Originally Posted by dmjlambert
Here is what I think you have going on. Tail and brake lights are supposed to work like this when you have a conventional bulb. The bulb has 2 filaments. One of the pads on the bottom of the bulb is for power supplied to a very dim filament, that is used for when your headlights are on. It is the running light or parking light. The other pad on the bottom of the bulb is for power supplied to a very bright filament. That is for brake or turn signal. So, if you have brake applied and while your brake is applied you turn on the headlights, you will notice almost no difference in brightness. That is because you are adding something like 2% more brightness to a bulb that is glowing at 98% brightness already because you have the brake applied.
So now imagine what would happen if the filament connections were reversed. You get a very bright running light because your headlights are on, and if you use the turn signal or apply the brake, it only gets 2% brighter. You probably wouldn't even notice the difference. Especially in the day time. I see a huge number of tail lights in cars that people have modified from factory, especially Jeeps for some reason. A large number of LED lights come from the manufacturer's wired backwards if they have a bulb base wired onto them. And if they are hard wired by the person making the "upgrade", that person just doesn't pay attention.
The knobs on either side of the base of the bulb plug are at different distances or heights from the bottom of the base where the contacts are. That is supposed to keep you from getting the bulb in there backwards. But if the bulb is made wrong, or if you force the base into the socket backwards, either of those conditions can cause the problem. Somebody helping my daughter with bulbs did that to her car, forced the bulb in the wrong way. It cracked the socket because the socket was plastic, and I did not appreciate that and had to repair it. But the filaments were reversed and it had the symptom I describe.
I see Jeeps in my area all the time that have LED lenses where the dim outer ring of LEDS is supposed to be for running light, and the bright inner spot of concentrated LEDS is supposed to be for braking, but they have it wired in reverse. The running light is bright as heck when following one of those guys, and if they apply the brake and that dim outer ring of LEDs come on, I can barely tell, unless I'm paying close attention.
Here is how to troubleshoot and resolve. Remove an LED light and take it to a bench with a battery or to the front of the truck. Use alligator clip test wires. Connect the negative battery post to the base of the bulb. Connect the positive battery post to a test clip wire and tap the other end to one of the 2 contacts at the base. Figure out which contact makes the bulb grow very bright, that is your brake and turn signal filament. Tap the positive test wire to the other contact on the base and very it glows but not nearly as bright. That is your running light. Use a marker to mark which is which. Go to the tail light socket on the truck. Use a test light or multimeter to figure out which of the 2 pads at the bottom of the socket gets power when headlights are on, and which get power when brakes are applied. Mark which is which. Then verify the correct contact on the bulb is engaging the correct contact in the socket when you insert the base and lock it into place.
It is also a good idea to make sure the truck works with conventional bulbs before trying LED bulbs. You may have done that already.
I think a lot of LED bulbs are made incorrectly and both of the "filaments" glow at the same brightness. In my opinion, those are cruddy. The ones that have different brightness filaments but are wired to the base pads backwards are extra cruddy.
Anyway, sorry that was such as long story. If it leads to resolving the problem, great. If it does not, then I'm sorry, but perhaps the troubleshooting ideas and thoughts will help somebody.
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