Electrical • Wiring
Automotive Relay Wiring Guide
Learn how common automotive relays work, what each terminal does, and how relay wiring is used to safely control higher-current circuits.
Overview
A relay allows a low-current control circuit to switch a higher-current load. This is commonly used for fog lights, cooling fans, horns, pumps, and other accessories.
Common relay terminals
- 30 — battery power input
- 85 — relay coil terminal
- 86 — relay coil terminal
- 87 — switched output to the component
- 87a — normally closed output on some 5-pin relays
How it works
When the relay coil is energized through terminals 85 and 86, the internal switch closes. This connects terminal 30 to terminal 87 and sends power to the load.
Common uses
- Auxiliary lighting
- Radiator cooling fans
- Horn circuits
- Fuel pump switching
- Accessory power control
Testing tips
- Verify battery voltage at terminal 30
- Check that the relay coil is receiving power or ground at 85/86
- Listen or feel for a relay click when activated
- Check for output voltage at terminal 87 when commanded on
Common failures
- Burned internal contacts
- Failed relay coil
- Corroded terminals or loose connector fit
- No trigger signal from switch, module, or fuse