Light Bar Wiring Diagram: Relay, Fuse and Switch

Why wire a light bar through a relay

A light bar can draw a lot of current. A relay lets a small, thin switch circuit control that big current, so the switch carries only the relay's small coil current. The heavy feed runs straight from a fuse near the battery to the relay and on to the light. Most light bar kits include a harness with a relay for exactly this reason.

What each relay terminal does

The diagram shows a four-terminal relay with the common numbering. Check the markings on your own relay and your kit's diagram, because harnesses differ.

The parts in the circuit

Step by step

This follows the pattern used by light makers' harness sheets, for example KC HiLiTES' sheet for its Pro6 bar, which mounts the relay close to the battery with its terminals facing down so it does not hold moisture. Follow your own kit's sheet where it differs.

1. Read the maker's diagram.
   Note the fuse value and wire sizes your light maker gives. Use theirs, not a number from a website.
2. Disconnect the negative battery terminal.
   Do this before you touch any wire.
3. Mount the relay near the battery.
   Terminals facing down, as the KC sheet directs, and away from heat and moving parts.
4. Route the switch wires inside.
   Run them through the firewall with a grommet, away from heat and edges, to where the switch will sit.
5. Connect the switch.
   Supply, load and earth as marked on the switch, with the switch off. Earth goes to a good grounded metal surface under the dash.
6. Connect the relay to the battery.
   Ground to the negative terminal; the fused feed to the positive terminal, with the fuse close to the battery.
7. Connect the light bar.
   Plug the light bar's cable into the harness and check that the wire to the light matches the maker's diagram.
8. Reconnect the battery and test.
   Switch the bar on and off. If it does not toggle, check that the relay connector is fully seated before anything else.

Legal checks before you drive on public roads

Rules about auxiliary lights differ by state. Check the number of auxiliary lights allowed, whether covers are needed on public roads, mounting height and color, and do not assume a light is "street legal". Our responsible lighting guide explains what to look for. Many drivers wire a separate switch so the bar cannot come on by mistake.

If the bar does not work

Work from the battery outward, with the switch off and the negative terminal disconnected when you handle wiring:

• Nothing happens: check the fuse first, then that the relay connector is fully seated, then the ground at the switch and at terminal 85.
• The fuse blows: there is a short or the fuse is wrong for the light. Find the cause. Do not fit a bigger fuse; the fuse protects the wire.
• The bar flickers: look for a loose ground or a loose terminal, and for chafed insulation where wire passes metal.
• The bar is dim: a long run of thin wire or a poor ground can drop voltage. Check the wire size against your light maker's chart.
• It stays on with the switch off: the relay may be stuck or the switch wired wrong. Disconnect the battery and recheck the harness against the maker's diagram.

A note on power

A light bar is a load on the electrical system. If you run several lights, a fridge and other gear, the starting battery can run low. See dual battery setup for a second battery that carries the extra loads, and battery maintenance basics for keeping the battery healthy.