While crimping has become a simpler and more convenient way to join automotive electrical wires, for some people it’s no substitute for a properly soldered joint. Done properly, soldering provides a strong, long-lasting connection that’s arguably better than any crimp connector.
Be it for connecting broken wires, replacing sections of old wiring, or installing accessories like auxiliary lights or audio systems, soldering may seem like a simple process, but it’s a skill that takes some knowledge and practice.
Amateur errors, like a poorly soldered connection, using the wrong kind of solder or mismatching different gauges of wire, can lead to more serious electrical gremlins or even fires in a worst-case scenario.
While you can practice your soldering skills with scrap wire, the knowledge of what wiring you can – or should – solder on your vehicle can be a little harder to come by. If you have any uncertainties over how, when and where to use a soldering iron, leave the job to a professional auto electrician.
The Right Gear
The first requirement for quality soldering is a quality soldering iron. Electric soldering irons are the most common, while butane-fuelled units up the ante, but up the price, too. For most automotive applications, a 40W or 50W electric soldering iron will be sufficient.
A cordless and rechargeable soldering iron offers greater convenience and flexibility than the corded equivalent, and if you can get one in kit form that includes extras like different soldering tips, a safety stand and other accessories, that’s even better.
You’ll also need soldering wire - of the correct electrical type - and a good pair of wire strippers. Safety glasses are highly recommended as solder can spatter. You should also have a sponge to both capture drips and clean the iron’s tip. A heat gun to shrink protective tubing over soldered connections is a handy addition, too.
Clean, Clear, Hot
Solder on a clean surface where possible, but if you’re soldering in the car (underdash wiring, for example), place something non-flammable under the working area as a precaution against burning the vehicle’s interior.
Always solder in a well-ventilated area, too, as the fumes from melted solder can be harmful.
Finally, make sure the soldering iron is hot before you start – if it’s not up to temperature, it won’t melt the solder correctly and you’ll make a mess.
With the solder and iron ready, slide a section of heat shrink tubing over one end of the wire, use the wire cutters to snip off around 2cm of insulation from each end of the wires you’re wanting to join, then twist the two exposed ends together.
Heat the wire with the iron, then apply the solder to the wire and allow it to melt through the strands of wire. Don’t apply too much and don’t use the iron to drip molten solder onto the wire.
Once coated sufficiently and evenly, allow the wire to cool, then tug on each end of the wire to ensure the soldered connection is strong. If it is, slide the heat shrink tubing over the join and gently heat to create a watertight seal. If you don’t have a heat gun for this part of the job, some soldering irons come with a heat shrink tip, or you can use a hair dryer at full blast.
Practice, Practice, Practice
With practice, the smoothness and evenness of your soldering will improve, ensuring electrical repairs and new connections that you’ve made in your vehicle are more conductive and last longer.