Brake Lights Odd Behavior

I went to get an inspection, and failed because of this. Brake lights work fine until the headlights are turned on. When the headlights are on the rear tail lights/brake lights are at full intensity, and don't get brighter when the brake is pressed, however the center high mounted light is operating normally, ie goes off an on with the brake. The guy said it was the first time he had seen it. Any ideas?

 

Depending on the year, wrong bulbs perhaps? Maybe there's a single filament vs double filament in there. I don't know the intricacies of single vs dual filament bulbs but that's where I'd look first as it's easy and cheap to test with new bulbs. What year truck and what kind of tail lights are on there?

 

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It's a 2015. Keep in mind, I bought the truck new and it has the same bulbs in it from the factory.

 

Next thing I would do then is take a multimeter to the plug. There are three wires for the tail/brake light portion. Tail, brake and ground. I would see if the tail and brake are getting the 12v when they’re supposed to or if it’s wrong. That will help troubleshoot if it’s the tail light or something further upstream.

Aside from that, I’m unsure and maybe others can chime in.

edit: so I assume you have the LED tail lights then. That’s a whole assembly and not an actual bulb.

 

You have aftermarket trailer wiring harness?

 

No, I have no aftermarket parts whatsoever

 

You say headlights, but is it the headlights or tail/marker lights.
That will help narrow down the paths to take looking for the problem.

 

Take one tail lamp bulb out and see if the other side works normally.
If it doesn’t, put the bulb back in and pull the other side and see if the first side works ok.
Check for corrosion etc while doing it

 

I think 2015 had LEDs. So he’ll have to unplug the assembly.

 

The inspector showed me that when the headlights are on and he presses the brake the tail lights don't change brightness indicating breaking like they are supposed to. However the middle light does. In essence it looks like I'm always braking.

 

Also, FWIW.
The difference between the Center top and rear brake lights is the ground.

 

You need to turn the headlight switch to the first notch.
NOT the second notch, the 2nd notch is the headlights. The first is for marker/tail lights.
What we are doing here is “no tools required” diagnosis. (For now)

See if the brake lights come on on the 1st notch

 

2015 should have LEDs. Can you confirm that?

And as stated, confirm whether this happens with the running lamps on, or only when full headlights are on. I suspect when running lamps are on (which are always on when headlights are on) will be when you have problems. Probably has nothing to do with the headlights themselves.

After confirming these details, my next steps would be to unplug one brake light, have somebody press the pedal and see if the other one works properly or if it's always too bright still. Then reinstall that side and switch sides. That might be able to isolate whether or not you have a bad tail lamp diode shorting the running lamp power to the brake light portion and backfeeding the other tail light on the brake portion via it's brake wiring.

 

Here are some pictures, this is with the full headlights on, Second one is brake depressed other is not.

 

I thought 2016 but you could be right.

 

I don’t see a trailer harness connection. So I’m going to assume you don’t have one.
It’d be nice to know if it were the headlight or tails circuit we are working with.
I have to head out to road call, at his point. I’d pull the stop fuse and see what happens.
If nothing happens pull the tail light fuse.

Let us know what you find.

I’m headed out on a road call to help a buddy. I won’t be able to help for a few hours.
Hopefully @Tacoma Mike can help you until then.

@Tacoma Mike this might help
https://www.tacomaworld.com/threads/2015-wiring.559639/

 

2016 went back to bulbs.

 

The photos confirm the problem but we still need to know if it’s
When running lamps are on or if it’s only when you go all the way to headlights.

 

Yea that’s no help.
I clicked on one number and my phone auto dialed that number lol

 

Also, point of reference.
Has the windshield been replaced ?