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Exclusive Article
Wiring Basics
by Curtis von Fange

One of the most neglected parts of old tractors is the wiring. After sitting in the elements for half a century or more much equipment wiring has deteriorated to sparsely covered strands of copper or other metal. Plastic insulation has cracked, mice have eaten through the older clothed style coverings and the exposed wires have reacted to winter moisture and salts by turning blue and powdery. Terminal ends have corroded, rusted or just plain evaporated away. Aged wires not only keep an engine from running but also pose a safety and fire hazard for the operator. Let’s take a brief look at wiring and what to do with it when it goes bad.

Most tractors need electricity to run. Diesel engines require vast electrical reserves for operating their huge starters. Gasoline motors need an electrical spark to ignite the gas/fuel mixture. Electricity operates the gauges, lighting features, and other tractor components. The transfer of electricity from the battery or generator to the energy consuming parts requires the use of some sort of transportation vehicle. That vehicle is wiring.

Wiring comes in all shapes, sizes and colors. Primary wiring transfers the battery voltage. It carries the 6, 12, or 24 battery voltage on the tractor. This wiring has ample insulation and wire content to carry the heavy currents from the starter to the battery and elsewhere on the unit. Secondary wiring is the conduit for high voltage in the ignition high-tension circuit. Its heavy layer of insulation gives protection against potential voltage leakage that causes cross firing between spark plugs. Secondary wires run between the spark plugs, distributor, and coil. This type of wire was covered in a previous ‘tune up’ article found in the YT archives. In this article we are going to continue our discussion on primary wiring.

Primary wiring is basically a length of metal conductor contained in a cloth, plastic, or rubber type insulation. The conductor can be of solid core or stranded metal. Most tractor and automotive applications have stranded wire for ease of bending around tight corners and for ease of terminal end crimping. The metal is usually stranded copper although stainless steel or aluminum is also used on occasion. Because copper is so universal it is best to it as the standard for all the wiring. If copper is mixed with aluminum there is a potential of future problems because the two metals may adversely react to each other over time. Don’t mix and match different wire types.

Wiring comes in different sizes depending upon the wire length, application, voltage, and amperage requirements. The total circuit length plays an important role in determining the size wire to use. As electricity moves through a wire it builds up resistance that, in turn, creates heat. The longer the wire, the greater the resistance. If the load carries a large amount of current and is a long distance from the battery, then resistance can cause excessive heat build up in the wire, melt the insulation and cause a short. A wiring chart found in an automotive service or manufacturer’s service manual will help in determining the size to length ratio. Make sure that the wire carrying capacity matches that of the total load for that circuit.

Application of a circuit and voltage play a large part in determining wire size. As a rule of thumb six-volt circuitry will have larger diameter wire sizes than twelve volt, and twelve volt larger than twenty-four volt. In other words, the higher the voltage, the smaller the wire size. Conversely, the larger the amperage load on a wire, the larger the wire diameter. For example, the primary wire running from the battery terminals to the starter or solenoid for a typical six-volt battery will be larger than that of an equal sized twelve-volt system because of the higher current demand for six-volt systems. In determining an appropriate wire size, total all the loads on that particular circuit. Then, considering the appropriate voltage and circuit length, use the wiring chart to establish the correct wire size. If no chart is available then use the old adage that ‘bigger is better’ and use a wire size larger than what one would consider normal.

When running wire for a circuit there are a few things to keep in mind. Make sure that the terminal ends are clean and properly crimped or soldered. Dirty posts or terminal ends will cause resistance and create heat. Tighten and secure properly using locking washers where appropriate. Use terminal boots to cover exposed terminal shanks. Keep the wiring from exposed heat sources such as exhaust parts or cooling system components and avoid oily areas along with other moving parts like fan blades or PTO shafts. Secure the wiring using mounting clips or clamps. When possible run multiple wires in a loom or harness that is mounted in secure locations. If passing through sheet metal install a grommet to avoid contact with the metal edges. Also be certain to leave enough slack in the wire to compensate for the rocking motion of the motor, wheels or similar moving components. Take care not to run wires in the slots between tractor parts as vibrations and part movement may wear through the insulation and cause a short.

Wiring problems can be frustrating and annoying. Proper mounting and protection of the wires can alleviate many of these hassles. If a circuit does not work the first thing to do is check for a blown fuse. A fuse is a protection device designed to open the circuit when it is overloaded. It protects the circuit load along with the wiring. Don’t operate the load without proper fusing in place, rather, locate the reason for its failure. Fuses blow because too much current is flowing through the circuit. The first thing to do is to scan the wiring for bad spots, grounds, frays or crimps to ground. Next, disconnect the circuit load to see if it is broken. If not, then check the wire integrity with an ohmmeter for grounds or cross wiring to other, larger circuits. On older tractors the problem is usually found to be a wire that has rubbed through, has broken or cracked insulation due to heat or oil breakdown, and is touching ground.

Sometimes the fuse may not be blown but there is no power reaching the load. Once again, first check the load to make sure it is not broken. Then check the terminal ends for breakage. On rare occasions wire that has been run near strong heat sources can sometimes break the copper strands without compromising the protecting insulation. In cases like this it is best to replace the wire for that circuit.

Occasional visual inspections of overall wiring conditions on your equipment will prevent many wiring problems. To reiterate, look for cracked and dried insulation, mice nibblings and nests, oil soaked wires, wires incorrectly routed with tight bends, and terminals with heavy corrosion and debris. Prompt and corrective action will ensure healthy wiring for long, trouble free use.

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