Trailer Harness Grounding

I’m looking to install a flat-4 wire harness from U-Haul to my SR5. The ground wire in the U-Haul harness has a warning sticker that says “attach ground wire to vehicle body or vehicle frame — do not attach ground wire to vehicle wiring”

…why? I don’t see why it would make a difference.

 

Because if you attach it to the vehicle wiring you risk overloading the wiring as vehicle wiring is typically only designed for the factory circuits, there's not alot of extra capacity there and truck's without the tow package don't have a ground back there for the trailer wiring.

It's always best to ground aftermarket stuff to the body or frame if you can't go direct to the battery.
There's typically always a bolt or hole nearby that can be used, not often a need to drill a hole.

 

that’s interesting. I had thought of that possibility but had dismissed it because I thought the factory fuses were on the “supply side”.. so no fuses to burn out on the way back to ground..
..or is it an issue of putting too much current through the ground wire itself?

 

The ground wire is part of a complete circuit so if you say add another 10 amp circuit from the battery and then tie the ground into a vehicle ground that's an aditional 10 amps that ground has to carry along with the factory circuits it grounds if that makes sense.
There's not alot of overhead built into most automotive wiring anymore, it's pretty much the minimum size it needs to be to safely carry the current of factory loads.

Exactly this.

 

thanks, I really should’ve known better. I guess I was just thinking in terms of voltage drop and not current.

 

However.

Be careful where you ground the wire, because of the magic of galvanic corrosion, which depends on how much current is passing through the wire and how far away the battery is from the ground point, and, and, and...

 

9 out of 10 times when I have an electrical problem it seems to be a ground problem. If the harness you have has a grounding warning, then do as it says. Ground it to the frame.

 

The frame or body just makes a better ground point, vs tapping into an existing wiring ground.