When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Burned Resistance Wire on 1972 C3 - Converting from External Regulator to 10SI In
Hi everyone,
I recently discovered that the brown/white resistance wire from the ignition switch got burned/melted on my 1972 Corvette C3 small block. Because of this, I suspect that either the original externally regulated alternator or the external voltage regulator may have failed.
I have therefore decided to convert upgrade the car to a Delco 10SI internally regulated alternator (63 amp) and completely remove the external regulator.
My current plan is:
Install a Delco 10SI 63A alternator
Remove/bypass the external voltage regulator
Replace all old charging system connections and damaged wiring
Upgrade the main charging wire to 6 AWG
Install an 80A fuse on the main alternator output wire
Replace the burned brown/white resistance wire completely
Run a short jumper from terminal 2 (sense) to the B+ output stud on the alternator
Connect terminal 1 (exciter wire) to ignition-switched 12V using a 10 ohm / 10 watt resistor in series
Add new engine/frame ground cables as well
I do NOT want to use the old resistance wire anymore because it was already damaged.
From what I understand, the resistor should prevent engine run-on/backfeeding when the ignition is switched off.
Does this setup sound correct and reliable for a street-driven C3 Corvette?
Am I missing anything important before I start wiring everything?
Thanks a lot for your help.
Last edited by MidnightC3Driver; May 19, 2026 at 12:08 PM.
Starting in 1969 all Corvettes had internally regulated 10SI alternators.
In addition they had fusible links on the main output wire.
Do you have the factory wiring diagram for the charging system?
Can you post a picture of the " external" regulator?
Please excuse my misunderstanding - it’s of course an internal regulated alternator..
Attached is the diagram an I’ve marked the brown-white cable which was attached to the ignition switch.
What I’ve understood is that the cable meltdown could be caused by a broken diode plate in the alternator- that’s why I want to replace the existing one.
Please excuse my misunderstanding - it’s of course an internal regulated alternator..
Attached is the diagram an I’ve marked the brown-white cable which was attached to the ignition switch.
What I’ve understood is that the cable meltdown could be caused by a broken diode plate in the alternator- that’s why I want to replace the existing one.
If the diode trio in the alternator is bad, you replace it.
As a matter of fact, all the internals of a GM 10si alternator are replaceable. And that includes the stator and the rotor, which is not shown in the photo.
Please excuse my misunderstanding - it’s of course an internal regulated alternator..
Attached is the diagram an I’ve marked the brown-white cable which was attached to the ignition switch.
What I’ve understood is that the cable meltdown could be caused by a broken diode plate in the alternator- that’s why I want to replace the existing one.
Did you check the DC output to see if it's 13.5 to 14.5 volts?
Did you check the AC output which if any would indicate a bad diode?
Did you look at the stamping on the current alternator to see what amperage it is?
No, unfortunately I didn’t take any measurements. What happened was that I noticed the brown-and-white resistance wire and immediately started disassembling everything. I removed the ignition switch, disconnected the wire from the switch, and also loosened the wiring harness behind the dashboard because it looked like there may have been an electrical short or wiring burn. I wanted to fix the issue as quickly as possible, so I also decided not to start the vehicle again afterward.
Regarding my old alternator, the number stamped on it is 1100921. After doing some quick research, I found that it was probably originally rated at 61 amps, although I’m not completely sure about that.
I have already ordered a new alternator, a 63-amp Delco 10SI. However, I’m still unsure about the best way to wire it, because I do not want to continue using the original wiring setup with the brown-and-white resistance wire.
No, unfortunately I didn’t take any measurements. What happened was that I noticed the brown-and-white resistance wire and immediately started disassembling everything. I removed the ignition switch, disconnected the wire from the switch, and also loosened the wiring harness behind the dashboard because it looked like there may have been an electrical short or wiring burn. I wanted to fix the issue as quickly as possible, so I also decided not to start the vehicle again afterward.
Regarding my old alternator, the number stamped on it is 1100921. After doing some quick research, I found that it was probably originally rated at 61 amps, although I’m not completely sure about that.
I have already ordered a new alternator, a 63-amp Delco 10SI. However, I’m still unsure about the best way to wire it, because I do not want to continue using the original wiring setup with the brown-and-white resistance wire.
picture of the old alternator below:
It's stamped 61 amps.i would expect bulb filaments to burn out if an alternator put out more than 14.5 volts for a prolonged period of time along with battery failure.
It's stamped 61 amps.i would expect bulb filaments to burn out if an alternator put out more than 14.5 volts for a prolonged period of time along with battery failure.
That makes sense and is actually a very good point.
I never experienced obvious overcharging symptoms like constantly blown light bulbs or a boiling battery, so maybe the issue was not simply excessive voltage from the regulator.
Could a failing diode trio or rectifier inside the alternator have caused excessive current/backfeeding through the brown/white resistance wire and eventually melted it?
Or are there any other common causes that could have melted the original resistance wire?
To be on the safe side, I decided to completely eliminate the old resistance wire circuit and replace it with a new exciter wire setup for the 10SI alternator - terminal 1 connected to ignition-switched 12V through a 10 ohm / 10 watt resistor.
yeah - something isn't adding up.
From my recollection the resistance wire for protecting points had nothing to do with the alternator wiring.
ALSO - if the 72 is anything like my 68 there is NO alternator malfunction lamp.....also, a higher voltage would mean less current through lamp filaments.
I've been running upgraded alternators for a good 20+ years and my voltage is routinely above 14 vdc. Lamp burnout has been a NON-PROBLEM
Originally Posted by Alan 71
Hello,
I too thought 72 engines had an internal regulator. ??
Regards.....
71 SB,
What part does the resister wire play in the ignition system?
I don't think I ever hear it mentioned?
Is this what MC3D asks about?
yeah - something isn't adding up.
From my recollection the resistance wire for protecting points had nothing to do with the alternator wiring.
ALSO - if the 72 is anything like my 68 there is NO alternator malfunction lamp.....also, a higher voltage would mean less current through lamp filaments.
I've been running upgraded alternators for a good 20+ years and my voltage is routinely above 14 vdc. Lamp burnout has been a NON-PROBLEM
I don’t get it.
I am talking about the resistance wire on the ignition switch on the steering column NOT the resistance wire for the ignition coil.
Does somebody know what could be the reason that this wire burnt down?
Attached the pictures again from the cable as well as from the wiring in a 72 C3 Ignition Switch Melted cable
Last edited by MidnightC3Driver; May 20, 2026 at 01:35 PM.
Perhaps I am misunderstanding something.
The only resistance wiring I see labeled as such in the 1973 diagram is for Ignition and it is EXPLICITLY labeled as such..... see image below circled in RED.
I followed the brown and white back to the ignition switch and it leads to this location indicated in the image (see brown arrow).... nowhere did I noticed anything labeled "resistance wire"?
WHY do you think it is a resistance wire?
Last edited by carriljc; May 20, 2026 at 03:51 PM.
Perhaps I am misunderstanding something.
The only resistance wiring I see labeled as such in the 1973 diagram is for Ignition and it is EXPLICITLY labeled as such..... see image below circled in RED.
I followed the brown and white back to the ignition switch and it leads to this location indicated in the image (see brown arrow).... nowhere did I noticed anything labeled "resistance wire"?
WHY do you think it is a resistance wire?
I think the term “resistance wire” may actually be incorrect in my case.
I originally assumed it was a „resistance“ wire because it has only a single wire conducter behind the brown-white insulation.
So to clarify, I am referring to the brown/white wire from the ignition switch that has only one single conductor.
edit: below the wire path:
Last edited by MidnightC3Driver; May 20, 2026 at 04:32 PM.
ah. ok then.
Just replace it with another wire of the same colors. Somewhere in it's long life it probably just shorted out to ground. If everything else if fine then it's just another wire. You can try looking around the location where it is burned and check to see if you can locate what it may have shorted to.....
ah. ok then.
Just replace it with another wire of the same colors. Somewhere in it's long life it probably just shorted out to ground. If everything else if fine then it's just another wire. You can try looking around the location where it is burned and check to see if you can locate what it may have shorted to.....
Honestly, I’m not sure whether I can reconnect this cable with a new wire, because I also don’t know where I would even get the correct cable from. From what I’ve read, you’re supposed to use the exact same wire gauge (single conducter), and I have no idea where to source that.
On top of that, even if I repair it now, that doesn’t necessarily mean the same issue won’t happen again in the future.
According to the wiring diagram, this cable goes to the alternator and connects to the exciter wire. That’s why I was thinking about replacing the alternator altogether with a 63-amp 10SI alternator and rewiring the exciter wire with a 10W 10-ohm resistor in between, so the engine shuts off properly when I turn off the ignition.
Your comments about exciter wires do not make any sense to me? BUT I don't know what I'm missing or why you're wanted to modify the wiring???
Do you have the AIM for your year?
On my 68 AIM it gives you Wiring Gauge for associated leads.
Note below that the ONLY Brown lead coming from my ignition switch is a 12 Gauge. You can find 12 gauge automotive wire at any car parts store....or online.
Since my original alternator was the same amperage (I'm using a 145 amp alternator now) as your year and it's a 12 VDC system I suspect you'd be just fine installing a 12 gauge lead as a replacement. Take a look at your AIM.
Here is a copy of GM's 1972 wiring diagram. It isn't as clear as I would like, but it is usable. I appears that the resistive wire that you referring to is 24 gauge. I don't know what the resistive value should be for that wire, but in the transistor ignition systems they use resistive wire and they are only 0.5 ohms.
Burned Resistance Wire on 1972 C3 - Converting from External Regulator to 10SI In
