1969 GMC alternator resistor wire

[1969 GMC truck]

I have a 69 GMC that has a resistor wire burned up under the dash. It runs from the ignition switch to the bulk head. I can not find a replacement any where. If I change to an internal regulator is this wire still needed? Where can I find a new wire with out buying a complete harness? Thanks in advance.

[67 chevelle]

The only resistor wire Im aware of is the cloth covered coil power wire {white} that runs from the engine bay bulkhead to the coil positive post

PS you might call american auto wire 800-482-WIRE (9473)

[1969 GMC truck]

I guess this is also called the exciting wire for the alternator. It is brown with a white stripe and should be hot with the ignition switch. I have gauges so no idiot light. The wire is single strand and very small diameter. It may be stainless I am pretty sure its not copper.

[demian5]

Originally Posted by 1969 GMC truck

I guess this is also called the exciting wire for the alternator. It is brown with a white stripe and should be hot with the ignition switch. I have gauges so no idiot light. The wire is single strand and very small diameter. It may be stainless I am pretty sure its not copper.

Resistance is exciting

[randy500]

Originally Posted by demian5

Resistance is exciting

I resist getting excited...

[VetteVet]

Originally Posted by firedemon

a resistance wire will lower voltage if you go internal you want full voltage to turn the alternator on . I would use a 16 gauge wire to replace it and you should be good either way

Actually the resistor drops the voltage so the diodes in the alternator don't burn up. The exciter voltage is a small amount which turns on the transistors in the regulator. The regulator in turn will control the amount of current that can charge the alternator fields. The more current the higher the charging voltage.

The resistor also keeps the small amount of internal voltage created in the alternator from back feeding the ignition switch which would keep the engine running when the key key is shut off.

The factory uses a 16 gauge wire for the exciter wire which is plenty.

I'm working with the OP in PMs and didn't know he had started this thread.

He is getting an alternator from LMC with a conversion kit so he should be good once he gets it set up.

[RichardJ]

GAWD! Every time I hear some one say "full voltage" , I want to puke my lunch.

The primary reason for the resistance wire, connected in parallel with the idiot lamp is in case the lamp should burn out. The résistance wire will continue to serve as the Exciter circuit and the alternator will continue to function normally until you replace the lamp.

I've wired some without the resistor and the idiot lamp would glow until you revved the engine and it would begin to charge normally. I have wired some with the lamp ONLY and the alt would begin to charge on its own. Seems to depend on lamp resistance.

The wire is a nickel-chrome material, but you can more easily replace it with a 10 Ohm 10 Watt resistor from Radio Shack and a length of copper wire as needed.

Be sure and find out what caused the resistor wire to burn before you replace it.

This lamp circuit is wired the same for the External Voltage Reg Alternator and the Internal voltage Regulated Alternator.

[67 chevelle]

Great learning experience here , I never knew there was a wire that went from the switch to the alternator , or most of the other stuff posted here .

[VetteVet]

Here's my reply to the PM that the OP sent me.
I only post it here because he started this thread and the same topics have come up.

the question is- what caused the wire to burn up. Most likely a shorted regulator or a diode in the alternator. The wire is a special resistance wire equivalent to 10 ohms. It runs from the key switch along with the 12 gauge, brown, accessory feed wire, which goes to the fuse panel.

The brown/white wire goes to the inside of the firewall block behind the fuse panel and connects with a brown 16 gauge wire on the firewall side of the block. This 16 gauge wire is the exciter wire for the alternator and it goes to the external voltage regulator plug, on one end, which is pin 4 if I recall correctly.

If you are going to convert to an internally regulated alternator, this wire will extend to the alternator and it will need a 10 ohm resistor soldered inline before it plugs into the IR alternator.

As it stands from the factory, it has the resistance in the burnt wire and it goes into the regulator and exits to the alternator via the white wire in the blue and white wire harness. The blue wire is the voltage sensing wire that tells the regulator in the alternator what the system voltage is, so the regulator can control the charging rate of the alternator. It is connected to the red wire in the external regulator which is hot all the time.

OK you ask, how do I go about fixing it.

You have some options. Obviously you have to replace the resistance,,and the wire, and it has to be a "key on" powered wire. Otherwise you'd run your battery down overnight.

You have to do this even if you keep the stock regulator and alternator, not a good idea. the internals are so much better.

So where do you get key on power?
You could splice in to the accessory wire at the key switch with any wire and solder in the 10 ohm resistor anywhere in the wire even in the engine compartment.

You could run a wire from the fuse panel Ign Unfused terminal and solder in a resistor. The Ign Unfused terminal is key on power.

The 10 ohm resistance is not hard and fast, it can vary a few ohms. In fact if you install an 85 ohm 5 watt resistor you can upgrade to the modern CS style alternator which is on most every GM car and truck made since 1989.

The SI series is very reliable and the 12 SI came on most mid eighties Camaros and Firebirds and some S-10s and all the big stuff. The CS 144 would be a great conversion and easy to do if you wanted more idle output.

If you wanted to be slick you could wire in a light that would replace the resistor and it would tell you if your alternator was charging or not. You should have a battery gauge in the dash cluster which also does that if it works.

The light clusters use the light, but they also have the resistance wire from the key switch, in case the light bulb blows, so the alternator will still charge. If you use a light just be sure it has 10 ohms between the terminals.

What ever you decide to do I can help you.

[VetteVet]

Originally Posted by firedemon

I see I was under the impression that the whole wire was a resistance from the key all the way to the alternator so just replacing the under dash with a regular wire he would of still had resistance my bad . I thought the resistance was just so the power wouldn't back feed and keep the truck running and that the regulator needed full 12 to turn on . is that lower voltage needed for both the internal and external or just the external ?

Not to worry, it's an honest mistake because the coil resistance wire runs all the way from the firewall to the coil, and strangely it is not resistance from the key to the firewall block. It's only 1.8 ohms though.

The stock alternator doesn't have the regulator inside so it isn't a factor for that but the resistor is still needed for the anti feedback and some guys use a diode. the voltage from the key will still energize the relay inside the external regulator.

Don't feel bad about Richard, he is a great guy and knows his stuff, I get a kick out of his reactions sometimes.

[RichardJ]

This idea of using a diode in this circuit is interesting. If someone is willing to cut into the original alternator wiring to install a diode, it makes me wonder what they have done to the original ignition circuit to require this band aid.

The resister/lamp in this circuit is used to supply a SMALL amount of current through the regulator to the Field winding. This SMALL current to the Field only allows the alternator to BEGIN charging. Once it begins charging, the voltage regulator takes over the job of supplying current to the field (rotor) winding.
The Ign switch/resister/lamp circuit is not designed to supply Field current after the alternator starts charging and producing an output.

Aside from that.
You have probably read this before. The Ign switch provides 12V to one side of the lamp. When charging the voltage regulator provides 12V to the other side of the lamp. 12 V on both sides of the lamp means NO current flow through the lamp and no light. A less than 12V signal from the regulator means the lamp will glow. Significantly less than 12V and the lamp will be very bright.

If the voltage regulator fails in such a way that it full Fields the alternator, the voltage on the lamp can be 16-17V. With 12V on the ignition side of the lamp it will light.

The resister/lamp circuit is designed to light the lamp in both under voltage and over voltage conditions.

Your diode negates any overvoltage warning, because current only flows through it, in one direction.

[VetteVet]

Originally Posted by firedemon

used a diode because previous owner must of had the same problem as the original poster and he replaced the resistor wire with a regular wire and must not have been able to shut truck off . so his solution was to make a small pig tail between the factory plug and the alt with a diode on the number 1 side . then when I converted to internal regulator I rewired everything from the bulkhead to the headlights using the recommendations from mad electric website and talking with mark himself that also uses a diode so it wont back feed and id be able to shut truck off . I don't have a light it only has the gauge so theres no resistance anywhere on the exciter wire now . it has been charging better than when I got it now for a couple months from when I rewired the engine bay . so my question is will this non resistance hurt the alternator

Diodes do have some resistance but I don't think they have 10 ohms. I can measure some that I have and if you unplug the alternator you can measure yours. If you have an SI style alternator you will have a diode trio in it, which is what the resistor protects. It may not fail right away but later on it probably will. Why take the chance when it is so easy to add the resistor.

[GASoline71]

Originally Posted by firedemon

sorry to the original poster I didn't mean to hijack the thread just trying to learn myself . hope this all helps you too

No need to apologize here. You're on topic, and are learning some new stuff. As did I when I read this thread. I've been working on old cars and trucks for 30+ years and learn new stuff all the time. Some people just get bent up about things that, to them, seems simple because it's something they view as easy for everyone.

I'm glad there are cats like VetteVet here who is a wiring wizard and takes the time to explain things beyond what most people would do.

Gary