What happens inside a 4-pin SPST relay when the coil is powered?

When the coil is powered, the switch changes position and pins 30 and 87 are connected. When the coil is not powered, pin 30 is not connected to anything, so the relay is in the 'off' position.

Which pins are the coil on a standard automotive relay?

The coil is connected across pins 86 and 85, as shown in the internal diagrams.

What is the difference between a 4-lead and a 5-lead Bosch-style relay?

The only difference is the addition of the 87a lead/pin on the 5-lead SPDT relay.

What is the pin layout on the bottom of a typical Bosch-style relay?

Viewed from the bottom: pin 87 at top center, 86 on the left, 87a in the center, 85 on the right, and 30 at bottom center.

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

4-lead relay schematic showing coil unpowered and powered states with pins 30, 85, 86, 87 and a 5-lead bottom view

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Schematic explaining a 4-lead SPST automotive relay in off and on states, plus a bottom view of a standard 5-lead Bosch SPDT relay.

AI caption April 2012 · Photo 1092 of 2216

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.

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Frequently asked questions

What happens inside a 4-pin SPST relay when the coil is powered?: When the coil is powered, the switch changes position and pins 30 and 87 are connected. When the coil is not powered, pin 30 is not connected to anything, so the relay is in the 'off' position.
Which pins are the coil on a standard automotive relay?: The coil is connected across pins 86 and 85, as shown in the internal diagrams.
What is the difference between a 4-lead and a 5-lead Bosch-style relay?: The only difference is the addition of the 87a lead/pin on the 5-lead SPDT relay.
What is the pin layout on the bottom of a typical Bosch-style relay?: Viewed from the bottom: pin 87 at top center, 86 on the left, 87a in the center, 85 on the right, and 30 at bottom center.