The material that follows applies to simple DC starting, charging, and instrumentation circuits. Cutting or splicing wiring harnesses used with electronic engine management systems can do strange things to the computer.
The first consideration when selecting replacement wire is its current-carrying capacity, or gauge. In the context of diesel engines, two more or less interchangeable standards apply: the Society of Automotive Engineers (SAE) and Japanese Industrial Standard (JIS).
For most wire, the smaller the SAE gauge number, the greater the cross-sectional area of the conductor (See Table 11.1). Thus, #10 wire will carry more current than #12. The schema reverses when we get into heavy cable: 4/0 is half again as large as 2.0 and has a correspondingly greater current capacity. Note that one must take these current values on faith.
The JIS standard eliminates much confusion by designating wire by type of construction and cross-sectional area. Thus, JIS AV5 translates as automotive-type (stranded) wire with a nominal conductor area of 5 mm2. The Japanese derive current-carrying capacity from conductor temperature, which cannot exceed 60°C (140°F). Ambient temperature and wire type affect the rating, as shown back in Table 4-2.
Vinyl-insulated, stranded copper wire is standard for engine applications. Teflon insulation tolerates higher temperatures than vinyl and has better abrasion resistance. But Teflon costs more and releases toxic gases when burned. In no case should you use Teflon-insulated wire in closed spaces.
All connections should be made with terminal lugs. Solder-type lugs (Fig. 11-5A) can provide mechanically strong, low-resistance joints and are infinitely preferable to the crimp-on terminals shown in Fig. 11-5B. Insulate with shrink tubing. When shrink tubing is impractical (as when insulating a Y-joint), use a good grade of vinyl electrician’s tape. The 3-M brand costs three times more than the imported variety and is worth every penny.
Figure 11-6 illustrates how stranded wire is butt spliced. Cut back the insulation 3/4 in. or so, and splay the strands apart. Push the wires together, so that the strands interleave, and twist. Apply a small amount of solder to the top of the joint, heating from below.