The infamous pink resistor wire

[Mach1 Driver]

As some are probably aware, there is a resistor wire in series with the ignition coil that is nominally 1.35 ohms. It lowers the voltage to the coil in normal operation to extend the life of the points. The wire is bypassed by the starter solenoid in START to give the ignition coil 12v, and makes the car easier to fire-up.
What I didn't realize about the wire is that Old Henry (in his in penny pinching ways) reduced the wire gauge to its absolute minimum- I mean it is REALLY undersize. I measured it in two 69 harnesses and found one to be 7 strands of .013, which equals 28 ga., and another that was 7 strands of .012 which equals 30 ga. The only way they can get away with this is because the ignition coil isn't on all the time, since the points are opening and closing when the engine is ON and running.
The exception is when the key is left ON with the engine stopped. The points will be closed about 2/3 of the time when the engine stops, so the coil will have constant voltage to it in this condition. Fortunately they are relatively short runs of wire and Henry just barely allowed for this. The problem arises when some smart guy changes the ignition coil to one with a low resistance primary- one of those really hot suckers with around 45kV of output. Instead of pulling 4.46 amps, now its pulling 6.46 amps, and that little old wire goes bye-bye.
Since Randy usually hangs out around here as a moderator, and nobody knows harnesses like Midlife, I'm wondering if this is a common gauge for the resistor wire from 1965 to say 1973?

 

[Midlife]

My experience is that it is more like 16 or 18 gauge in total. When I use a wire stripper, I set it to 16 gauge and it seems to do just fine.

 

[Mach1 Driver]

OK, but not what I get- not even close. It has what is probably silicone insulation- it has a fairly large OD and is spongy, so I used the 14ga strip hole, and didn't cut any of the strands. I was really surprised to have only 7 strands in each harness. Maybe they didn't do this until 69?

 

[Midlife]

The nichrome wire strands are probably not equivalent to copper strands in size. I use 16-20 butt splices on the pink resistor wire without any difficulty; if it were truly 28 to 30 gauge, they would never crimp in place.

Another point that everyone misses is this: once the resistor wire transmits current, it heats up. What happens to current when passing through heated wire? The resistance goes up! So...when this happens, the current reduces and the resistor wire becomes a choke for current further reducing the voltage; basically it sets the maximum current based upon the rest of the circuitry that can possibly pass.

 

[Mach1 Driver]

I verified that it is indeed nichrome wire, and yes, nichrome wire is still measured by American Wire Gauge (AWG).

Technology is great when it works. I have asked the Google AI numerous times, what is the CMA for 7 strands of .013” diameter wire? I have gotten replies ranging from 28 AWG to 20 AWG.

So I opened two tabs and asked the Google AI what was wrong with it’s calculation and got this answer:
The Error Dissected:
The other AI claimed that 24 AWG has 1,024 circular mils and 23 AWG has 1,291 circular mils. If you look closely at the numbers, it took the true values for 20 AWG (1,022) and 19 AWG (1,288) and accidentally labeled them as 24 and 23 gauge.

How the HELL can the freaking AI mislabel anything?? Also, the CMA numbers don't exactly match either. So, I personally calculated the CMA, and found a real world table not AI derived for the CMA of 19 and 20 AWG. It works out like this:

19 AWG= 1,288 CMA and is 105 CMA higher than the resistor wire
Resistor wire= 7 strands of .013” dia. wire = 1183 CMA
20 AWG= 1022 CMA and is 161 CMA lower than the resistor wire

So the resistor wire is closer to 19 AWG than 20 AWG. It is important to note that the current carrying capacity of nichrome wire is different than copper wire, but now I have the correct AWG figured out. Since the points open and close, the average current through the ignition coil's primary at a 650 RPM curb idle is approximately 2.5 to 2.8 Amps. The average primary current through the ignition coil at 5000 RPM is 1.1 Amps. BUT, if the key is left in the ON position and the engine is stopped and the points are closed, the coil's current is 4.47 amps.
Henry's engineers obviously got it right, but I was alarmed at how small the actual resistor wire was. Thanks for the help Randy.

 

[Mach1 Driver]

I verified that it is indeed nichrome wire, and yes, nichrome wire is still measured by American Wire Gauge (AWG).

Technology is great when it works. I have asked the Google AI numerous times, what is the CMA for 7 strands of .013” diameter wire? I have gotten replies ranging from 28 AWG to 20 AWG.

So I opened two tabs and asked the Google AI what was wrong with it’s calculation and got this answer:
The Error Dissected:
The other AI claimed that 24 AWG has 1,024 circular mils and 23 AWG has 1,291 circular mils. If you look closely at the numbers, it took the true values for 20 AWG (1,022) and 19 AWG (1,288) and accidentally labeled them as 24 and 23 gauge.

How the HELL can the freaking AI mislabel anything?? Also, the CMA numbers don't exactly match either. So, I personally calculated the CMA, and found a real world table not AI derived for the CMA of 19 and 20 AWG. It works out like this:

19 AWG= 1,288 CMA and is 105 CMA higher than the resistor wire
Resistor wire= 7 strands of .013” dia. wire = 1183 CMA
20 AWG= 1022 CMA and is 161 CMA lower than the resistor wire

So the resistor wire is closer to 19 AWG than 20 AWG. It is important to note that the current carrying capacity of nichrome wire is different than copper wire, but now I have the correct AWG figured out. Since the points open and close, the average current through the ignition coil's primary at a 650 RPM curb idle is approximately 2.5 to 2.8 Amps. The average primary current through the ignition coil at 5000 RPM is 1.1 Amps. BUT, if the key is left in the ON position and the engine is stopped and the points are closed, the coil's current is 4.47 amps.
Henry's engineers obviously got it right, but I was alarmed at how small the AI called the resistor wire. Thanks for the help Randy.

 

[PSIG]

I'm not sure how my info is helpful to the OP, except for the odd reporting of AI for the estimated AWG of the nichrome, and how we would handle crimping or other mechanical concerns? Assuming that, and only knowing enough about resistance wire to be dangerous for other uses and reasons; I'll dump this out there if it's useful to anyone.

The OEM resistance wires of the era and through the '70s I have seen have been all over the map, for various assumed reasons. One is alloy supply, of which there are many alloys of different resistance and diameter, requiring gauge and length adjustment (e.g., around 20AWG at 25.52" for Ni80Cr20 at 1.35 ohms).

Another is reliability schemes, such as using lower resistance (larger diameter or gauge, or different alloy) but longer length, in order to spread the heat out. This would distribute the heat across greater length reducing the temperature on the insulation at any specific point. This all relates to the insulation, which is the weak link, as the wire can typically survive 1200 to 1400°F (750°C) assuming no damage. The insulation fries, so the wire fails. If the wire failed at a specific point without large-area insulation failure, that would indicate wire damage and a hot-spot.

Some of these are assumptions, but if valid might help to explain why some OEM wires were 30" or more, while others of similar resistance spec were 18" or less, and the AWG varied. <shrug> There's more info for those looking to create resistance wires for a specific application such as including the small increase in resistance when hot of less than 7% from 20°C to 400°C (NiCr 80/20), but enough to be part of calculations.

For less theory and more practical application (it just needs to work well); there are workarounds for wire overheating conditions or current limiting, and while not stock, can be both more effective yet relatively hidden or disguised.