Replacing Aging Wire With Like

Mar 06, 2024

Few things can be as perplexing – or frustrating – as electrical wiring on a vintage tractor. More often that not, the best solution is simply to replace it front to back.

"Typically, if you find sections of wiring that are badly deteriorated, it's usually bad throughout," says John Brillman, owner of The Brillman Company (brillman.com), which stocks both wiring harnesses and bulk electrical wire. "Quite often, you'll find that the insulation on the original wiring, which sometimes consisted of nothing more than waxed paper, has deteriorated to the point it's only being held together by the cotton braid. When that happens, there's not much to keep electric current from jumping through the cotton to a nearby frame rail or component."

"Rarely will you find a good section and a bad section. So now you need to decide whether you want to locate a wiring diagram and rewire the tractor on your own, or if you'd rather purchase a preassembled wiring harness. But you also have to consider your experience and the amount of time you have available," he says.

Your purchase will depend on whether you're restoring the tractor as a working machine or you're doing an accurate restoration.

If you're going for historic accuracy, you'll want to locate the appropriate gauge of lacquer-coated, cotton-covered wiring, which was used on most tractors built before the mid-1950s. Brillman says the wiring harnesses his company stocks have all terminals soldered in place and sealed, and all wires are numbered and color-coded to correspond with an installation guide that's included.

Up to current grade

All reproduction harnesses are also made up of the appropriate gauge wire covered by PVC, which, in turn, is covered with cloth loom, cloth braid, nylon braid, or tape to match the factory original, giving you the best of both worlds. Several other companies offer similar products, including Walt's Tractor Parts (waltstractors.com) and Agri-Services (wiringharnesses.com).

Brillman says the biggest thing to watch for when replacing wiring or installing a new harness is to make sure the ground points are free of paint. "Unlike the original manufacturer, most restorers paint all the parts and then assemble the tractor. So make sure you don't paint over your grounds. Or else clean off the paint from those areas before rewiring."

Stay with 6-volt?

In the process of rewiring a tractor, you'll also need to decide whether to restore the original 6-volt system (which was used on most vintage tractors) or upgrade to a 12-volt system and/or electronic ignition.

When asked about converting a Farmall tractor to a 12-volt electrical system, T.W. Cook, a Farmall enthusiast from Georgetown, Texas, generally answers, "Why? These tractors were designed to start perfectly well on 6 volts and will start today if you have proper cables and a good battery."

He says the problems can often be traced to the battery cables, noting that many tractors he's seen have had the cables replaced with thin 12-volt cables. "If you have these, you need to throw them away and replace them with big 2-gauge or larger cables," Cook says.

If you do convert to a 12-volt system, Brillman says it's important to install the correct wiring for the system. "It depends on how it's been done," he says. "Many times, it's possible to rebuild the generator for 12 volts, in which case, the wiring harness layout will be identical. However, if the system has been upgraded to use an alternator, it's a totally different layout."

Either way, don't forget to install new spark plug wires as you finish up the project.