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The basics of how electricity works and why you need to solder wires together

04-18-2009, 05:14 PM
A lot of people have problems when it comes to electrical installations or repairs. The electrical system is one of the most difficult to understand on a car. It can be simplified significantly by thinking of electricity as water and the wires as pipes. Voltage is water pressure, the amount of water flowing is amps, and any restrictions in the pipe is resistance. The battery is a pump, and the electrical load is a turbine. For the pump to spin the turbine, you need pressure and flow. It won't spin without enough of both. If there are restrictions in the pipe, there will be a loss of pressure after the restriction, but full pressure before it. The restriction will also cause less water to flow. There may just be a trickle trying to spin the turbine, which won't work. OK, the water analogy's gone. It's all volts, amps, and resistance now.

The most important thing you need to understand when doing anything electrical is resistance. Every conductor has resistance. An ohmmeter will read zero or close to zero on any wire as long as there is some conductor throughout. You can cut all but one strand on a battery cable, and guess what? It will show zero ohms from end to end. This is because an ohmmeter uses almost no current to measure resistance, so just about any load will draw more amps than the ohmmeter uses. That cable isn't going to run much more than a dome light, but your meter won't tell you that. If you are having a problem and suspect a bad wire, don't use an ohmmeter. Connect the leads of a volt meter to each end of the wire and operate the circuit. It should read less than .5v on very high amp circuits (starter, winch, offroad lights, etc). .5v is the MAX, not what you want. Usually it will be closer to .1-.2v, but "they" say .5v won't cause problems. For low amperage circuits, the max is .1v. This is also why you need to select the proper wire size. Too small a wire will have a volt drop across it, which turns the wire itself into an electrical load. This makes the wire very hot and can cause a fire. The hotter a wire gets, the more resistance it has, so it just keeps getting hotter and hotter. It also drops the voltage to what you are running, which can cause improper operation in electronics, can cause some motors to go bad prematurely, and will make lights dimmer. A lot of people don't give any consideration to the ground side of the circuit, but it is just as important as the power side because all the current has to flow back to the battery. I can't tell you how many times I've seen 4-0 cable going to an amplifier with 4-0 cable for the ground screwed to some body sheet metal. What's wrong with that? Take a look at your battery cables. The ground usually has a ~10 gauge wire that bolts to the body. The body ground on the car is intended only to give a ground to some sensors, headlights, taillights, and other fairly low amperage accessories. This means essentially that the amp has 4-0 going to it, but only 10 gauge going back to ground, which electrically speaking is the same as having 10 gauge going through it because the curent can travel back to the battery only as the smallest wire in the circuit allows.

The most important thing to do when you install something electrical is solder the wires together (this is assuming you selected the proper wire size, if your project didn't come in a kit ( Many people will simply twist the wires together and then wrap electrical tape around the twist. This is a very bad thing to do. The wire will loosen and you will have an intermittent open wire. Whatever that wire powers will turn off. Solder is also needed because if all you do is twist the wires, you have very little contact between the two which will cause high resistance if there is enough load (amp demand). This is why sometimes it will work for a while and sometimes it wont. The larger the strands, the truer this is. The solder fills in the gaps and gives you one solid wire, electrically speaking. Solder also makes the wire as strong as a solid section of wire. If something pulls on a wire that is only twisted together, it will pull apart. A soldered wire will break on the wire, not where it is soldered, and it takes a lot of force to do that. Before you solder something, plan what you are going to do. Heat shrink tubing is the best way to protect the solder joint, from here on out, called a "splice". The heat from soldering will shrink the tubing if it is too close to the splice. If you can, give yourself some extra wire so you can slide the heat shrink to one end, 4" is usually enough to keep the heat away. If heat shrink is impossible for some reason, use good electrical tape. The cheap stuff cracks and will let moisture into the splice which will cause corrosion which will cause resistance. "Crimp only" connectors should be avoided whenever possible. The ONLY time you should even consider using them is on the interior of the vehicle where they cannot get wet. Even there humidity can cause a problem over time. Under dash work will sometimes require the removal of the dash to solder a wire. In that case, I will use a crimp connector, but be sure you use one that comes with heat shrink already on it! To summarize, if at all possible, solder it!

How to solder: Follow the tool manufacturers safety instructions! Strip approximately 3/8" to 1/2" of insulation off each wire to be soldered. Place the heat shrink tubing over the wire and as far away from the splice as possible, at least 4". If the strands in the two wires are roughly the same size, flare out the strands on each wire and push them together. Push the strands back flat once the wires are together. Unless your skin is thicker than mine, use pliers or you will puncture your fingers. It is very important to be sure you don't have any conductors pointing out that will pierce the heat shrink tubing. Using a soldering iron, heat the wire from the bottom and hold the solder on the top of the wire. When the solder starts to melt, keep feeding it into the splice until you can see solder all the way around the joint. Don't use so much that it forms a drop on the bottom or it flows into the wire inside the insulation. Do not feed the solder onto the iron, feed it directly into the wire. Feeding it into the iron will let it start flowing into the wire before the whole wire is hot enough for the solder to flow, and possibly causing it to not flow throughout the splice. If the insulation starts to burn before the solder melts, you need a hotter iron, one that will heat the wire hot enough before the heat dissipates further down the wire. Once the splice is cool enough that you can hold it with your bare hands, slide the heat shrink over the splice and shrink it using a heat gun. Of course, for more than one wire, solder them all, then heat shrink them all. Or wait for each individual wire to cool, I guess it's really up to you.

Here are some pictures, I have all my good tools at work, so the way I did it is not entirely right. All I could find at home was needle nose pliers and sidecutters. You should really use wire strippers to strip the wires. The best set I have is Gardner-Bender that I got from Menards, they cost $14 and are better than my $40 Snap-on strippers. Lacking the proper tool, I used a very sharp knife and rolled the wire over the blade to cut the insulation and then pulled it off. That's a good way to cut yourself, and not always possible in a real-world scenario, so use the right tools! Having forgot my heat gun at work as well, I used a lighter, not the best way, as explained later.

These are some of the tools you need. Not pictured are wire strippers and a heat gun.
A- Soldering iron set. This one cost about $75, but worth the investment for me. You can get a cheap electric one for about $7, but as always, you get what you pay for. Butane is also more expensive than electricity, but it heats up so much quicker that it's worth it. The heat is also infinitely adjustable, opposed to even the best electric that have only a few heat settings. That little can of butane cost $2.50 from a gas station... normally I buy big cans in bulk and get a can 4x that big for 1/2 the price but I also forgot to fill the soldering iron before I left work.
B- Butane for the iron, not needed for electric, of course
C- Wires to be soldered. Having forgot automotive wire too, this is part of an old power cord. Similar enough for a demonstration, just thicker insulation than typical automotive wire
D- Needle nose pliers
E- Sidecutters
F- Heatshrink butt-connector
G- Heatshrink tubing
H- Electrical tape (won't be using it, but it was already sitting on my "workbench" when I started)
I- Finished stereo adapter I made last summer
That black cap looking thing next to the electrical tape is the cap from the soldering iron
Here I am holding the wire, so far so good!
Using a sharp knife to cut the insulation and pull it off. Don't use a knife! Always use the proper tool.
The wires are stripped and the strands flared out slightly
Flared wires pushed together and flattened again.
Ready to solder the wires together. Note that I am resting the wires on the open needle nose pliers. This will (hopefully) prevent my improper work surface (upside-down plastic storage container) from melting or catching on fire! If soldering inside a vehicle, have something on the carpet to protect it from possible damage. A damp rag has always been sufficient for me.
The wires are now soldered together. I couldn't take a picture of the actual soldering because I need both hands for that.
The heatshrink is in place. Remember to slide it at least 4" from the splice while soldering.
The heatshrink is now shrunk over the splice, keeping out any moisture that will cause corrosion.

That's all there is to soldering. Anyone with two hands can do it, but it may take a bit of practice on scrap wire before you dive into the real thing. Now, if you cannot solder the wire, here is the "most proper" way to use a heatshrink butt-connector:
Strip about 1/4" of insulation off the wire and twist the strands together tightly.
Here is the heatshrink butt-connector, notice the heatshrink already around it.
Insert the twisted strands into the butt-connector and crimp it using the crimping part of the wire strippers. Repeat with the other wire. I used pliers to "crimp" it, but I wouldn't trust that in a live circuit. Once you have both wires crimped, pull fairly hard to be sure the crimp is solid. You will be unable to pull the wires out if the crimp is good. If a wire pulls out, cut the connector out and try again.
Here it is crimped and shrunk. Notice the melted part on the right side. That's because lacking a heat gun, I used a lighter. An open flame is not good for shrinking the tubing. This particular one has two layers of heatshrink. The outer layer is just regular heatshrink, but there is a glue-like inner layer that melts and fills in any gaps, such as if you had two wires going in one side. That is what you should do if you ever need to "T" into a wire, DO NOT use scotchlok connectors, they damage the original wire and provide no protection from corrosion.

04-18-2009, 10:26 PM
Now we just have to hope the people who come to ask use the search button =P.

Nice comparison though, to water. If they only knew how much more there actually is to it all

04-19-2009, 01:14 AM
The what button? Sadly, I can't say I've ever heard anyone ask how to solder anyway... it's easy money making a minimum of $32.15/hr fixing something that almost anyone could have soldered to begin with... everyone remember that when the proper tools seem too expensive. And that's my pay, the shop gets the other half. I just wish I could get 8 hours of work in a day. The message here: buy the tools, save money. Someone could have almost bought my nice butane soldering iron set for what they'll pay to have me fix it.

I cannot take credit for the water analogy, I heard that from one of my electronics instructors. I started thinking about it more in-depth after he said that, and there's a surprising number of things that work until you get into transistors, logic gates or AC stuff. Switch=valve, diode=check valve, capacitor (in a DC circuit)=one of those vertical things that traps air to keep the pipes from banging when you shut the water off.

I originally typed a bunch about ohms law, watts, and got more in depth on what's in the post. I figure what's there is all most people need to know for most things, and can easily research if they need more. I started out only writing about soldering only and then had to add on, then took a lot out when it started looking like a book. I'm hoping people read it, not scroll though and decide not to :)

Blue Bowtie
04-19-2009, 11:59 AM
Excellent post. You have demonstrated at length and with graphics what I have been preaching for years.

The only difference is that personally, I don't even trust the compression connections in the long term. I've worked on far too many older vehicles, machine tools, and installations to have any faith in compression alone, except in HV installations. Even those are somewhat suspect, especially on aluminum and nickel. Cold flow happens, no matter the material. It occurs faster with aluminum and annealed copper, but happens to all materials eventually.

04-19-2009, 10:11 PM
Thanks for bringing that up, I didn't mean to imply that I trust them, in fact you will never see one on any of my vehicles. Ultimately it's the customers decision, and when there's no room to get both hands in there without hundreds of dollars for removing part or all of the dashboard, they'll pick the cheap route every time. If I were the service writer that would change, but I'm not. It's never an option except for underdash work. I haven't seen the heatshrink crimp connectors fail, I think the inner glue layer might hold the wires and reduce the cold flow by keeping tension off the conductor a bit :dunno:

04-19-2009, 10:56 PM
I tin both sides before butt soldering two wires together. In tight places this is my go to connection!

I also like doing the twist and solder connection on easier to get to wiring jobs

If you use a self sealing butt connector make sure you see the "ooz" squeeze out the ends when you heat shrink them!

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