Mustang Info Mopar Info
Shop Cars Store
Help Keep This Site Alive!!!!
This section has diagrams that will
explain how a relay works, what the terminals on the relay control
and how to wire a relay to control nitrous systems, line locks,
transbrakes, etc. Toward the bottom is a diagram showing how
to wire a transbrake and nitrous system together so the nitrous
system deactivates while the transbrake is activated.
The standard 12 volt relay is
single pole, double throw. There are five terminals; two to activate
the relay and the other three are the switching terminals. A typical
relay has three terminals that are facing up and down, and two that
face side to side. The top terminal (#30) is connected to a 12 volt
power supply. The two side terminals (#85 and #86) are the
"activation" terminals and have no polarity. They simply require +12
volts on one and ground on the other to activate the relay. The
terminal on the top will be the common "switching" terminal. This
means that it will be the one which is always connected to one of
the other two switching terminals. When the relay is not "active,"
the top terminal is internally connected to the middle terminal
(#87a). As soon as the relay becomes activated, the top terminal
will break it's internal connection to the middle terminal and
become connected to the bottom terminal (#87). It's just like
flipping a switch, but the flipping action is controlled from a
One of the main reasons to use a relay is to control a high power
source from a lower power source. Let the relay carry the load, not
the switch. As an example, lets look at an electric cooling
fan. For argument purposes lets say this fan draws 30 amps.
A typical rocker or toggle switch isn't going to last long for this
application without a relay. Sure you can get a big bulky
switch that will carry the load safely but why not run a relay and
use a nice looking switch that will look nice in your dash, console
or where ever you choose to put it. Most standard relays can
safely handle loads of 30 amps or more. If you tap
into another switch for activation a relay becomes useful as well.
An example would be using a headlight switch to also control a set
of fog lights. You can also keep your relays close to the
device which they control so you can keep the main power wires all
in one area and close to the device you are controlling if you so
desire. You can also get creative with relays and use the
ground wire as the activation wire which adds another degree of
safety because you have ground wires running thru the vehicle which
will not short out if they rub thru.
In high current applications, the life span of a switch can be
drastically shortened. Even though it may not seem like it, when you
flip a rocker switch, it actually takes a long time for it to make
contact inside. A long time in electrical terms is still very short,
but a switch takes much longer to make its contact than a relay.
This long switching time causes internal damage to the switch as an
arc is passed across the terminals as they come in contact. The
longer it takes to make contact, the longer that arc will burn in
the switch. As a switch deteriorates, the internal connections start
to build up resistance. This resistance can offer intermittent
connections and voltage drops, both of which can be disastrous.
Because a relay is activated much faster, the arc causes minimal
Another use for relays is the ability to control multiple switches
at the same time like head lights and fog lights, two cooling fans,
a cooling fan and electric water pump, etc. Yet another use is
being able to supply one device from one of two sources. Example,
you have a temperature gauge and you want to be able to monitor the
engine coolant temperature and the temperature of the coolant in the
radiator. You could use two gauges but using a relay you could use
one gauge which would be cheaper and possibly be less cluttered.
With a relay, you could have a small switch control to control relay
with the gauge going to the top terminal (#30) and each
temperature sender going to each of the other switching terminals
(#87a and #87).
Two of the top quality relays are Bosch and Potter & Brumfield.
Stick with those two brands to minimize any problems you might run
into. Most products that are supplied with relays contain
cheap relays. If you buy a product that includes relays and
they are not one of the above mentioned brands, throw the relay(s)
away and buy a Bosch and Potter & Brumfield. A good relay will
run you around $8, well worth the money.