The protective insulation often gets worn through on the wiring loom of 135 tractors. This usually happens after years of vibration, the wires rubbing against the engine. It is also quite common for rats to chew through wiring looms! The result is at best an inoperative electrical system, but it is not unusual for an electrical fire to start – particularly if there is oil stuck to the side of the engine.
Prices for a wiring loom are around the £25-£30 mark, and you can compare prices here.