A soldering iron is a tool that heats up to melt solder, a soft metal alloy of tin and lead. In automotive electrical work, soldering is the gold standard for joining wires. Unlike crimp connectors, which can work loose over time due to engine vibrations, a soldered joint fuses the copper wires together. This creates a permanent connection that resists corrosion and maintains low electrical resistance.
To use the soldering iron, you first slide heat-shrink tubing over one of the wires. You twist the bare copper ends of the wires together, then press the hot tip of the iron against the joint. You apply solder directly to the heated wire, not the iron tip. The hot copper draws the melted solder into the strands, sealing the splice. Once the joint cools, you slide the heat-shrink tubing over the repair and shrink it with a heat gun to seal out moisture.