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
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.
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.
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
Relays and Fuses
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
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.
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
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.
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.
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
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!
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.
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.
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.