In certain locations within a vehicle and in environments where strong electromagnetic interference is present, wiring harnesses are subject to a situation where unwanted electromagnetic induction occurs. This interference is referred to as noise. To prevent noise, some vehicles use shielded wiring harnesses. The type of shielding used can be one of three forms: twisted pair, Mylar tape and drain lines.
Twisted pair uses two wires delivering signals to a common component. The wires are uniformly twisted through the entire length of the harness. The twist has the effect of cancelling any noise that occurs in the wires.
Mylar is an electrically conductive material that is wrapped around a wiring harness inside the outer harness layer. Any noise that attempts to reach the wires inside the shield will be absorbed by the Mylar where it will be conducted to ground via a ground connection. If the harness is exposed, the Mylar will have to be rewrapped so that noise cannot penetrate into the harness.
A Drain line is a non-insulated wire that is wrapped within a wiring harness. The drain wire is connected to ground at the harness source end and conducts any noise to ground, negating the noise effect. If the drain wire is cut, it will be inoperative so it is important the wire is not cut.