To wire a light bar, run a fused power lead from the battery to a relay, connect the relay output to the light bar, and use an in-cab switch to trigger the relay. The switch carries only a small control current, while the relay and fuse handle the light's full load. A plug-and-play harness combines all these parts into one loom.
This guide walks through each part of the circuit, the checks to make before you start, the wiring steps, and how to choose a harness that matches your HELLA lights. Your light and harness instructions always come first: where they differ from this guide, follow them.
What parts does a light bar wiring circuit need?
A light bar circuit has five parts. Each one has a single job.
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Fuse: protects the power lead from the battery. It sits as close to the battery as practical, so a short anywhere downstream blows the fuse instead of damaging the wiring.
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Relay: an electrically operated switch. A small current from your dash switch closes the relay, and the relay then sends full battery current to the light.
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Switch: mounted inside the cab. It only triggers the relay, which is why the switch wiring can be lighter than the power lead.
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Ground: the return path to the battery negative or a clean chassis ground. A poor ground is one of the most common reasons a light flickers or will not turn on.
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Harness: the loom that ties the fuse, relay, switch and light connectors together, so you are not building the circuit from loose wire and crimp terminals.
What do the relay pins mean?
Most automotive relays use the same four terminal numbers. Check the markings on your relay or harness diagram before you connect anything.
|
Relay terminal |
What connects to it |
|---|---|
|
30 |
Fused power from the battery positive |
|
87 |
Output to the light bar positive |
|
85 |
Coil ground |
|
86 |
Coil trigger from the switch |
When the switch sends power to terminal 86, the coil pulls the relay closed and connects 30 to 87. The light comes on. Turn the switch off and the relay opens.
Do HELLA light bars come with a wiring harness?
Not always. The HELLA FAQ puts it this way: "Most HELLA light sets include a wiring harness, but requirements vary by product." Check the individual product listing for what is in the box before you order a separate harness, or ask the MyHellaLights team if the listing does not say.
If you need a harness, match it to your lights:
|
Harness |
Listed for |
What the listing states |
|---|---|---|
|
HELLA Blade Series and HELLA X-TREME Series (3" and 2") |
12V DC; up to 200W total (100W per light max); 30A inline fuse; one 12V automotive relay; two switches; DT connectors; two cables, each 7.5 ft (230 cm) |
|
|
Dual light setups |
Plug-and-play kit with relay and switch |
|
|
500 and 450 halogen lights |
Plug-and-play kit with relay and switch |
The MULTIFUNCTION HIGHPOWER HARNESS is not listed for HELLA Black Magic light bars. For a Black Magic bar, check that bar's own listing for harness and power details before you buy.
How much power can a light bar harness handle?
Add up the rated wattage of every light on the harness and keep the total under the harness rating. For example, the HELLA X-TREME XT-4C lists a power rating of 60W. Two of them on the MULTIFUNCTION HIGHPOWER HARNESS draw 120W, inside its 200W total and under its 100W per light maximum.
To estimate current, divide watts by volts: 120W on a 12V system is about 10 amps. Never fit a larger fuse than the harness specifies to stop a fuse from blowing. A fuse that keeps blowing is pointing at a fault.
What should you check before you wire a light bar?
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Voltage: confirm the light, harness and vehicle system match. Some HELLA lights list 12-24V DC, while the MULTIFUNCTION HIGHPOWER HARNESS lists 12V DC.
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Power rating: total light wattage stays under the harness rating.
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Connectors: the harness plugs match the light's connectors.
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Cable length and route: the leads reach from the battery to the light and from the relay to the switch, away from sharp edges, hot surfaces and moving parts.
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Mounting: the bracket and mounting point can hold the light securely and let you aim it.
Disconnect the battery negative before you make any electrical connection. If the job involves vehicle electronics you cannot identify, use a qualified auto electrician.
How do you wire a light bar step by step?
1. Mount the light bar
Fit the light bar with its mounting brackets and hardware. Leave enough slack in the light's lead so the connector is not under tension, and set the angle so you can aim the beam once it is wired.
2. Lay out the harness
Lay the harness along its route before you fix anything. The light connector should reach the bar, the power lead the battery, and the switch lead the cab. Mount the relay and fuse holder somewhere dry and away from engine heat.
3. Run the switch lead into the cab
Pass the switch lead through an existing firewall grommet where you can, and seal any new hole. Mount the switch where you can reach it without blocking controls or your view.
4. Connect the light bar and ground
Plug the harness output into the light bar and check polarity. Connect the ground lead to the battery negative or a clean, bare metal chassis point.
5. Connect power last
Connect the fused power lead to the battery positive last, with the fuse installed in its holder. Then reconnect the battery negative.
6. Test the circuit
Turn the switch on and off a few times. The light should come on and go off cleanly every time. Check the relay, connectors and fuse holder for heat after a few minutes of running. Secure the harness with cable ties every few inches so vibration cannot pull on the connectors.
Why won't my light bar turn on after wiring?
Work through the circuit in order, with the battery disconnected before you touch any wiring.
|
Symptom |
What to check |
|---|---|
|
Light does not turn on |
Fuse, battery connection, ground, light connector |
|
Switch does nothing |
Switch lead to relay terminal 86, coil ground on 85 |
|
Relay clicks but light stays off |
Power on 30, output on 87, light connector, ground |
|
Light flickers or cuts out |
Loose connector, corroded or painted ground point |
|
Fuse keeps blowing |
Pinched or chafed wire shorting to the body, total load over the harness rating |
|
Connector or relay gets hot |
Loose terminal, load over the harness rating |
Is it legal to run a light bar on the road?
Many HELLA driving lights and auxiliary lamps are designed for off-road use only and must comply with local and state regulations when used on public roads. At the federal level, Federal Motor Vehicle Safety Standard No. 108 sets requirements for original and replacement lamps, reflective devices, and associated equipment. Check your state and local rules before you run an auxiliary light on a street-driven vehicle.
Which HELLA light bar should you wire up?
The HELLA Black Magic collection is HELLA's off-road LED light bar range, in slim, double row and curved styles from 20" to 50". If you are wiring a pair of compact driving lights instead of a bar, the guide to HELLA X-TREME LED driving lights compares the spot, flood, and combo beams and covers what to check before wiring.
MyHellaLights is the official US online store of HELLA Automotive Sales, Inc.
Frequently asked questions
Do I need a relay to wire a light bar?
Yes, for any light bar that draws more current than a dash switch is built to carry. The relay lets the switch control the light with a small current while the full load runs through the relay and fuse.
Where should the fuse go on a light bar harness?
On the power lead, as close to the battery as practical. That way the fuse protects the whole length of wire from a short.
Which lights does the MULTIFUNCTION HIGHPOWER HARNESS fit?
Its listing names the HELLA Blade Series and HELLA X-TREME Series (3" and 2"), with up to 200W total and a maximum of 100W per light.
Can wiring a light bar myself affect the warranty?
It can. The HELLA FAQ states: "Improper installation, modification, use outside the product's intended application, or physical damage unrelated to a manufacturing defect may void the warranty." Follow the product's instructions and use a harness rated for your lights.
Will a light bar drain my battery with the engine off?
Yes, a light bar running with the engine off draws from the battery and can flatten it. Wire the switch so you can see when the light is on, and turn it off when the engine is off.