Bosch Relay Wiring Diagram - Pin 30, 85, 86, 87, 87a Explained (SPST Relay)

This document is a general reference explaining the pin numbering and wiring of standard Bosch-style automotive relays. It answers the common question of what the numbered terminals (30, 85, 86, 87, and 87a) mean on a 4- or 5-pin relay. Pin 30 is constant positive power, usually wired directly to the battery. Pin 85 is the coil ground, connected to the negative battery terminal or any grounded metal panel. Pin 86 is coil power, wired to the control source such as a switch or the vehicle's IGN or ACC circuit. Pin 87 is the switched positive power output, which connects to pin 30 when the relay coil is energized. Pin 87a, found only on 5-pin relays, connects to pin 30 when the coil is NOT powered. The document includes a labeled bottom-view diagram of a typical 4-lead Bosch-style SPST relay, showing the physical arrangement of the terminals: pin 87 at the top (horizontal blade orientation), pins 86 and 85 on the left and right, and pin 30 at the bottom. This is a universal reference not specific to any vehicle year or model, useful for wiring accessories, lights, horns, or other add-ons through a relay.