Electrical mystery, chasing the ground.

[jangoforhire]

So here's the deal.

I get a lot of use out of my roof lights. However, ever since I installed them, I noticed one thing - the more lights I have on at once, the dimmer they all get. I didn't really mind this, as practically speaking I almost never have all the roof lights on at once.

But, I have some time to kill this week so I figured I would fix it as I was pretty sure what the culprit is.

I'm almost positive it's the ground. The lights on the rack are grouped into four circuits.

• Front (two 6" SR's and one 20" SR).
• Left Side (one 6" SR).
• Right Side (one 6" SR).
• Back (two 10" SR's).

Each one of these groups has it's own dedicated 14 gauge power wire that runs back into the truck. Each one also has it's own ground. However, the ground wires for each of these 4 groups meets at the back of the rack, and is joined into a single 14 gauge wire that runs into the truck for ground.

Intuitively, I figured the ground was my problem as I was probably just bottlenecking the current with the single 14 gauge wire from all the lights back into the truck.


So, I unwrapped the wires on my roof rack where the 4 ground wires are joined into one and disconnected them. I then ran a 10 gauge wire up from the inside of the truck. Just to be safe - before I crimped and covered everything in heat shrink I tested one light. So I connected the 10 gauge wire inside the truck to a ground point via an alligator clip, and then connected the 10 gauge wire at the roof rack to one of the lights ground wires. Flipped all the switches to and...

ALL of my roof lights came on. Somehow this is suggesting that all of my roof lights are on the same ground circuit even before where I joined them together.

I tried connecting all the going wired to a ground and it always behaved the same way. I'm very confused as to what's going on here.

How it should be setup:

How it appears to be working right now:

It's almost as if every ground wire is shorting out and connecting them all together.
No idea how that's happening though...

 

[Ducmonsta]

It's possible they are sharing a ground through the GOBI rack. If I'm reading this correctly, you only have one light grounded via the 10ga and you can turn on all the lights without their respective grounds attached?

I thought the custom mounts from GOBI use the mounting studs. I don't know for sure if Rigids can be grounded through them, but it appears from your findings that they can.

 

[Ducmonsta]

Just thought of something, unbolt one light from its mount and you'll confirm if they're using the GOBI as a ground. Of course, don't pick the one that's connected to your 10ga.

 

[jangoforhire]

It's possible they are sharing a ground through the GOBI rack. If I'm reading this correctly, you only have one light grounded via the 10ga and you can turn on all the lights without their respective grounds attached?

I thought the custom mounts from GOBI use the mounting studs. I don't know for sure if Rigids can be grounded through them, but it appears from your findings that they can.

Just thought of something, unbolt one light from its mount and you'll confirm if they're using the GOBI as a ground. Of course, don't pick the one that's connected to your 10ga.

I had this exact same thought too. Maybe somehow the lights are grounding themselves through their mounts. It's really the only thing that makes sense at this point. I'll go back out the garage and do some more trouble shooting soon.

 

[jangoforhire]

Another interesting fact, when I installed all the lights I tried to ground them to one of the Gobi mounting brackets. However this wasn't a sufficient ground as none of the lights would come on - I had to run a ground from inside the truck.

So if the lights are grounding themselves, I may have to connect my 10 gauge wire from the inside ground to the rack itself to properly ground it.

 

[Jeremy556]

Another interesting fact, when I installed all the lights I tried to ground them to one of the Gobi mounting brackets. However this wasn't a sufficient ground as none of the lights would come on - I had to run a ground from inside the truck.

So if the lights are grounding themselves, I may have to connect my 10 gauge wire from the inside ground to the rack itself to properly ground it.

Pretty sure the light you have connected to your 10ga ground wire is grounding the entire rack through its mount. When you tried to use the rack as a ground before, there probably wasn't any connection back to ground.

I would ground the rack directly (drill a ground screw into it and cover with silicone), in addition to running a ground wire to the lights.

 

[bear74]

It sounds like a ground loop to me. DC can get ya since you are only using one negative pole and multiple paths to ground, and if you add the second wire and the ground plane caused by the rack mounting bolts (highly resistive path to ground), shazam the magic happens and you get all the lights on.
Pulling a ground that is sized appropriatly to the amount of amps total (FLA) by the lights is the way to go no matter if you dont have them on at the same time. Your inadvertant lighting just shows you the importance of that fact, a shorted negative lead will be a pain to fix depending on the routing.
I would either put some rubber or micarta spacers in the mounting flanges or do as you said and mount the groud wire to the rack. You can experiment my jumpering the rack to the chassis with the 10AWG disconnected. Let me know if you need any help.
Back to keeping all of your garages lit.
Oh, and Happy New Year

 

[eddiebx]

Please forgive me if I'm just missing the obvious here, but is there any reason why you are running constant power and switched ground? I have zero experience installing LED light bars, so take it easy on me, but why not just change the setup to switched power wires

This way it wouldn't even matter if the individual housings are grounding out on the rack or not?

 

[Antman]

The rack is definitely grounded. All my lights go to a single ring terminal on one of the racks mounting bolts covered in liquid electrical tape. No need to run a ground inside- yay!

 

[jangoforhire]

Please forgive me if I'm just missing the obvious here, but is there any reason why you are running constant power and switched ground? I have zero experience installing LED light bars, so take it easy on me, but why not just change the setup to switched power wires

This way it wouldn't even matter if the individual housings are grounding out on the rack or not?

I am running switched power wires, I'm just trying to figure out what's going on with my ground so I can get adequate current flow through the entire circuit. I may have written it in a confusing way as to test the lights I'm leaving all the switched on (giving constant power) and just "switching" ground by connecting a ground to each ground wire to see what happens. That's just for testing though.

The rack is definitely grounded. All my lights go to a single ring terminal on one of the racks mounting bolts covered in liquid electrical tape. No need to run a ground inside- yay!

Mine seems to not be...not sure how. I just went out to check again and confirmed this, the bolts at the back going into the roof give me no ground at all.


After playing around a little more, Bobby called it. The lights are all grounded through their mounting tabs to the rack. The rack itself isn't grounded until I connect one wire, which grounds the entire rack, and thus all the lights. I tried connecting a ground directly to the rack and directly to a lights mounting post and it worked perfectly. I'll probably just ground the rack AND connect all the individual lights ground wires to that same ground point just to be safe.

I'm amazed this hasn't been discovered before. It would make connecting these lights a lot easier if it was known they can be setup to ground themselves through their mounts.

 

[eddiebx]

Oh okay, I must've mis-read your original post. Maybe the reason why Jeff's rack is grounded and your's is not is because you're not getting a clean metal to metal contact at the mounting point? Are the bolts you used coated?

Pretty cool that you can just self ground through the mounts, but yeah I agree with you, certainly couldn't hurt to overkill and connect all the individual ground wires too