I have a 78 F150 4x4 and I added those tractor type backup lights under the rear bumber so I can see when in reverse. well my problem is I tied into the factory back up lighting harness and they work fine, but I also ran a wire up to the cab so I could have them on a switch, this way I could see when hooking up to my car hauler, or other various things. I found that when I put power to the lights through the wire I ran to the cab, the back up lights get much brighter am I doing something wrong, or is this because the factory harness was not designed to carry this much lighting? and my added wire will carry enough current to let the lights light up to their full potential? I'm stumped thanks all who helps.

 

Yes and you should not run the tractor lights off of the Truck harness, Use a Relay back at the back and run the power from the battery with an inline fuse and run a wire from the Backup lights to activate the relay to the tractor lights. Just go to autozone and get a relay from Blazer off road lights and it has the Destructions on the back of the package

 

I have separate circuits for additional front, side, and rear lights on my 79. Front lights are Daylighters, side lights are frame mounted to shine on the area where you step out of the cab, and rear lights are under the bumper. We're in the country and it gets DARK out there!
You should never tie in to factory harness wiring for the power feed to aux. lights.

As noted above, run a wire (10ga) from the + battery side to a fuse or circuit breaker, then from circuit breaker to a relay. Run another 10 ga wire for 12v+ power to the lights. Take the - lead from the lights and ground to frame/body.

Run a 14 or 16 ga wire from the circuit breaker to the switch you are going to use, or you can use an ignition controlled source to power the switch. You can use the smaller gauge wire and ignition source power because if you use a relay the amperage draw from the lights does not go through the switch, it goes only through the relay. The switch only needs 12v and milliamps to energize the coil inside the relay - much safer, especially when you start using big lights. It's not the voltage that can fry the switch, it's the amperage draw.

The relay you get will have 4 or 5 terminals. 30 is 12v+ from the battery, 87 is 12v+ to the lights, 85 is ground to frame/body, 86 is 12v+ from the switch. If there is an 87a terminal just ignore it - it is a normally closed contact that opens when the relay is energized.

If you've done it right, when you flip the switch ON, 12v power is sent to the relay terminal 86, which energizes the coil and closes the normally open contact 87. This sends power directly through the relay from terminal 30 to the lights connected to terminal 87. The circuit breaker or fuse protects the whole thing.

In my case, I have a circuit breaker protecting a power feed to a fuse box under the hood since I have several lighting circuits, all with separate relays, etc.

Good source of info - www.the12volt.com
I do a lot of auto wiring and I've been pleased getting electrical stuff from www.delcity.net, but then I order a bunch of stuff at one time. You can always buy local for small projects.

 

Great info guys,,,, Leave Them Factory Harnesses Alone!! They have enough trouble as it is

 

Thank you guys so much! I never realized how relays work therefore is the reason I didn't install one. I'm not truck stupid I just was taught that if it works then it's good enough, but this taught me alot about relays and I have a bunch of ideas in my head with lighting. Thanks again it all makes alot of sense.

 

It is good you are wiring it up correctly with the relay. The load will burn out the factory switch. Even the normal load will burn it out way too easy!

 

you can even use a capacitor if you run it right