In an automotive electrical system it is undesirable to have high current and thick wiring running through the dash panel, horn, switches, the PCM, etc. The vehicle’s PCM uses a low current signal or ground to activate the fans, idle control devices and other actuators. Yet high current is necessary to drive so many components. Manufacturers use relays for this purpose.
Electromechanical relays are simple mechanical switches. They require low current from a component like the BCM or PCM to activate. So, instead of having to manually activate any component like an A/C clutch or fan, the BCM does so by passing low current through its relay. When current is passed through the relay’s coil, a magnetic field is created that connects the high current circuit.
This requires two separate components: the switch and the coil to activate it. Viewed from the bottom, the #85 and #86 terminals are used as electromagnet control. The coil usually contains a clamping diode to protect the contacts and other circuits from the sudden collapse of current when power is removed. On some circuits, it’s important not to reverse these two pins. The #30 pin is connected to a high current source and number #87a (SPDT) is the terminal where the arm parks (NC). The #87 pin sends the power to and activates the circuits load (component).
Automotive manufacturers have been using them in place of switches for years. Their great because they are simple to diagnose and conveniently placed in a central location for easy identification and replacement.