I have a weird problem and I'm hopping someone has seen it before. I have a 85 Lariet and when my truck is running with the head lights on the brake lights stay on. But if the headlights are on with out the truck running or if I'm driving without the headlights they work perfectly. Does anyone know what would cause this?

 

To be clear...
Key off, headlights off, brake pedal released: brake lights off.
Key off, headlights on, brake pedal released: brake lights off.
Key on, engine off, headlights off, pedal released: brake lights off.
Key on, engine off, headlights on, pedal released: brake lights off.
Key on, engine on, headlights off, pedal released: brake lights off.
Key on, engine on, headlights on, pedal released: brake lights on.

Is that what you've got?

 

Yes sir.

 

That would take one of a few really complicated & specific failures. Are you sure it's the brake lights, and not just the parking lights getting brighter with the alternator raising the voltage? Are you sure both taillight bulbs are installed correctly? What do the brake lights do if you only turn the PARKING lights on - not the HEADlights?

With the engine running, headlights & brake lights off, what happens if you hold the brake pedal - do any parking lights come on, or do the headlights come on dim? What if you hold the brake & turn the headlights on - do the brake lights change brightness?

You need to put every detail about the truck into your signature so it's visible with every post. Click this & read the caption:


(phone app link)

 

Will due, I will look up more specific information on the model and update my signature when I get home tonight. What I can tell you off the top of my head is its a 1985 XLT Lariet with a 351 Windsor and a motorcraft 4180C carburetor. The only modifications from original equipment is the bed was swapped out to a flatbed last year and I put aftermarket LED taillights on a couple days ago but this problem was an issue with the original tail lights and sockets to.

I also noticed when I was messing with it last night that the lights also stay on when the key is turned to the accessory notch before turning the key to crank it. I don't think it is the parking lights, when I hit the brakes they don't get any brighter. It does the same thing with the parking lights on but not the head lights, as long as the key is off everything is good but once you turn the key to accessory or crank the engine they stay on.

If the engine is running and the headlights are off I can hit the brake and everything works perfectly, parking lights and head lights don't turn on. If I hold the brakes and turn on the headlights the brightness doesn't change.

 

...put in your signature...

With this symptom, there must have been something changed besides the bed. Look closely for ANY wiring modifications or damage: radio, alarm, RKE, remote start, TRAILER connectors hanging over the sharp edge of the bumper... Be thorough. Even cab steps bolted through the thresholds are notorious for shorting out the wiring there.

 

Sorry for the delayed response. That is a very good point, time to start tracing wires. Thanks for the help.

 

Did you figure out the problem. Mine is doing the exact same thing. I to just recently took my bed off to replace fuel pumps.