Electrical Connections: The Importance of Soldering
By Chris Wantuck
So much depends on the electrical system that during restoration or maintenance, extra care should be given to not only the wiring, but more importantly, its connections. This restoration tip examines good connection techniques and the importance of soldering.
Electric currents run through the vehicle's wiring from many different components. There are sources such as the battery or generator and there are loads, the components that require the current to operate. Ignition coils and breaker points, headlights and taillights, instrument panels, horns, and heater fans are all examples of electrical loads. All of these have one thing in common, they use wire between them and all use some sort of connector to transition the wire to a fastener. The most common connector is a wire terminal or lug. They can be open type such as a spade or hook type and the closed type is called a ring terminal. Regardless of which type is selected, electrical connections to these terminals have two properties associated with them: mechanical and electrical. A mechanical connection is a friction mating of the terminal metal to the wire itself, crimping or crushing the metal around the wire. All terminal types make this mechanical connection to the copper wire and in some cases include the wire's insulation for greater strength. The mechanical connection is intended to provide an electrical connection while also providing strength if the wire is pulled or strained. Electrical connections are those that use solder around the wire to improve the electric current flow and should not be confused with a mechanical connection. While strong most times, a solder connection is not a substitute for a good mechanical connection. The best electrical connections include both mechanical crimping of the terminal around the wire followed by soldering.
Wire is rated in sizes usually called American Wire Gauge (AWG) which is essentially the wire's diameter, expressed in either inches or millimeters. A 10 AWG gauge wire is approximately 1/8 inch in diameter and 36 AWG gauge, one of the smallest gauge wires, is a mere 0.005 inches or about the size of a human hair. The higher the AWG number, the smaller the wire. Wire gauges can be a single piece of wire called solid core or made up of multiple strands of smaller gauge wires called stranded core. The benefit of stranded gauge wire is that it is more flexible than solid core. Stranded wire can be made up of a few medium size wires, a couple dozen smaller size wires or in certain cases over fifty very small (human hair) size wires. Stranded wire examples of a 14 gauge wire include 7/22 (seven 22 gauge wires) and 19/27 (nineteen 27 gauge wires)
Cross section example of two types of 14 AWG stranded wires, before and after crimping. Left a 7/22 stranded wire and on the right a 19/27 stranded wire. The 7/22 would be more common for an automotive application.
Overall, the total diameter of the wire doesn't change, just its composition. Stranded wires are tightly twisted together while the insulation is applied during manufacture. The significance of a wire's gauge is the amount of current it can safely pass through it while the number of strands indicates its flexibility. The following table shows this for some common size wires.
The flow of current through the wire occurs evenly among the strands and when meeting the terminal, predominantly flows through the outmost strands of wire where they make contact with the terminal's metal collar. The contact of the single strands — their round edges against the terminal's collar — doesn't provide much contact area to carry the current.
Illustration of current flowing in individual wire strands. Note each strand's minimal contact to the lug's metal collar.
When crimped, the wires flatten out to conform with the collar, but the outer strands are still the connection point for current flow.
Crimped #10 terminal, but notice how much space remains between the strands and the collar. The gaps between the collar and strands let air reach the surfaces of the wire and consequently, corrosion will occur and deteriorate the connection.
Even if the crimp is perfect and all strands are making contact, there is limited contact between the strands and the terminal. Furthermore, a long term looming problem began the minute the wire's insulation was stripped: Oxidation.
Oxidation occurs on wire strands that are exposed to air. It deteriorates the wire much like rust does to exposed sheet metal. It begins on the wire's surface and etches into the stand's joints to include around the contact area at the metal collar. If sufficient current flows, it heats the oxidized area which in turn accelerates and creates more oxidation. This oxy-heat cycle continues to the point where the wire's contact to the terminal is reduced and current ceases to flow (through the affected strands). This puts more current through the other remaining strands accelerating their corrosion. The result is a corroded terminal that inhibits current flow and depending on the wire size and amount of current, can become hot enough to start a fire. Using a stranded wire with more strands like the 19/27 shown provides more strands to make contact and less air for oxidation. However, the same process will occur, it just may take longer to fully corrode. The simple way to 1) improve the contact surface area of the strands to the terminal and 2) prevent oxidation from forming (by blocking air from getting to the strands) is to add solder to the connection. Solder will fill in all around each strand as well as the entire surface area of the terminal's collar.
Same #10 terminal with solder filling in all the gaps. Solder enhances current flow from all the wire strands to the metal collar.
The result is a way for the current to flow through all the strands of the wire and pass through the terminal evenly without any loss. The noticeable improvement will be brighter head lights, tail lights, and brake lights. Even the horn will sound louder and the heater fan will spin faster. What is equally important is that by taking the time to evenly crimp and solder a wire's terminal, it will provide years of corrosion-free service. A terminal that is just crimped may not even pass the pull test.
Looks can be deceiving. This ring terminal is the right size for 10 gauge wire and was crimped, but all it took was a light tug to pull the wire from the terminals collar. A tighter crimp would have required more tension to pull it apart. More importantly, if soldered, it would not have come apart at all.
The final part of a good electrical connection is insulating the terminal's collar. The yellow or blue plastic shield found on terminals may become loose after crimping and almost certainly slides off when heated during the soldering process. The simple answer is to just remove the plastic shield altogether (or purchase terminals without the plastic shield) and use a special insulator that will conform to the terminal.
#10 Gauge ring terminal with and without plastic shield and stripped wire.
Shrink tubing is a vinyl hollow sheath that when heated, reduces its diameter to half of its original size. Two variations are available, thin wall and a thick wall. Both meet the electrical insulating needs, but the thick wall version "just looks better" for auto restoration.
Completed ring terminal connection with two thick layers of shrink tubing applied.
Actually using two or even three layers of thick wall shrink tubing over one terminal provides the cosmetic appearance of the rubber insulator originally used in the 1920's — 1940's.
Still not convinced soldering electrical connections is worth your time? Lets do the math. If each connection that hasn't been soldered loses a mere 0.05 volts across it (measurable only with a digital volt meter) and there are say fifteen connections between the battery and the ignition system (includes connections through a firewall, circuit breaker, amp gauge, ignition switch, coil and return ground leads), that's a 0.6 volt reduction. That may not sound like much, but while engaging the starter that draws a sizeable amount of current and on a 6 volt system, the amount of voltage reaching the coil is at best 4.0 volts, that 0.6 volt reduction would now be 3.4 volts at the coil. Add to this a cold day where the engine isn't spinning very fast (low compression and weak spark), this may make the difference if the car will start. Isn't it worth some extra time to put all the factors in your favor?
A Soldered Connection Provides 3 Times Better Current Flow Than Just A Crimped Connection
- Use new wire when possible. This can be cloth covered over vinyl to maintain original appearance that is now commonly available from specialized suppliers.
- Strip wire using wire strippers that have gauge settings or notches for the size wire you're working on. Using a knife to cut around the outside insulation may cut into the strands which defeats the purpose of making a good connection.
- Crimp the terminal using flat nose electrical Lineman's pliers. This ensures that the collar is flattened evenly and not just in the center.
- Use only rosin core 60-40 solder that is intended for electrical applications.
- Use a heat gun or hot-air hair dryer to apply the shrink tubing. Never use an open flame.
- Select the proper gauge wire which is especially true for 6 volt systems. Generator and ammeter connections may very well carry 25 amps total which calls for 10 gauge wire. Use the largest gauge wire where possible, especially for high current loads such as the head lights.
- Ring terminals provide better contact area over spade or hook terminals and are less likely to come off if the screw or nut securing it becomes loose.
- Select the proper terminal for the screw size, matching up both the terminal's hole (screw size) and wire size. Using a #10 ring terminal for a #6 or #8 size screw is too big and current will not flow properly.
Sources: A listing of some suppliers for wire and terminals:
McMaster-Carr Industrial Suppliers,
Warehouses in NJ, IL, OH and GA
Rhode Island Wiring Service,
567 Liberty Lane, PO Box 434,
West Kingston, RI 02892
P.O. Box 51, 107 Woodville Road,
Wood River Junction, RI