The Bosch Relay Unraveled
 
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 fail.
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.

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.

 
 
Back