Relay wiring diagram: how to wire 4-pin and 5-pin relays. Learn the pattern, and you can wire lights, fans, or horns on any loader, excavator, or skid steer. A 4-pin relay is simple on/off. A 5-pin adds pin 87a—powered when the relay is off, useful for changeover circuits. For most add-ons, leave 87a unused. The standard layout: 85 to ground, 86 to your switch, 30 to fused battery, 87 to your accessory. Keep the fuse close to the battery. Size wire and fuse for your accessory’s draw—not the relay’s sticker. Avoid common mistakes: connecting to 87a instead of 87, skipping the fuse, or weak grounds. Test with a multimeter before finalizing.