A relay wiring harness lets a small dashboard switch control auxiliary LED lights without carrying the full lamp current through the switch. This guide explains a 12V setup with one main fuse, a four-pin normally open relay and two lights connected in parallel. The same arrangement can power the main lighting function of compatible work lights, driving lights and light bars.
Check the instructions supplied with your lights and harness before making connections. This guide uses an independent on/off switch. High-beam control, illuminated-switch connections and additional lamp functions may require different wiring.
How the circuit works
With the switch off, the relay keeps terminals 30 and 87 disconnected. Turning the switch on energizes the coil and closes that contact, supplying both lights. The switch carries the relay coil current; the lamp current passes through the relay contacts. Both lights receive the supply voltage because they are connected in parallel.
Auxiliary LED light wiring diagram
Relay connections shown in the diagram
| Terminal | Connection in this diagram |
|---|---|
| 30 | Battery positive through the main fuse |
| 87 | Positive input of each LED light |
| 85 | Positive feed through the on/off switch |
| 86 | Battery negative return |
This diagram uses 85 for switched positive and 86 for negative. A plain, non-polarized relay coil without an internal diode can operate with either polarity. Many automotive diagrams instead show 85 negative and 86 positive. For a diode-protected or electronic relay, follow its marked polarity. Identify the printed terminal numbers on the actual relay; their positions in this schematic are not a socket layout.
1. Check the lights and harness
Confirm the supply voltage and the total operating current of all lights that will run together. Use a relay with a 12V coil for this circuit. Check the harness, relay contacts, connectors and fuse holder against the load, including any specified startup current. Select wire sizes for current capacity and voltage drop over the complete positive-and-negative circuit length. A high amp rating printed on the relay does not establish the rating of the complete harness.
2. Fit the main fuse and power feed
Isolate power according to the vehicle manufacturer’s procedure and leave the fuse removed while wiring. Mount the fuse holder close to the battery positive connection. Route its output to relay terminal 30. In this diagram, the switch feed branches off after the fuse, so it is also on the protected side.
The fuse protects the wiring. One main fuse is sufficient only when its rating protects every downstream wire, including the switch-control wiring and each lamp branch, and stays within the component ratings. If a smaller branch needs a lower fuse rating, add suitable protection at that branch’s supply point or revise the wiring. Never fit a larger fuse to compensate for undersized cable.
Read our guide to choosing a fuse for LED work lights
3. Connect the lights and switch
Connect terminal 87 to both lights’ positive inputs, and connect both negative inputs to the negative return. Wire the two-terminal on/off switch between the fused positive feed and terminal 85; connect terminal 86 to the negative return. The drawing uses dedicated return wires. If using chassis ground, follow the vehicle’s approved grounding arrangement. An illuminated switch may have additional terminals: follow its own wiring diagram.
4. Secure the wiring and test
Keep cables away from exhaust heat, sharp edges and moving parts. Protect pass-through holes with suitable grommets, secure the loom and use connections suited to the installation environment. Follow the component instructions for mounting the relay and fuse holder.
Before restoring power, check polarity, terminal numbers, insulation and connection security. Fit the correctly selected fuse, reconnect power following the vehicle procedure and test both switch positions. If the fuse blows, the lights flicker or a connection heats abnormally, switch off and find the cause before continuing. A relay click alone does not confirm that power is reaching the lights.
Driving lights and additional functions
This independent switch arrangement can power compatible driving lights and light bars, but it does not make them follow the vehicle’s high beam. For high-beam-linked operation, use the correct vehicle-specific trigger connection or interface and the lamp or harness instructions. Do not assume a headlamp wire is a simple positive trigger.
DRL, position lights, backlighting and selectable modes may use separate inputs. Identify those inputs from the product instructions instead of joining them to the main light feed. The diagram also does not cover warning-light synchronization or flash-pattern control.
Common questions
Can I connect just one light bar?
Yes. Use one load connection instead of the two parallel branches, provided the voltage, current, wiring and protection are suitable for that light bar. Secure and insulate any unused harness output as its instructions require.
Will the lights work with the engine off?
Yes. The switch in this diagram is battery-fed, so the lights can operate with the ignition off and drain the battery if left on. Ignition-controlled operation requires a suitable control supply and revised wiring.
Can I use this on a 24V vehicle?
The basic arrangement can be adapted, but this drawing is for 12V. Verify the lights, relay coil, switch and other components for the actual supply voltage, then select wiring and protection using the current at that voltage.
Get help with your installation
Send your light model, quantity, vehicle voltage and harness model, plus a clear photo of the relay markings if you need help identifying the connections.
Contact Primelux for installation support
Technical references
HELLA — Relay types and common terminal designations
Blue Sea Systems — DC circuit protection principles
HELLA — Driving lamp installation and high-beam control examples
