Brown Wire Fix Question

[donerehart]

I've been reading up on the Brown wire fix and it appears that there are two ways to do it. Is this correct? Either you can run a wire from the starter stud to the ignition switch, or you can run a wire from the battery to the ignition. Which is better, what would you all recommend?
Thanks!!!
Don

[coise]

I was asking myself the same question .
Thanks for asking.
coise

[brianpainter]

Starter to ignition would be a shorter run, and easier to do in my opinion. I'm going to try and tackle it on mine in the next couple of weeks.

[azruss]

Personally, i think the battery cable is a little wimpy for that length. That cable has to supply all the power to the car. this isnt the brown wire fix, but would be a benefit to go up a cable size or double up the cable.

[Fiat88]

Personally, i think the battery cable is a little wimpy for that length. That cable has to supply all the power to the car. this isnt the brown wire fix, but would be a benefit to go up a cable size or double up the cable.

I agree. I did the brown wire fix going from the battery but also replaced the old green cable with a new 1 gauge one made by Painless. Heres the kithttp://www.summitracing.com/parts/PRF-40100/

[aj81spider]

According to my wiring diagrams connecting to the starter stud would parallel the existing brown wire. Connecting to the battery would mean current had to flow from the starter stud back through the battery cable then through the wire you connect to the ignition switch. I did some quick math, and made some bad assumptions to make the math easier. However if you assume:
- all wiring is 12 AWG with a resistance of 1.6 ohms/1000 feet.
- it's 6 feet of wire from the battery to the starter (approx 0.01 ohms)
- it's 4 feet from the starter to the ignition switch (approx 0.0064 ohms)
- a wire from the battery to ignition would also be 6 feet (approx 0.01 ohms)
- there is no additional voltage drop induced by the battery charging (which would make things even worse)

The source of your current is the alternator connected to the starter stud. To get to the ignition switch the existing brown wire goes 4 feet (0.0064 ohms). If you connect your wire to the battery then that path is back along the battery wire then through your new wire for a total distance of 12 feet (0.02 ohms).

With 30 amps of current the drop from the stud to the ignition switch through both paths would be approximately 0.145 volts.

If I did the math correctly (always a dubious proposition, so weigh in and correct me if wrong) then that means the current would split 22.7 of the 30 amps down the existing brown wire and 7.25 of the 30 amps down the wire connected at the battery.

If you connect the wire to the starter stud and assume you can make it the same length as the existing brown wire then the current would split 50-50 down the two wires.

The real situation is more complex, for example the wire to the battery is actually a thicker wire (hence less resistance) - but the general principle holds.

Either connection will help, but attaching at the starter stud will result in less stress on both wires and a lower total voltage drop to the ignition switch.

[donerehart]

That was very helpful AJ81! That is what I was wondering...makes total sense. I couldn't understand why taking the wire to the battery was the best option... Much appreciated!
Don

[coise]

Are you an electrical engineer aj81spider? This is great stuff, thanks a lot.
I´m always amazed how much we can learn by just chatting in this forum.

[aj81spider]

I actually am an electrical engineer by training and practice early in my career. Now I manage, so I don't do any real work. The electrical is about the only area of working on the Fiat that I'm truly comfortable (although that doesn't stop me from breaking other stuff!).

[TronDD]

Since I hate chasing electrical gremlins, I may need to stop by and give you the opportunity for more electrical work on a Spider. I won't even charge you for the privilege.

Tim.

[bjbrown]

So, it sounds best to first improve the wire from the battery to the starter with a heavier gauge, then add the second "brown wire" from the starter to the ignition switch. That sound about right?

[blurple124]

Boy, I am a mechanical engineer at Milwaukee School of Engineering right now, and that post up there brings back some bad memories from steady state circuit analysis.

[divace73]

current usually takes the path of least resistance......

[blurple124]

True, but if two wires are connected in parallel, you have to treat each as if it has its own resistance (as explained above, due to the resistance in the wire), then the current is divided between those two wires based on a formula. If you wanted to, you could figure out how much current would be going through each wire: Ia (current in wire) = RT/Ra (total resistance of both wires divided by the resistance of the wire for which we want to find current) X IT (total current at the source or going into the two wires as they split).

In all honesty, I don't think you would actually need to do the calculations. If you were to, you would see that yes, the current is travelling along the path of least resistance, but that does not mean that current through the more resistant wire is zero.

(yay math....)

[RRoller123]

Just a word of caution when making up wires such as these being discussed. I am a former V.P./Plant Manager/Mfg Engineer in electronics. I retired (sort of) 10 years ago, but this stuff is still current. For heavy cables, soldering in the home environment is very difficult. You really need a good solder pot and no-clean flux to pre-tin the wire, and it is very hard to get enough heat applied with an iron to the connector interface to make a good solder joint. If you use a flame, chances are you will toast the insulation pretty badly. It is best to go with a crimp, but the absolute correct tool and dies for the connector (ring, stud, whatever) must be used. If you do solder it and add flux, DO NOT use plumbers flux, as it is extremely acidic and will attack the solder system over time. No-Clean is the best, since they don't need to be cleaned (duh) but these are generally pretty wimpy fluxes. A reactive flux is going to be driven by capillary action back up the strands behind the insulation when the solder joint is formed, and no amount of cleaning in the home environment will get it out. The acid in the flux will stay active and attack the joint over time. Bottom line is the crimp is best, if done with the proper dies.