How Car Wiring Looms Work – a Rough Guide

Electrical wiring in vehicles is colour coded and the bunch of coloured wires is called a loom. If there are a number of wires running together, they’re fastened together with a plastic sleeve, or insulating tape.

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Modular Looms

Automotive wiring looms are the electrical webs that control many car functions, and keep a lot of the switches and controls talking to each other. A good automotive wiring loom retailer will have a selection of modular looms for various vehicle types. These often consist of front and rear looms and a main loom. Let’s take an example of a modular loom and see what it comprises. The Main Loom will contain the Dash Loom. This has half a dozen branches, so that the dash can be wired separately from the vehicle. The separate wires are for:

•       Switches

•       Lights

•       Gauges and dials

•       Warning Lights

•       Hazard lights and Signal Indicators

•       Washing function / Windscreen wipers

The loom also connects to the Electronic Control Unit (ECU) which has the following functionality:

Picks up the tachograph signal, the radiator fan signal for the relay that controls the cooling fan, and the signal for the fuel pump.

A relay controls a function, and the main loom will control the Rad Fan signal for the cooling fan relay and the fuel pump signal pick up for relay. It will also have a live positive switched feed for:

•       Fuel injector

•       Lambda sensor (controls tolerances for exhaust and catalyst)

•       Pick-up for distributor

•        Amplifier and coil

•       Idling valve

•       Permanent Live positive feed

•       Earth

A good automotive wiring loom provider will have diagrams to help with the job.

Before You Start

You’ll need to equip yourself with a set of tools before you start – have a look online to see an illustrated guide to what you need.

Before starting work on a car’s wiring, disconnect both the battery terminals. Be careful if any individual wire comes loose and when you unclip a wire make sure you put it back in its clips. Beware short circuits – one cause of these is where a grommet has come out of its hole and the hole edges have worked through the insulation. Keep an eye out for corrosion too, because it might be causing a bad contact, with the result that something isn’t connecting.