Tail And Reverse Light Ground

I am wiring up my tail lights and reverse lights with a painless performance harness and I think I’m about to do something stupid.

16ga to left turn/brake
16ga to right turn/brake
16ga to each tail light
18ga to splice to 2 18ga to each reverse light

That is all “stock” in the harness.

There is a black 18ga intended for ground for done light that I can run all the way to a firewall ground connected to the battery negative. I would love to be able to splice that 18ga to two 18ga wires running to the reverse and tail lights on each side, but that would essentially put one 18ga carrying all the grounds back to my firewall ground. The lights are still going to be “grounded” to the painted panels on the tub.

Is 18ga good enough for that or do I need to run a 14ga back to the firewall (which would be a huge pain in the neck since all my dash wiring is done) or run 14 or 16ga directly to the frame at the rear of the jeep?

 

Thanks As I was making electrical connections, I thought of the line from Spinal Tap: “There is a fine line between clever and stupid” and I was pretty sure I was crossing that line!

 

If you have a fiberglass tub you would want to have a healthy ground wire that everything can tie into. Avoid circular ground loops, you want branches so bad connections don't cause havoc. Like left tail lights grounding via right turn signal element.

Self tappers and star washers are your friend.

My steel tub is my tail light ground but my trailer lights have the typical molded ground going all the way to the main ground lug that the battery ground strap connects to. The power to my body lights comes from the trailer connector but the ground paths are seperate.

 

I stashed the 18ga black into loom after heat shrinking each end. Man, there are a lot of extra wires with a basic harness - AC, fuel pump, dome light… Then I removed the paint and used a lot of dielectric grease.

From what you guys have written, it sounds like running a 14 ga ground from driver’s tail and reverse to the rear driver’s frame and another 14 ga from the passenger left tail and reverse to the passenger side frame would be ok if needed.

My grounds are battery negative to 3/8” brass all thread in firewall, battery negative to starter engine (and therefore engine block), and engine block to frame. I’m going to power my headlights through relays so will ground them to the frame up front.

 

JMO - connection to the body steel is fine if you make a good connection. Recommend new hardware, new shakeproof (aka star, toothed) washers, and a dab od dielectric grease for these ground connections. Electrically I think choosing a connection to the frame or the body will not matter, if both have good connections to battery ground.

I understand Howard's caution about multiple paths (ground loops), and potential unexpected backfeeding through some unexpected circuit. However, this is all DC electricity and the car body is one big ground plane. Electricity could seek an alternate path to ground, but the existing grounds would have to go open first from dirt, salt and water. I contend duplicate ground connections to the car body will not matter if the ground connections are sound.

Your original question about why one 18 ga ground for 14 ga power wires does not consider the length of the wires. If you have 72" of 14 ga from the dash and 12" of 18 ga to body steel, likely those are more equivalent than you would imagine. The body or frame steel is not as good a conductor as copper, but it is massive compared to either wire, and will have low low resistance because its a whomping big conductor. Comparing the wires according to their length, the short 18 ga wire will have about 40% of the resistance of the long 14 ga wire. Anyway, these tiny resistances won't make any noticeable difference in the performance of your lights and accessories - just make your ground connections good and solid.