What is the BROWN Alternator Wire for?

[Gregski]

first of all guys nothing is broken I am not trying to fix anything, this is just to help me and anybody else who comes across this thread to understand what the brown alternator wire is for

second, I would like to keep this discussion on point and I will start a new different thread to go over other wires for the alternator so that it is easier for folks to search for and find specific information that they need (finding stuff can be frustrating sometimes) even though its been covered here and discussed already

also posting this specifically in the 73-87 area since I don't know if the functionality is the same for the prior generations ie 67-72 for example

so on a 1974 1/2 ton truck 350 V8 4 spd manual 2 wheel drive no AC, I have removed the wire harness conduit and traced the BROWN alternator wire all the way from the alternator to the plastic black square bulk head connector on the firewall

and I have to ask what is the brown alternator wire for?

[franken]

Alt light I'd imagine.

[enaberif]

Its if you have a volt gauge which a 74 doesn't.

The brown wire goes to the bulkhead and through up to the main cluster plug. If you had a cluster with a volt meter this wire would be used for that.

[Gregski]

Originally Posted by franken

Alt light I'd imagine.

I don't think I have an Alternator Idiot light on my 74 or at least haven't found it yet, LOL

showing you a pic of my stripped down gauge cluster, as those who have seen my threads know I like to be very detailed (for my own sake) as well as hoping to help others who may come across this wealth of knowledge you all are helping me drop on them, hee hee

[Gregski]

Originally Posted by enaberif

Its if you have a volt gauge which a 74 doesn't.

The brown wire goes to the bulkhead and through up to the main cluster plug. If you had a cluster with a volt meter this wire would be used for that.

I'm with you, but if there were no Volt gauges in 1974 then why run the wire? Was GM being clairvoyant? LOL

[ray_mcavoy]

The brown wire is the alternator "exciter" wire. It provides power to the alternator's field windings on initial start-up. Once it's up and running, the alternator's internal diode trio provides the field current.

For trucks without gauges, the "GEN" indicator light bulb is part of this circuit in the cab harness.

For trucks with a factory ammeter (73-75) or voltmeter (76+), the cab harness contains a 10Ω resistance wire (usually brown with a white stripe) that takes the place of the "GEN" bulb. The resistance is necessary to protect the alternator's diode trio and to prevent feedback into the ignition that could keep the engine running after the key is turned off.

[greg64]

Okay, here's how it works. These alternators we're using are Delco 10SI and 12SI. These were the first Delco alts with built in regulators (that were solid state too). They replaced the 10DN that uses the external mechanical regulator from the 60s. The SI is really good, and only tends to fry if it overheats.

The SI has 3 wires. The heavy red wire, that comes off the threaded terminal is positive power. Obviously the housing is negative. The other two wires are part of a common spade lug connector. These wires are red and brown. The red is smaller in gauge compared to the main power red.

The red wire is "voltage sense". Wherever this wire is terminated will be held by the regulator at 14.4V. This is a great feature, because this can overcome any voltage drop in the main power wire from the alt. This wire should make it back to a junction block on the firewall.

The brown wire is "energize". See, the problem is that when the engine first starts, the alternator is spinning, but the rotor is not yet energized, so the alternator puts out nothing. The heavy red wire can't be used for this job, because it is connected to the battery always, so the rotor would drain the battery when the engine isn't running. If brown isn't connected, like a "one wire alternator" (dumb, I might add), the alt will eventually self-energize if it is spun fast enough. Instead, the brown wire is hot when the ignition is on, which feeds a bit of power to the rotor and energizes the magnetic field. Brown also passes through the Alt idiot light, which causes the Alt light to be lit when the alternator isn't producing any output. This works because juice is flowing through brown to energize the rotor, but the alt isn't putting out any power.

Ask any questions you like about the SI alternators.

[Gregski]

Originally Posted by greg64

Okay, here's how it works. These alternators we're using are Delco 10SI and 12SI. These were the first Delco alts with built in regulators (that were solid state too). They replaced the 10DN that uses the external mechanical regulator from the 60s. The SI is really good, and only tends to fry if it overheats.

The SI has 3 wires. The heavy red wire, that comes off the threaded terminal is positive power. Obviously the housing is negative. The other two wires are part of a common spade lug connector. These wires are red and brown. The red is smaller in gauge compared to the main power red.

The red wire is "voltage sense". Wherever this wire is terminated will be held by the regulator at 14.4V. This is a great feature, because this can overcome any voltage drop in the main power wire from the alt. This wire should make it back to a junction block on the firewall.

The brown wire is "energize". See, the problem is that when the engine first starts, the alternator is spinning, but the rotor is not yet energized, so the alternator puts out nothing. The heavy red wire can't be used for this job, because it is connected to the battery always, so the rotor would drain the battery when the engine isn't running. If brown isn't connected, like a "one wire alternator" (dumb, I might add), the alt will eventually self-energize if it is spun fast enough. Instead, the brown wire is hot when the ignition is on, which feeds a bit of power to the rotor and energizes the magnetic field. Brown also passes through the Alt idiot light, which causes the Alt light to be lit when the alternator isn't producing any output. This works because juice is flowing through brown to energize the rotor, but the alt isn't putting out any power.

Ask any questions you like about the SI alternators.

Greg that was some serious know how you dropped on us, thank you very much.

Couple pesky things to follow up on.

My spade lug connector does not have a red wire it has a yellow wire and a brown wire. But sure enuff about three feet later the yellow wire splices into a red wire and runs to the bulk head.

Now back to that Alternator Idiot light, if there is one on my truck I sure would love to know where?

[greg64]

Originally Posted by Gregski

Greg that was some serious know how you dropped on us, thank you very much.

Couple pesky things to follow up on.

My spade lug connector does not have a red wire it has a yellow wire and a brown wire. But sure enuff about three feet later the yellow wire splices into a red wire and runs to the bulk head.

Now back to that Alternator Idiot light, if there is one on my truck I sure would love to know where?

My Dad was an automotive electrical systems teacher at our local college. He's long retired, but I've been mucking around with these alternators for 30 years.

Ya, as you guessed, the yellow wire is the small red wire I spoke of earlier. It's good that it goes to the firewall, because this lessens the voltage drop. I've seen some cars where this wire is just looped around on to the main power terminal. This works, but abandons a nice feature these alternators have.

Since you have an amp meter, there wouldn't be an alt idiot light. The brown wire has some built in resistance. My truck had all idiot lights, so when I switched to gauges, I just wired a dash indicator bulb in series with the brown wire and connected it to an ignition hot source. If the bulb lights with the engine running, the alternator has problems.

I should share another trick with you guys, but I have to dig out a picture first.

[greg64]

Okay, on with the trick... See how this picture shows a Test Point?

Okay, if you ever suspect your regulator is bad, or just that your alternator isn't putting out any juice and you don't know what's wrong, take a paper clip and stick the end in this test hole and touch the case of the alternator. This will bypass the regulator, temporarily, and fully energize the rotor, giving maximum output from the alternator. Now, you don't want to leave it like this for more than a few seconds, because the output voltage is likely to jump to 18V. Watch on a meter to see what the output voltage does. If it jumps up, way past normal 14.4, then the regulator is bad. If the voltage stays at 12V, then the stator windings or output diodes are bad.

This trick is useful, because the regulators are cheap and very common, and they are easy to change. Might save you the cost of a reman alternator.

Oh, and something else for guys with older cars. The 10SI is a bolt for bolt compatible replacement for the earlier 10DN, and works far better. A great and easy upgrade, and still looks mostly stock.

[hatzie]

Originally Posted by ray_mcavoy

The brown wire is the alternator "exciter" wire. It provides power to the alternator's field windings on initial start-up. Once it's up and running, the alternator's internal diode trio provides the field current.

For trucks without gauges, the "GEN" indicator light bulb is part of this circuit in the cab harness.

For trucks with a factory ammeter (73-75) or voltmeter (76+), the cab harness contains a 10Ω resistance wire (usually brown with a white stripe) that takes the place of the "GEN" bulb. The resistance is necessary to protect the alternator's diode trio and to prevent feedback into the ignition that could keep the engine running after the key is turned off.

Beat me to it.
I read somewhere the resistance of that wire is closer to 45Ω. I would guess anything close to the resistance of the GEN bulb filament would do the job.

[Gregski]

OK, as my buddy Tony used to say "this is neither here nor there" but I just went out to the pick up and took two measurements with my trusty yeller voltmeter

Battery Voltage with the truck off, not running (fyi I have a brand new battery)
12.63 volts

Battery Voltage with the truck on, running, idling
14.56 volts

[Gregski]

OK, not to beat a dead horse to death, but if I don't have an idiot light then does the brown wire go to the ammeter? I don't think so as there is a black wire with a white stripe that houses a 4 amp fuse I believe that feeds the ammeter and runs from the junction block on the firewall, is that correct?

[enaberif]

Originally Posted by Gregski

OK, not to beat a dead horse to death, but if I don't have an idiot light then does the brown wire go to the ammeter? I don't think so as there is a black wire with a white stripe that houses a 4 amp fuse I believe that feeds the ammeter and runs from the junction block on the firewall, is that correct?

Should be two black wires with fuses for the ammeter.

[greg64]

Greg, the 12.6V is good, that's what a fully charged battery should read.

The brown wire will make it back to an ignition on point, not the ammeter. If you're worried about it, pull the plug at the alternator, then turn the key to on (the engine doesn't have to be running). Then probe with your voltmeter between the brown wire at the alt and ground. It should read 12V.

[Grandma’s 68]

Can the brown wire from an alternator be hooked up to the ammeter on a 68 c10?

Original wiring diagrams attached.

At the ignition, switch power seems to goes via a brown/white wire to the bulkhead connector where it splits, going externally to the alternator via a brown wire and internally to the ammeter also via a brown wire. Is the brown/white wire a ‘resistance wire’ as mentioned above?

Also at the switch there is another brown wire that eventually disappears into the fuse panel. It’s not clear what happens there. I guess it powers all the other stuff that’s only on when the ignition is on.

I may be reading the circuit all wrong, so any knowledge sharing would be appreciated.

Thanks.

[Richard2112]

Tons of good info in this thread. Where does one get an internal regulator for an alternator?

[Gregski]

Originally Posted by Richard2112

Tons of good info in this thread. Where does one get an internal regulator for an alternator?

Wow I started this thread five years ago! So what year is your truck and what alternator do you have cause as Greg64 pointed out in post #7 above the Delco 10SI and 12SI already have them built in.

[Richard2112]

Originally Posted by Gregski

Wow I started this thread five years ago! So what year is your truck and what alternator do you have cause as Greg64 pointed out in post #7 above the Delco 10SI and 12SI already have them built in.

Ive upgraded my alternator to a 105 amp and it works fine. I just never saw internal regulators listed or sold in auto parts. If/when mine eventually fails, I'd be interested in just installing a new regulator.

[Gregski]

Originally Posted by Richard2112

Ive upgraded my alternator to a 105 amp and it works fine. I just never saw internal regulators listed or sold in auto parts. If/when mine eventually fails, I'd be interested in just installing a new regulator.

Oh, I'm not sure they do that I don't think they sell rebuild kits these days I may be wrong, so you would probably trade yours in for a core charge and have to buy a new one or a refurbished one, that's my guess and observation over the last 20 years or so

[rodstored-72]

I like this.. a lot of good information here. it is falling in line with a question I have. I have a '86 c10swb that had a 305 and Silverado package. If I understand right the same brown wire on my harness would be the volt meter supply (?) wire? also I am curious how it plays in relation to the current LS swap, which I am in the middle of? it is connected to a 2 prong connector that has a H.D. red wire that I believe ran directly to the batt. or junction block that was attached to the firewall above the engine.
thanks!!

[Katrina/10]

Rock Auto sells the regulators separately.

[1976gmc20]

Some years ago I replaced the alternator on my 76 GMC because it was noisy and I thought the bearing might fail and lock up out on the road somewhere.

I put the old one on my old Case tractor (yeah, "converted" to 12v). I had to install two toggle switches, one for the ignition and one for the exciter wire else you couldn't shut off the engine Amp gauge won't come up unless the second switch is turned on, then I usually would turn it right back off.

Neither of the above are actually "in service" right now

[ray_mcavoy]

Originally Posted by Grandma’s 68

Can the brown wire from an alternator be hooked up to the ammeter on a 68 c10?

Original wiring diagrams attached.

At the ignition, switch power seems to goes via a brown/white wire to the bulkhead connector where it splits, going externally to the alternator via a brown wire and internally to the ammeter also via a brown wire. Is the brown/white wire a ‘resistance wire’ as mentioned above?

Also at the switch there is another brown wire that eventually disappears into the fuse panel. It’s not clear what happens there. I guess it powers all the other stuff that’s only on when the ignition is on.

I may be reading the circuit all wrong, so any knowledge sharing would be appreciated.

Thanks.

Welcome!

No, the brown wire for the alternator can not (and should not) be hooked up to the ammeter.

The diagrams you attached are a bit on the small side to read, but it appears as though the first one is for trucks with a warning light instrument cluster while the 2nd diagram is for trucks with a factory gauge cluster. That could explain why you are having a hard time following the routing of the brown wire.

In 67-72 trucks with the factory gauge cluster, the brown wire should run from the voltage regulator to the firewall bulkhead connector. Then on the under-dash side it changes over to a resistance wire that is connected to the ignition switch. The battery gauge (ammeter) has it's own separate wiring ... black and black w/white stripe wires that each have small 4 amp fuses where they tie into the charging system wiring.

And yes, that 2nd brown wire that runs from the ignition switch to the fuse panel is to power other circuits that are live with the key in the run and accessory positions.

[Grandma’s 68]

Thanks. I’m sorry about the picture resolution. I have not found where you can control the upload resolution. They just seem to upload at around 40k. Some advice there would also be appreciated.

The images were of page T5283 and T5292 from the document 1968_GMC_Wiring_Diagrams_After_December_1_1967. After your observation, I did have the wrong circuit for the instrument panel. It should have been T5290, which has the gauge. This was my main mistake.

So it would seem the brown wire from the alternator changes at the bulkhead to brown/white, then splits somewhere to two brown wires, one going to the switch and another to the fuse panel. So my mistake was to think that the brown wire went to the gauge. The gauge is actually wired to the black and black/white wires. So the brown wire, which had not physically been connected to the alternator, is okay to hook up. If my ammeter doesn’t work it’s likely the fuses I the bl &bl/w wires.

Thanks for your help.

I have a new NAPA alternator https://www.napaonline.com/en/p/RSE2...ywordInput=213 on the 1968 c10.