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My Toyota Vitz 2007 has some trouble with its engine wiring harness. Engine idles roughly and wire harness damaged in several places and I had to solder wires several times. I suspect the rough idling is also probably due to the problems in the harness.

I tried to find an an aftermarket wiring harness but the best one I could find has its wires that are going to the fuse box cut. I found the fuse box and could buy the socket that connects to the fuse box too. Now I am going to solder the wires.

There are about 20 wires I have to solder. My question is, would it be appropriate to solder wires? Would it make the resistance of the wires different and cause problems?

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  • depending on what the wire is doing using but connectors with heat shrink tubing is acceptable as well.
    – Ben
    Commented May 14, 2016 at 10:49

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Soldering wires in a particularly old harness will actually reduce resistance in that portion of the wire. It won't be noticeable except for allowing for greater signal/line integrity. I.e. In a wire with failing spots (where conductor has been damaged or has broken) or exposed conductors, rejoining via soldering will restore the reliability of that particular wire.

Soldering broken wires, extending stretched wires to reduce stress, etc. are very common processes. When done correctly, you will not affect the performance of the conductor. What actually will affect your harness more is the age and growing parasitic resistance of the harness itself.

Ideally however, best practice dictates that whenever replacing the harness, it's best to replace it connector to connector instead of splicing wherever possible. This is because for every solder joint, you theoretically introduce a possible failure point. Is that of major concern? Not particularly if you are skilled enough to produce good solder joints without cold jointing, insufficient wetting, and the numerous problems that can plague a solder joint. This will affect reliability, as well as introduce interesting anomalies.

In summation, provided you can terminate solder joints effectively, and protect them adequately (please don't use electrical tape to cover your solder joints - heatshrink is certainly a worthy investment for the longevity of your harness to prevent moisture and corrosion from penetrating the insulation) you should have absolutely no issues. Ideally you should just replace the harness from one major connector to another if possible, however in some cases this is not possible, and soldering wires is the only remaining option. This isn't to say soldering can't be as effective or durable, there are just many variables that can make it particularly difficult for novices or the inexperienced to maintain quality, repeatable, and effective solder joints.

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  • +1 for heat shrink! You can buy it very cheaply and readily these days. It just takes a little forethought when working on the joints. Don't leave home without it.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented May 14, 2016 at 13:10
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A word of caution: soldering creates a stiff section in an otherwise flexible wire. If the wire flexes, the soldered/un-soldered transition is a stress point, where the wire can break. This is one reason that crimping is (usually) preferred (eg). Which is not to say, never solder; just, think about the mechanical support/strain relief.

And, clean off the flux; it's corrosive.

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  • Very valid point to bring up, however, crimped connections introduce pressure that can suffer the same kinds of breaks due to inflexibility that a soldered joint can. From the ease of utilization standpoint, crimped connections are easier and much quicker. But neither are superior to pure unadulterated stranded wire in comparison for durability where there is frequent flexing motion.
    – Jeep Cherokee Rescue Society
    Commented May 17, 2016 at 9:48

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