I installed a led light bar and for life of me can’t get it to work. I have switch connected with 3amp add a fuse and it’s grounded with dimmer white/green wire. Switch lights up appears fine. The power runs from switch to relay, relay is wired to battery grounded to bracket in front of radiator and I get a good reading. When I turn the light on the reading is very weak and doesn’t appear to be getting any power to light itself. I think I’ve narrowed it down somewhere between relay and bar. Checked all wiring which is solid. Any suggestions ? Tia

What do you mean by "the reading is very weak"?

Trouble shooting 101... Make sure the individual components work first...

Apply 12v to the light bar direct, check function, perhaps measure amp draw to confirm performance against the specifications

Hook up relay, check output voltage using DMM, make sure its switching the load on/off correctly.

Are your wire sizes appropriate for the load and distance? What is the voltage at the light bar when its running & what is the supply voltage at the battery?

I agree with Bumbo. Ensure that the light bar will work with 12V (assuming that's it's voltage specification) directly.

Are you hooking the wires of the LED bar in the right configuration? LED's, unlike lamps, must be wired with the correct wire to positive and the other to negative/ground.

I forgot to mention...light bar is 12-24v I’m utilizing a 12v relay. I believe the wires at the light were correct because there was a red and black, I did assume the Black was the ground. How do you recommend I supply power to the light to test it? When everything is off I’m getting a 12v ready when I turn on the switch it seems to draw down to about 3-5 range. Wires are all correct size as far as I know they came with the harness. Something I forgot to mention the harness is configured for two fog lights so I am only using the one side and not the other but they are split and incased in a shrink tube. I tried getting a reading off the unused plug and get nothing at all. Would it be wise to open it up and remove the wiring for the side I’m not using? I figured it wouldn’t matter. I’m pretty novice when it comes to electrical work just the basics.

So I was just reviewing a wiring diagram and realized my relay isn’t grounded. The ground runs from relay to where you would hook up light. So I hooked ground from relay to ground on light bar and then to frame. Im guessing the relay and bar have to be grounded separately and not together. Doesn’t seem like it would make a difference

Why are you continuing to test with a harness and relay? ... just follow my directions.

Troubleshooting 101... Make sure the individual components work first.

Connect it (the light bar) directly to the battery... no relay, no harness, no fuse. Red to red, black to black...

You didn't answer my question. What do you mean by "the reading is very weak"?

If you are talking about voltages, add the units as it makes it easier to understand. What is 3-5 range? Volts? Amps? Watts?

When directly connected, what is the voltage of the battery resting, what is the voltage under load with the light connected to it.

You either have a voltage drop thought the cable, a problem with the light, or a battery that is dropping voltage under load and not capable of supply the current for that load, or a wiring problem you created.

Ground is ground... everything needs a ground otherwise how do you expect to complete the circuit.

The ground for the relay is a signal ground, meaning it can be a very small wire like 18-24 AWG

The ground for the light must be capable of passing the current, its going to need a larger wire. Generally speaking when using something like a Bosch automotive relay, it would be direct to ground/chassis with the relay only switching the positive leg.

If you are getting a 12v to 5v drop, and its wired correct, you have a serious voltage drop or resistance issue somewhere. Most likely from an undersized wire for the load.

I’m just thinking the relay had a blue and yellow running to a plug for fog light which I cut off. I assumed the yellow was a ground but perhaps it wasn’t since there was a seperate black ground wire coming from harness. So I should probably unhook ground from lightbar to yellow relay wire. The yellow wire may just be a daisy chain for other side fog light plug.

I was able to check everything today and it turns out the relay had to be grounded separate from the light bar itself...not sure why. Now my problem is the actual light bar not emitting bright at all. I directly connected it to battery its dim and if I go through the relay its dim. I'm using 12 gauge wire maybe that's too small? The length is not an issue any shorter and it wouldn't reach. Could it be because of the cold(20f) or maybe it takes awhile to warm up. I should mention I am using the 20" light bar from Amazon for $30 that has been raved about in other threads, I don't expect it to be the best.

I suggest you do some research into how relays function and should be wired up, as it doesn't sound like you had a solid grasp when you tried to install it.

Relays have two functional parts: an electromagnet and a ferrous switch. When 12V (12 gauge wire is overkill for such an installation, so that's not the problem) is applied to the hot lead of the coil and completes a circuit to the chasis ground, it magnetically activates the switch portion, which, in this instance, will allow a second, seperate 12V lead to continue from your source - the car battery - through the load - your LED light bar, which should then itself be connected to ground.

Appreciate the response...the problem wasn’t so much the relay but rather the harness I was using was set up for two seperate fog lights. I ended up tearing it apart and just running the wires for to the bar. Anyways even if I directly hook the bar up to the battery it’s still dim. My battery is practically brand new and I’m getting a 12.5v reading on it.

I’ve tested everything with the multimeter the battery is reading 12.23 just before the relay it’s down to 11.80 The switch wire coming in is also 11.80 when engaged. The reading to the lightbar then drops down to 10.80. I’m guessing that’s why it’s so dim. I don’t understand the loss in voltage. I suppose I could shorten the power lead wire from battery to relay by 12” would that make that much of a difference?

Doubtful. Wire, under most practical uses, has virtually no voltage drop.

What are you using for the connections between your wires? Can you test providing power to as close to the bar as possible, such as carefully using a set of jumper cables to clamp to the OEM wires coming from the light bar?

There's always the possibility that you have a malfunctioning product.

I would change your switch ground to something other than the dimmer switch ground...that is where I think you're experiencing some flaw...

To test the light first, I always put the red wire to the battery positive terminal the black wire tot he ground terminal...it should be bright...if not, there's something limiting your light from within the light itself...

Its called voltage drop and line loss. That is why I mentioned wire size and taking the measurements to gain insight.

That is a pretty large voltage drop... what is the size of the wire? Chances are that harness is not up to the task.

Shorter the length, the less drop... and larger/thicker the wire, the less voltage drop.