Here's a diagram of a Bosch relay. They come in many types,
and are used in quite a few different vehicles. Ford's version is a gray
relay with no mounting ear, and is used in the new underhood wiring modules.
GMC and Chevy uses them too, but are usually black. There are look-alikes
too, but the numbers on the terminals give them away. Most after market
relays will have a mounting tab for mounting it with a screw. Always mount
them with the terminals facing down, or they will get water inside and
The innards of the relay. When power is applied across
the 85 and 86 terminals, current flows through a coil of small wire. This
wire is about 100' long, and is usually 28 gauge wire. This builds up a
magnetic field in the bar it's wrapped around, and the steel plate snaps
to it. When the power is off, the spring pulls the plate back away from
the magnet bar. The "click" is the plate slamming into the magnet as it
turns on. It doesn't "click" when turned off, because the plate swings
away from the magnet without hitting anything.
The Bosch Relay Unraveled
Note: When the power is applied, the coil sets
up a magnetic field in it's windings. When the power is removed, the field
collapses, and a reverse current of high voltage will "kick back" This
is called counter EMF, and is how your ignition coil works. If your fingers
are across the coil terminals when the power is removed, you will get shocked!!
Below is a couple of circuits to
help understand how the relay works in real life. Relays are used to transfer
high current. A Lot of vehicles make use of the ground-to-turn-on circuit.
If one if the relay coil terminals have battery power all the time, the
ground-on circuit is how it's wired. Most horn relays are wired in the
ground-on method. The steering wheel contact touches ground and turns on
the horn. The horn relay is used because the 15-20 amps from the horn would
arc and quickly destroy the contacts in the steering wheel.