A relay is an electrical switch that allows you to provide either a power source (+) or a ground source (-) to a circuit. There are
many different types of
, from very small to very large, but the most common relay used in the automotive environment is the
Single Pole Double Throw (SPDT) relay. The second most common relay found in modern automobiles is the Single Pole Single Throw
(SPST) relay. These two relays are very similar in design, the primary difference being that the SPST relay does not have
normally-closed terminal (more on that later). Relays used in the automotive
environment are typically rated at 30, 40 or 50 amperes, which is quite a lot of power.
How a Relay Works
Relays have an electro-magnetic coil inside them. When this coil is energized, it creates a magnetic field that flips a
switch to connect two terminals. The naming convention for the terminals found in a Single Pole Double Throw relay is as follows:
85 and 86 are the terminals that attach to the electro-magnetic coil.
30 is the common lead, which provides the positive power source or negative ground lead for your switch.
87a is the normally closed lead and 87 is the normally-open lead.
When 85 and 86 are not energized, the 30 lead is connected to the 87a lead. When the coil is energized, 30 is connected to the 87 lead.
In a Single Pole Single Throw relay, 87a does not exist. 30 isn't connected to anything until the coil is energized, at which point it connects to the 87 lead.
How are Relays Used?
Relays are used for a numerous electrical connections in your vehicle. They are used to turn on your
headlights,
tail lights, and
fog lights. If you look at your
fusebox diagram, you'll see a number of different
fuses, as well as spots for a fuel pump relay, a
blower relay, a cooling fan relay, an ignition and starter relay. All of these systems need triggers for activation
and the automotive relay is the number one choice for an electrical switch in the automotive environment. This is
mainly due to their simplicity; there is only one moving part, so it the failure rate among relays is extremely low.
Relays can also be as simple or as complex as a manufacturer needs them to be, which allows engineers to design extremely
complex electrical circuits that will be reliable for the life of the vehicle.
When to Replace a Relay
Relays rarely need replacing, but if you have an electrical device on your car that isn't
getting power, the likely cause is either a fuse or a relay. If you checked and replaced all fuses
in the circuit and you are confident that the relay should be getting power, you can listen to the
relay to see if it's working. Put your ear close to the problem relay and turn on the power to the
problem part; if the relay makes a 'click' sound, the relay is working. If it does not make any sound,
it is possible that the relay has failed and needs replacement. If you are experienced in electrical circuits,
you can also use a Digital MultiMeter (DMM) to ensure the relay is getting power. If you are not experienced
with circuits, seek help from someone who is.
Common Relay & Electrical Switch Components
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