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The Right Connection

DIY car wiring basics

by Julian Edgar

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This article was first published in 2008.

There are lots of Dos, Don’ts and some Maybes in wiring cars. Don’ts include things like short circuits and too-thin wire, Do’s include the use of fuses and relays, and Maybes include crimp connectors and using adhesive electrical tape.

One of the most important things to remember when wiring cars is that the car is subject to heat and vibration, movement and (in some areas) dust and water. Wiring that would be fine in a house or some other static object can fail when subject to the rigours of automotive use.

Wrapping up the Subject

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Always corral wires which have been added to the car - especially multi-wire looms.

There are several ways in which this can be done. The factory method in many cars is to wrap the loom in adhesive tape, over-lapping the tape by at least a third each time around. Doing this will give a factory-appearance in many cars, and the adhesive wrap goes some way to protecting the new wiring.

Another way is to use convoluted tubing, available from auto accessory stores. This flexible tubing has a slit along its entire length, allowing the wiring to be inserted without it having to be threaded through the tube. Many original equipment manufacturers use this tubing, and so the new wiring can look factory-neat in this form too.

Finally, wiring can be protected in nylon wrap, which comprises a long thin piece of plastic that has been formed into spiral. The wire needs to be threaded through the tube which is formed, or the plastic unwrapped and then re-wrapped around the wires. This unwrapping/re-wrapping is practical only on short lengths of loom.

All protective wraps are useful, with convoluted tubing giving the best flexibility of use combined with a good appearance.

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Any alarm wiring should be ‘dressed’ to look like it is part of the car’s original wiring - there’s nothing more obvious to a thief than a pair of red and black wires disappearing in the direction of an operating siren!


All the wires that are added to a car’s electrical system must be sufficiently thick in gauge.

The wire gauge must be capable of handling the required electrical current, and in addition, even when carrying only small currents, the wire must be physically strong. The low-current side of relay might only require computer ribbon-wire in terms of handling the electrical load, but vibration and the resultant flex will break this type of thin wire.

Wire should always be multi-strand not solid core - multiple strand wire is much more flexible.


The wiring should be securely fastened into place. Loose wires can get tangled in fans and moving belts, or come into contact with hot surfaces or moving suspension or steering components. Loose wires routed badly can even jam throttles...

Cable ties are easiest to use, and tie-off points can be obtained by using existing wiring. Clamps making use of self-tapping screws can also be used, especially where the new wires are thick or heavy. However, these holes become points from which rust can start, and often look pretty bad as well.

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Using the original cable mounting points is the best way to control wire movement. For example, some cars have flexible metal tags welded to the body - these usually have enough extra length to allow them to be used with added wiring. Other cars have cable securing bolts or clips.

Where cables pass through sheet metal - for example through the firewall - rubber grommets must be used. Not only will these stop chafing, they’ll also prevent engine noise transmission into the cabin. If no grommets are available, a piece of cut-off fuel hose can be slipped over the wires instead.

Relays and Fuses

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Where a current flow of over 5 amps or so is being used, a relay should be installed. A relay is simply a high current electric switch which can be operated by a low current mechanical switch. You operate the dashboard switch, which sends a signal to the relay to switch the high current circuit.

Extra driving lights, up-rated headlight bulbs (from 50 to 100 watts, for example), electric radiator fans or an aftermarket engine management ECU should all be wired-in with a new relay.

New heavy current-draw circuits should be protected by a fuse, preferably an in-line one located as close as possible to the battery. Drawing current through an existing fuse is fine – but only if the factory fuse has its capacity uprated to suit and if the original wiring can handle the new demands. The cigarette lighter wiring and fuse, for example, are usually heavy duty.

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If you plan to install lots of new electrical items like extra instruments, new courtesy lights, a high-power sound system, and electric radiator fans then consider buying another fuse box. Buying one from your model of car at the wrecker will keep the car looking factory in appearance, and will mean that the same type of fuse is used throughout the electrical system of the car. The same applies to buying relay boards. Some cars run up to six relays in an engine-bay box, and this can be purchased and then installed - giving a neat appearance and also generally being a cheap way of sourcing relays.

While you’re at the wreckers, have a good look at the electrics of the top-line version of your car. If it has electric windows, mirrors or seats, or it has a better instrument cluster or better interior lighting then you’ll be able to upgrade your own car relatively simply.

Making Connections

Connections to existing wiring can be made in a number of ways.

Crimp terminals are widely used and if the crimping is done with a high quality tool, these connections can be very secure.

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Another approach is to solder connections. Electrically, these connections are better than crimping but the downside is that the wire is made more brittle and if flexed, may break near the soldered join. People who don’t like soldering in car wiring often point out that manufacturers rely on crimping rather than soldering on wiring looms, but that isn’t always the case – some switches, for example, use OE soldered connections.

A final benefit of soldering is that it is much cheaper and easier – crimp terminals quickly add up in cost (especially if male and female terminals are being used to make connections) and invariably you run out of the terminals just when shops are shut!

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A good rule of thumb is to use crimp terminals in wiring subjected to a lot of vibration, and soldering in areas where movement is less. However, in both cases, make sure that the wires cannot be flexed back and forth near to the connection.


Insulating of connections is vital. Without appropriate insulation, short-circuits and other faults will occur.

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Two types of insulation are widely employed - insulating tape and heatshrink. In most cases, I am a fan of insulation tape. Insulation tape is easily applied, easily removed if a wiring change is needed, can be put on T-type junctions (where the original wire has not been cut), is cheap and doesn’t involve applying heat. If quality tape is used, it is also durable – I recommend Nitto tape.

However, heatshrink is especially good if the connections are small and fiddly. For example, if soldering an extension cable to a small electronic component like a thermistor, heatshrink will allow a much neater and more compact job to be done.


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The difference between wiring connections that fail and those that last the life of the car isn’t much in work time or effort – it doesn’t take much longer to do a neat and durable job than to do a sloppy one. So think-through the job first; make electrically and mechanically strong connections; use appropriate gauge wire, relays and fuses; and carefully secure the wiring into place.

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