10 or more just add the resistor in the brown wire in series. You won't need to come off the fuse panel with a separate wire. Sometimes we do that and add a charging light in the circuit for extra resistance and to be cool.

Here's a diagram of the key and the 24 gauge resistance wire (10 ohms), It's the fourth one down and it connects with the 12 gauge brown ACC wire terminal. It runs to the inside of the cab firewall block and connects to the 16 gauge brown wire on the engine side that goes to the alternator L terminal.

Ignore the blue arrow on the light switch.



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Old 12-30-2017, 07:13 PM   #5
putnam00
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Re: alternator excite wire

Would this work? i think it would take the heat better than just soldering a resistor in line

never mind I need to read thats for a coil ignition

https://www.summitracing.com/parts/m...hoCR1AQAvD_BwE

Last edited by putnam00; 12-30-2017 at 07:48 PM.
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Old 12-30-2017, 08:06 PM   #6
VetteVet
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Re: alternator excite wire

Well you have been sleeping in class again LOL. You could use that but you would need about 50 of them because they are only .8 ohms. They are used to protect the ignition coils and points in the points ignition system. Your resistor will work fine. You can put it anywhere in the brown wire but be sure to use heat shrink insulation on it after you solder it in place.

Did you do the voltage test to see if the brown wire has power with the key on?


Here's a diagram of how simple it is to install.
You won't need the red S wire because the alternator does it's voltage sensing internally.


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Old 12-30-2017, 08:28 PM   #7
putnam00
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Re: alternator excite wire

yeah i have 12 volts with key on, i'm searching for a resistor that size and only have found one from painless for 30 bucks, and I'm still searching for one but I have a different problem on top of the alternator
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Old 12-31-2017, 12:06 AM   #8
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Re: alternator excite wire

////////////////////////////////////////////////////////////////////////////////////

Quote:
Originally Posted by putnam00 View Post
yeah i have 12 volts with key on, i'm searching for a resistor that size and only have found one from painless for 30 bucks, and I'm still searching for one but I have a different problem on top of the alternator


Where are you looking?


https://www.amazon.com/s/?ie=UTF8&ke...l_5yko9hlk13_e
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Old 01-05-2018, 03:54 PM   #9
putnam00
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Re: alternator excite wire

Got a resistor hope it works
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