Relays & terminal designations
A general guide to how automotive relays work and why their terminals carry odd numbers like 30, 86 and 87b. This is not specific to any one car — the numbering is a European standard used on almost every relay you will meet.
What a relay does
A relay is a remote-controlled switch: a small current through an electromagnet (the coil) pulls a set of contacts closed, letting a much larger current flow through a separate, heavier circuit. That is why the wire feeding the coil can be thin while the wire through the contacts is thick — they carry very different currents.
Most relays have four or five terminals: two for the coil and two or three for the contacts. Energise the coil (apply voltage across its two terminals) and the contacts snap over; remove it and a spring returns them.
A normally-open (NO) contact is open at rest and closes when energised. A normally-closed (NC) contact is the opposite. A changeover relay has both — a common pole that moves from the NC output to the NO output when the coil pulls in.
Why the numbers — DIN 72552
European cars label terminals by FUNCTION, not position, using a standard called DIN 72552. Every relay, switch and control unit built to it uses the same numbers, so a wiring diagram reads the same across makes — a “30” is battery-positive on a Porsche, a VW or a Mercedes alike.
For a relay the ones that matter are the coil pair (85 and 86) and the contact outputs (87, and on changeover or multi-output relays 87a / 87b). 30 is the permanent battery feed the contacts switch; 31 is ground.
So “87b” is not mysterious: it is simply a second switched output on a relay that has more than one contact. A twin relay (like a fuel-pump / DME control relay) is really two relays in one housing sharing the 30 battery feed — one closes 30→87, the other closes 30→87b.
Common DIN 72552 terminals
| Terminal | Meaning |
|---|---|
| 30relay | Battery positive — permanent +12 V (unswitched) |
| 31relay | Ground — battery negative / chassis |
| 15 | Switched +12 V — live with the ignition on |
| 85relay | Relay coil — ground (control) side |
| 86relay | Relay coil — supply side |
| 87relay | Relay contact — switched output (normally-open) |
| 87arelay | Relay contact — normally-CLOSED output (changeover relay) |
| 87b / 87crelay | Additional switched outputs on multi-contact relays |
| 50 | Starter control (solenoid) |
| 53 | Wiper motor |
| 56 / 56a / 56b | Headlight / high beam / low beam |
| 58 | Side & tail lights |
| 61 | Alternator charge-warning lamp |
| 49 / 49a | Flasher unit input / output |