Bosch-Style Automotive Relay Diagram - 4-Pin SPST & 5-Pin SPDT Pinout Explained

This document is an educational schematic explaining the internal operation of standard automotive relays. The first illustration, titled '4 LEAD RELAY,' shows two side-by-side internal diagrams of a 4-lead/pin SPST relay: on the left the coil (pins 86 and 85) is unpowered and the switch contact from pin 30 is open ('RELAY OFF'); on the right the coil is powered, the switch closes, and pins 30 and 87 become connected ('RELAY ON'). The accompanying text explains that when the coil is not powered, pin 30 is not connected to anything and the relay is in the 'off' position. The second illustration shows the bottom view pin layout of a standard 5-lead Bosch-style automotive SPDT relay, with pins labeled 87, 86, 87a, 85, and 30, noting the only difference from the 4-lead relay is the addition of the 87a lead/pin. A reader can learn how relay pins 30, 85, 86, 87, and 87a function, how the coil switches the contacts, and the physical pin arrangement of a typical Bosch-style relay as seen from the bottom.