Tonight Mrs. Fungus and I decided we'd go to Portillo's for some roast beef sandwiches, and when we got into the Jeep, I found that the rear brake lights wouldn't go off. Or at least it seemed like that.
The defogger indicator light is staying on all the time now, whenever the engine is on. I went out and checked it a little bit ago and the brake lights are not on with the engine on, which is a good thing.
Anyway, I needed access to the wiring diagrams in the book I own but can't find, so I paid Haynes $20 to let me look at the images of it on-line. So naturally, now, the book will come crawling out of the woodwork.
Here's what I found: the damned manual does not include the rear defogger. The only mention of it is the instrument panel light in the switch; the defogger and its relay etc are utterly absent from teh wiring diagram. I checked every last page. I did a search in Googe and found something that contains it, and printed that out so I could refer to it.
Now, I never use the defogger, so I think I could just yank the relay for it and stop that behavior. THe problem is, I don't know if that fixes anything else. Probably not.
But it would give me information about where the short circuit is. What I know is, when I step on the brake, current is backfeeding through that light somehow. There are two or three possibilities that make, from here, any sense at all, but the most likely one seems to be that the issue is in the liftgate wiring.
In cars, just about everything uses a chassis ground. The negative terminal of the battery is connected to ground and everything else's negative terminal is also connected there, because the body of a car is usually metal and metal conducts electricity. It's cheaper than running ground wires for everything that needs power, it's just as good as using wires, and it's actually safer to boot.
The indicator light for the rear defroster connects between the defogger's relay's switched output, to ground. Period. The only way for it to get power and light up is for electricity to flow from the positive end of the defogger; and the only place those wires are in anything like close proximity should be at the back of the vehicle. If I take the relay out and see exactly the same behavior, that proves it.
The bothersome part is that the front turn signal and marker lamps, as well as the dashboard lights, are all going on when I step on the brakes. Going off of the wiring diagram, it looks as if there are only two wires that can cause all the symptoms I'm seeing: the wire from the brake switch, and the wire from the headlight switch to the fuse block. Because if you supply power to the headlight switch terminal that supplies the front running lights, it will also light up the instrument panel.
So it's got to be in the absolutely most inconvenient place one could have a short circuit: in the driver's side dashboard.
The truck is old enough that I would not be terribly surprised to have wire chafing issues. It means taking the driver's seat out and digging under the dashboard, though, and there's no way in hell I'm doing that with tomorrow's high temperature slated to be zero frigging degrees.
The real problem is this: the fuse for the brake lights is 15A. This wiring diagram suggests that the headlights are getting power, too, and their fuse is 20A.
The other thing: what I can try is to remove the trailer wiring harness and see if that makes a difference. I'm not sure what's in it; it's a black box you connect between two connectors in the electrical system. It'd be nice if that was the source of my problem. But it's not the way to bet, because that solution would be far too easy.
Anyway, once it's a reasonable temperature outside, I've got a couple of things to check: remove the relay for the rear defogger, and remove the trailer wiring. That should help me narrow things down, at least a little.