Lighting Issue has been driving me nuts for weeks

[HunterH]

Hey guys, I’m hoping someone can help me figure this out because this lighting issue is driving me crazy.


I’ve got a 2005 Ford Ranger. The rear bulbs are stock and I have aftermarket switchback LED front turn signal bulbs that have worked fine for about a year and a half.


Here’s what’s happening:


When the headlights and running lights are OFF, everything works perfectly. Brake lights, third brake light, turn signals front and rear, hazards — all normal.


As soon as I turn the running lights or headlights ON, everything goes nuts. The third brake light stays on dim. When I press the brake pedal, the two lower brake lights shut completely off and the third brake light just gets a little brighter. The turn signals don’t work front or rear. If I turn the hazards on, they come on solid for a couple seconds, then dim and flicker. The turn signal indicators on the dash glow dimly. Sometimes a turn signal will flash once and then stop, and the dash indicator just stays dimly lit.


So far I’ve checked all the bulbs and completely cut off the old broken and corroded trailer wiring harness (it didn’t even have a plug and was missing a pin). Pulling trailer fuses changed some behavior, but even with the trailer wiring removed the issue still happens any time the headlights are on.


Another weird thing — with the lights off, pressing the brake sometimes turns the running lights on. This hasn't happened since I removed the trailer wiring harness.


With the lights off, everything works like it should.


At this point I’m thinking it has to be a bad ground somewhere in the rear, maybe a shared body ground for the lighting circuits, but I haven’t tracked it down yet.


Has anyone run into this before on a Ranger? Any specific ground locations or harness spots I should check first?

 

[Curious Hound]

Grounding is a string possibility.

Give me some time to think about this one.

 

[85_Ranger4x4]

Ground issue is my vote as well.

Usually when lights come on halfway on their own it is a ground issue for me.

 

[lil_Blue_Ford]

It’s either a ground issue or damaged wiring most likely.

My F-150 had a trailer plug put on like 15 years ago and had liquid electrical tape put on the soldered connections. Apparently that stuff doesn’t work long term on vehicles and I had similar issues with the rear lights. Found that corrosion got in and trashed the entire rear harness. Like it rotted the wires apart inside the insulation. I got an undamaged/unmodified harness in a junkyard and because I got lucky and was able to pull it off a truck in line for the crusher, and had been pulled over and told the junkyard about that, I got it for free. Then I sat down and did my modifications with glue lined heat shrink.

 

[RobbieD]

Agree with a ground issue being most likely causing your issues.

So far I’ve checked all the bulbs and completely cut off the old broken and corroded trailer wiring harness (it didn’t even have a plug and was missing a pin).

Was the trailer harness factory-equipped, or was it added later?

If it was added, ESPECIALLY if squeeze-type Scotchlok connectors were used to splice the wires, check carefully were the Scotchloks "bit" into the original wires.

Scotchloks are notorious for letting moisture into the insulation, and rotting the host wire strands.