A 5-pin SPDT Bosch-style relay (30A or 40A) lets a small switch control a large load. Critical for any high-amp accessory — never run 20A through a dash switch directly.
Bosch 5-pin relay terminal numbering: 30 = battery+ feed (load source), 87 = normally-open output to load, 87a = normally-closed output (when relay is off), 85 and 86 = coil terminals. Standard usage: 30 to fused B+, 87 to your accessory, 86 to your switched +12V trigger (through fuse), 85 to ground. Many relays have a diode between 85/86 — observe polarity if so.
Why bother? A 100W light bar pulls ~8A — fine for a quality switch. A 150W bar pulls ~12A — borderline. A pair of 100W lights plus a fridge plus a winch all on one switch line is a fire. Relays let you run 22 AWG signal wire to the dash switch and 10 AWG power directly from battery to accessory through the relay's 30/87 contacts.
Use Bosch/Tyco/Hella OEM-grade relays, not the $1 Amazon ones — they fail welded closed in heat. Use a Hella or Littelfuse weatherproof relay socket (mini-ISO 5-pin) so you can replace the relay without resoldering. Bundle multiple relays in a Bussmann RTMR or 'relay block'.
| Part | Vendor | Est. price |
|---|---|---|
| Hella 30/40A 5-pin relay | Hella | ~$10 |
| Weatherproof relay socket | Del City | ~$5 |
| Bussmann RTMR relay/fuse block | Bussmann | ~$35 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.