Thanks to Bogie for the FAQ!

The easiest way to switch  high current accessories on & off is to use a relay.Here is the proper way wire one.
There are two diffrent relays that will be used for the most part in automotive situations,SPDT & SPST.
Single position single throw which will switch both output terminals (87 & 87a) on or off when power is applied to the trigger terminal.
Single position double throw which will switch between the two output terminals (87 & 87a) when power is applied to the trigger terminal.

Rather than trying to make a graphic of the text, it will be always printable, searchable and readable as text. I've marked up the text like the original. I'll be adding the graphics to this post once I have them the way I like them.



ISO relays were designed to try to standardize relay connections and to make it easier to test and design systems.

An ISO relay is an electromechanical device that consists of a coil, a resistor or diode, and electrical contacts. The electrical contacts are held in a de-energized position by the spring pressure. When voltage and ground are applied to the coil, the coil becomes an electromagnet and pulls the contacts to the energized position. When either the ground or the voltage is removed from the coil, the magnetic field collapses and the spring tension opens the contacts to the de-energized position. The resistor or diode is there to stop the voltage spike when the coil is being de-energized.
(See diagrams )

• Terminal 30 is usually connected to battery or ignition voltage. However, on circuits that require a motor to change direction, for example, two relays are used.

• In this situation, Terminal 30 is connected to the motor, Terminal 87 to the voltage source and Terminal 87A to ground.

• Terminal 87 is connected to Terminal 30 in energized position.

• Terminal 87A is connected to Terminal 30 in de-energized position.

• Terminal 86 is connected to the electromagnet and usually connected to a switched battery voltage source.

• Terminal 85 is connected to the electromagnet and usually connected to a switched or unswitched ground.

In the past, the ISO relays were made only by BOSCH. Now there are many companies making the ISO relays, and they are used by almost all car lines. You will find ISO relays on every car that Chrysler has made since 1988. On some cars you will find as many as 25 relays — or more. They are so popular now that THEXTON makes a tool designed just to test the ISO relay and its circuit (Tool #136).