Wire gauge at ignition switch

[stuartrubin]

TLDR:
What gauge wire should I use for wires to the ignition switch terminal #15?

Full version:
I am about to replace a (finally) defective ignition key switch. (Some of you may remember a previous post, http://www.fiatspider.com/f15/viewtopic.php?f=5&t=39516), where I thought I had a defective switch, but it turned out that one of the terminal was fried. I made a new terminal and things started working. Until it didn't...)

The after market replacement has one less connection. The instructions tell you to connect the #15 blue/black and INT blue/pink bundles together into one and connect that to #15 on the new. (Ref: https://fiatspider.com/f15/viewtopic.php?t=37135). My switch did not come with a "doubler" so I need to make one.

I have a kit of "quick connects", both male and female. I plan to wire two males to one female to create a "Y" connection for the #15 and INT wire bundles to go into the new switch #15.

This is electrically pretty straight forward, but I want to make sure I'm using an appropriate gauge wire.

And should these be stranded or solid (or does it matter)?

Thank you!

[18Fiatsandcounting]

This is electrically pretty straight forward, but I want to make sure I'm using an appropriate gauge wire.

And should these be stranded or solid (or does it matter)?

Are you asking just for your "Y" connector, or for other planned wiring jobs?

If just for the Y, I think 12 gauge would suffice. For comparison, I believe the "brown wire" to the ignition switch is 10 gauge, so you could go with that size as well. 14 gauge and smaller is probably getting risky if you need to carry a lot of current.

Use stranded wire; solid can break over time with vibration. I use stuff called "hookup wire", but note this isn't ideal for use in the engine compartment where you get lots of heat, gasoline fumes, oil, etc. But, since you're below the dash, hookup wire should be fine.

-Bryan

[stuartrubin]

Bryan, thanks for the reply. I assumed the stranded was the way to go for mechanical reasons, but sometimes there are weird reasons to pick solid. The stranded should be a lot easier to work with.

And, yes, I am talking about just the "Y" connector / adapter I'm rigging up. I think I actually might have some 12-AWG at home, which is nice. 10-awg seemed like overkill, but like you said, it's what the infamous "brown wire" uses.

I'm just not sure how much current flows through each of these strands. In this particular case, it will actually be 4 wires coming together into one, so I need enough to handle the sum of those currents. I would actually like to make a proper calculation. Of course, we're really talking about handling an "average" current since the issue for wire gauge is heat, and bursts are not to important.

Thanks again

[18Fiatsandcounting]

Stuart, I think you're looking at this the right way. A couple things:

- I actually have never sat down to calculate what the current draw on each of the wires is, but I suppose you could get a rough upper limit by combining the total of the fuses ratings served by a particular wire. In my older cars, there is one 16 amp fuse and nine 8 amp fuses, so the total would be 88 amps for the entire car, but in practice this would never happen because it assumes both high and low beams are on at the same time, all directional indicators are on at the same time, the wipers are running, the horn is blowing, the fan is on, the heater blower is on, the cigarette lighter is on, etc. Driving fast in the cold rain going all directions, while flashing your headlights and blowing your horn and lighting your smokes, in the true Italian style, I guess...

So, figure about half this in actual practice, for a maximum, or about 45 amps. And this is indeed what the early alternators were rated at. But it doesn't all go through one single wire. My sense for an individual wire in a spider is a maximum of 20 amps (except for the starter motor), and so 12 gauge would work. 10 gauge would as well, but it might be overkill as you mention.

- In general, solid wire is used in point to point applications where there is no flexing. In addition, solid wire is better for alternating currents, especially at higher frequencies where stranded wire bundles can do weird things with your signal. However, since neither of these apply for a car which is by and large all DC current, stranded wire is the way to go. I vaguely remember that the original wire from the coil to the ballast resistor in my car was solid core, but that may have something to do with the short run and the weird AC signals you get with a collapsing magnetic field in the coil. Like the way old cars used to drive AM radios crazy with interference!

-Bryan

[SteinOnkel]

Looks like you're on the right track with AWG12.

I can highly recommend www.wirebarn.com for your automotive needs. It's what I use to build all my harnesses and they supply a very good product at a reasonable price.

[18Fiatsandcounting]

I can highly recommend http://www.wirebarn.com for your automotive needs. It's what I use to build all my harnesses and they supply a very good product at a reasonable price.

Thanks Steiny. Another source that I have used is:
https://www.remingtonindustries.com/hoo ... utomotive/

-Bryan

[spider2081]

So I'm reading all the replies to the original post and agree
Then the I was thinking how does this logic compare to the logic of the "brown wire fix" where adding a second # 10 wire enhances performance because the wire resistance is halved.

Also On some Spiders I tried using # 10 crimp terminals on the brown wire and found the terminal opening was too smal for the brown wire. I guessing metric wire sizes do not directly cross to AWG,

[SteinOnkel]

So I'm reading all the replies to the original post and agree
Then the I was thinking how does this logic compare to the logic of the "brown wire fix" where adding a second # 10 wire enhances performance because the wire resistance is halved.

Also On some Spiders I tried using # 10 crimp terminals on the brown wire and found the terminal opening was too smal for the brown wire. I guessing metric wire sizes do not directly cross to AWG,

It doesn't make any sense. brown wire goes to one terminal with two external posts. It makes no difference whatsoever.

[18Fiatsandcounting]

It doesn't make any sense. brown wire goes to one terminal with two external posts. It makes no difference whatsoever.

I have to admit I have never understood this "brown wire fix". If you run multiple wires, but run them through the same single contact points inside the switch (even with different external spade lugs), all you have done is lower the current in each wire, but you've done nothing to reduce the current through the single contact points, which is what matters.

I'm always open to being corrected, so have at it guys. This from the guy with the original Sipea switches in 52 and 50 year old spiders, with no switch problems yet. Maybe I need a totally righteous stereo with some subwoofers in the trunk to draw some serious amps...

-Grumpy Bryan

[aj81spider]

For a small leg connecting your switch to several wires 12AWG should be fine. The resistance of 12 AWG is 1.588 ohms/1000 feet. The resistance of 10 AWG is 0.9989 ohms/1000 feet. If we assume you have a 1 foot pigtail then the difference in resistance between a 10 AWG pigtail and a 12 AWG pigtail is 0.6 milliohms. If you are drawing 100 amps through the pigtail the difference in drop will be 0.06 volts (and will be swamped by the resistance of the connections).

The brown wire fix is sadly one of my pet peeves. It's not aimed at the ignition switch, which as has been pointed out, sees the same currents and stresses with it and without it. It's intended to improve a bad voltage drop situation in the harness, and is popularly thought of as a way of splitting the current between the old wire and the new wire. In reality the new wire will carry most of the current and the improvement comes from reducing the resistance of the run by bypassing connectors that are 40 years old and frequently corroded and high resistance. You could get the same results by fixing the connection problems on the original circuit.

Making the faulty assumption that I can do the math correctly, here's an example: Let's assume that you are inspired to do the brown wire fix because you are seeing a 2 volt drop on your circuit. If we assume 45 amps drawn then there is 0.044 ohms of resistance in the original circuit, largely found in the connectors (wire generally doesn't increase resistance over time). If you add a 10 foot 10 AWG wire in parallel that with no intermediate connectors, the new wire will have a resistance of 0.01 ohms. With this, the new voltage drop will be 0.36 volts, vastly improving your situation ("Yay! the brown wire fix worked!"). However, the current will split proportionally (by resistance) down the two paths and the new wire will carry 36 amps while the old wire will carry 9. If your situation is worse than this and the resistance of the old wire is even higher, the proportional split also gets worse.

For all practical purposes you are replacing the original wire with the brown wire fix, not providing a supplemental path, and functionally you are simply bypassing old connectors that have developed high resistance over time.

[stuartrubin]

I have read about the infamous "brown wire" fix. I think the idea is that other electrical systems suffer because under heavy load, their respective voltages drop.

The brown wire is singularly responsible for carrying current into the ignition switch. The switch then distributes the loads as needed to ignition, accessories, etc. out of the switch assembly. The fix works because the switch has two #30 terminals, one of which is unused. So you just run a second brown wire and connect it to the other #30. This effectively drops the resistance to the brown wire in half, the voltage drops under load by half, and increases the power capacity of the wire by 4x.

This is a great reference for the "brown wire" fix: https://www.mirafiori.com/faq/content/bwfix/bwfix.htm.

As for the lifespan of the ignition switch itself, without seeing the internal mechanical/electrical workings of the switch itself, I doubt the brown wire fix will help. (In some modern electronic systems with "switching" power supplies, a voltage drop will cause a large current increase, so there may be some hypothetical improvement if you, say, some sick 1000W stereo....)

(And, just so we're clear, I'm actually not implementing this. I'm just replacing the switch and need to make an adapter to shove four wires into one.)

[18Fiatsandcounting]

A.J. and Stuart, I agree with all you've said. I just feel that if you have an issue with the original wires or connectors along the way, it would be better to fix those rather than just put in a new parallel current path. And, as noted, if the end result is that all the current still flows through a single point of contacts inside the ignition switch, that's still a common failure point.

I'm looking at my earlier spiders ('69 and '71) and the wiring is a bit different. Power comes to the ignition switch by two ways: 1) the brown wire, and 2) the black wire to the + output of the alternator. Different circuits are powered by each of these, so the brown wire doesn't carry all the current for the electrical devices in the car. I figured this was to avoid some circuits seeing a lower voltage at the brown wire due to the large voltage drop across the battery to starter cable, when starting the car. But maybe it's more complicated than that.

-Bryan

[stuartrubin]

Bryan, nice work on the math. These "back of the envelope" calculations are a lot more than I've seen in some real engineering applications! (I am an EE, but mostly do software now.)

To stress my point again through, I am not doing the brown wire fix. I have no reason to believe that I have any problem with current draws, bad connectors, etc. I have every reason to believe that there are internal problems in the ignition switch.

Nonetheless, while I'm in the area, I will at least inspect and clean the "quick" connectors, and replace any that are suspicious.

[18Fiatsandcounting]

Sounds like a plan Stuart. Carry on, good Sir!

Although my education is in chemistry, I have a pretty strong background in EE as my dad was a EE professor at the local college, and I was playing with oscilloscopes by the time I was 10. So, as some on this forum may have noticed, I'm always digging around in the electrical aspects of my various Fiats. This week it's alternators!

-Bryan

[spider2081]

I figured this was to avoid some circuits seeing a lower voltage at the brown wire due to the large voltage drop across the battery to starter cable

The alternator output connects to the starter solenoid battery wire with a #8 wire. So electrically they are the same potential except for wire loss. Voltage drop in a 2' piece of #8 would not be very much