3rd brake light problem

[Jermu]

Soooo... I had noticed in my truck that the 3rd brake light isn't working, but thought that it was due to the stupid euro regulations that forbid the lamp, so I didn't investigate it, thought it was just deactivated. But then, some time ago I got an LED 3rd brake light with them cargo lights to replace the old one because the regulations have loosened in Finland lately (Finally we can have DOT lights, yeeeey). Aaand I opened up the light to take it off and noticed that there were no bulbs inside the light, and that the base was blackened from above where the bulbs would have been sitting. Odd, I thought to myself. Then I went on to plug in the LED light and was very happy to see it come to life. That's when I realised that the power was off. So it seems as though it is getting electricity from somewhere, but there are no auxiliar cables coming from the batteries or anything and as the harness travels inside the cab, I haven't had the time nor place to start investigating it. Any ideas?

 

[revvychevy]

Soooo... I had noticed in my truck that the 3rd brake light isn't working, but thought that it was due to the stupid euro regulations that forbid the lamp, so I didn't investigate it, thought it was just deactivated. But then, some time ago I got an LED 3rd brake light with them cargo lights to replace the old one because the regulations have loosened in Finland lately (Finally we can have DOT lights, yeeeey). Aaand I opened up the light to take it off and noticed that there were no bulbs inside the light, and that the base was blackened from above where the bulbs would have been sitting. Odd, I thought to myself. Then I went on to plug in the LED light and was very happy to see it come to life. That's when I realised that the power was off. So it seems as though it is getting electricity from somewhere, but there are no auxiliar cables coming from the batteries or anything and as the harness travels inside the cab, I haven't had the time nor place to start investigating it. Any ideas?

You’d have to look at a diagram to confirm but as far as I can remember, the cargo lamps are wired backwards. The brake lights are wired from brake switch to lamp to ground. But I think the cargo lamps are wired from fuse to lamp to switch to ground, meaning that since the switch is on the ground side of the circuit, you’ll have one wire that’s always hot going to the cargo lamps. That wire may be crossed or shorted or the plug itself was plugged in wrong. I’d take a volt meter and test each wire in the plug, then again with someone holding the brake switch down, and again with the cargo lamp switch on. Record which wires change in each step and that may tell you where the problem is. Also sometimes the aftermarket LED lights themselves are wired wrong. Normal bulbs are just resistance wires. They’ll work with electricity flowing through them either direction. But since LED’s are diodes, they’ll only work in one direction and sometimes you have to reverse them to match factory wiring. But if anything, that should be a light not working at all, not a light coming on when it’s not supposed to.


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