Anyone ever heard of this?

Just resolved an interesting electrical issue. When I pushed the brake pedal the marker lights would come on. Not the headlights, or at least not enough current to be able to see anything from them in the garage. But while driving when I hit the brakes the clock would dim and the dash lights would turn on. If the key was out and I opened the door and pushed the brake pedal I would get the headlight warning tone. I was thinking it was a short in my trailer wiring harness, or maybe that little diode box on it went bad. It turned out to be a tail light bulb of all things. Before getting into it, I thought possibly the brake filament flexed down and was making contact with the other filament, but the bulb looks normal. No sagging filament, burn marks or anything. Just a tiny piece of what looks like welding slag, but it doesn't look like it could possibly touch anything. I even tried both bulbs individually in each housing, and it was consistently this one bulb. Replaced both of them for good measure and problem solved. Damnedest thing...

 

Things like this are what drive us crazy. lol

 

Damn I'd a never figured that out. Good find though

 

is it brand name bulb? I have seen similarly odd electric behaviour when 2 different bulb manufacturers are installed on the same circuit (turn, brakes etc.) First time to figure it out took a while, after that it is the first thing to check. Good catch.

 

This is quite common with LED bulbs where there is a "leak" from one circuit to another but never seen a short in classic bulbs. The only time I've seen the similar symptoms was when ground was not connected (the wire soldered to the metal "foot" of the bulb rot out or was not solder correctly) but then the touch with soldering iron fixed the thing..

 

Just standard Sylvania Long Life bulbs. I always replace bulbs in pairs and these were less than a year old. There is continuity between all contacts, I'm assuming due to the common ground, but is there any sort of small diode inside the base of these things that could have gone bad? I'm kind of curious how this happened.

 

Well looks like it might be something else. I just noticed it again. Only once or twice, not as regular as before. Maybe a bad socket? I'm fairly certain it's something back there in that area. As soon as I pulled out that bulb it stopped. Put it back in and it was doing the same thing. It seemed to be fixed, but unless there's something else back there frying bulbs I'm stumped.

 

Inspect your ground at each taillight (from the plastic housing to the body). I had your exact symptoms and after scratching my head for a long time, I realized that without the ground, those bulbs are just 2 filaments in series. So when you hit your brakes it sends 12V through 2 filaments, and energizes the marker light circuit with reduced voltage (due to resistance across the filaments). Also inspect the socket itself, sometimes it's corroded or overheated.

 

Clean the sockets most often a bad ground .

I have found a fitting brush works great chucked in a drill

Causing that circuit to ground back through the tail light circuit

 

Good deal, I was thinking it might be an issue with the ground. The outsides of the sockets have some surface corrosion on them, but the insides looked decent enough. At least for a truck that just turned old enough to buy cigarettes and lotto tickets. MikeWH, which ground specifically do you mean? The one on the harness going into the housing?

 

I was worried you'd ask for specifics! It's been a couple years since I have been into mine so I can't remember the exact configuration.
Basically, check for continuity with your meter between negative battery terminal and the shell of the socket. I think there are a few connection points there. If you post a picture of the area I could get more specific.

 

The bulbs ground in the inside of the socket that is what needs to be cleaned

The socket is the only place the brake light and tail light circuits can do this unless you have physical damage to the wiring harness

The ground for the brake light circuit on my 2000 is on the left Door Post behind the plastic trim