My brake lights (but not the 3rd brake light) on my pickup are constantly on, with the truck running or not. You're probably thinking brake light switch, right? Well, that's not the problem. I replaced it a while back because I was having this same problem. It seemed to fix it, but every once and a while the brake lights would stay on. I could jam the brake pedal once or twice and the lights would go off. Today, they just won't turn off no matter what I do, aside from pulling the brake light fuse. I've wiggled the brake pedal and brake light switch with no difference. My turn signals still work with the brake light fuse installed or removed. The hazards work (flash) with the brake light fuse removed, but not when the fuse is in place. The brake lights will stay on, and the front turn signals will light up and stay lit constant. I've pulled both tail light bulbs and put them back. No change. Also, the brake switch works. When I hit the brakes, the 3rd brake light turns on, and turns off when I release the brake, but the tail lights are still on.

Any ideas?

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[This message has been edited by Fiero_Adam (edited 09-19-2008).]

We had a Blazer misbehaving like that and it turned out to be the turn signal switch.

Short somewhere in the system, good luck finding it.

Start at the rear brake light bulbs.
steve

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Technology is great when it works,
and one big pain in the ass when it doesn't.
Detroit iron rules all the rest are just toys.

quote Originally posted by Cooter: We had a Blazer misbehaving like that and it turned out to be the turn signal switch.

This is a common place to have problems. The signal for the brake lights actually go through the turn signal switches. Or you might have a problem in the wiring to the brake light switch, like the wires are shorted together, maybe. Replacing the switch, you may have moved the wires into a place where they weren't touching anymore. Fixed a Ford Tauras that had a similar problem, was teh wiring to the switch.

Some of those have two switchs one for the ecm and the other to the lights, or one switch that has two switches in it, more than 3 wires. The brake lights do some how get mixed up in the turn siglal switch though.

When the turn signal is off and the hazards are off, the turn signal allows the brake light signal to pass through the switch.
When you operate the turn signal or hazards, the turn signal interrupts the signal to one or both sides and allows that light to flash from the turn signal or hazard.

Fiero_Adam, I just reread your original post. And since you say the front turn signals illuminate with the brake lights I would almost gaurantee it is the combo switch, maybe a wiring issue in the harness to the combo switch. You would need to check continuity between wires at the switch to see for sure.

I found the problem (with a little help). I tried the multi-function switch first, but that didn't fix it. I found that those switches do fail often, though. It ended up being a relay of all things. For some reason, power goes through this relay (labeled CHMSL (Center High Mount Stop Light)) but doesn't affect the 3rd brake light. The contacts are stuck closed and it causes just the tail light brakes to stay on. I found this bit of info after searching the net for an hour. I guess it's not as uncommon of a problem as I thought. I found a lot of questions concerning this problem, but nobody had a definite answer. Then I just happened across one answer that suggested this.

Thanks for the help everyone. I'm glad it was this cheap relay instead of the multi-function switch.