all tail lights work, Brake lights don't work.

I was told my brake lights were not working so i went and replaced my bulbs, they both looked bad. went to check brakes after replacing and the lights still don't come on when pedal is engaged. All other lights in the back work including the bulbs I just replaced when I turn on head lights, the brake lights don't come on when pedal is engaged. I don't get why the bulb comes on when all lights are switched on, but don't indicate the brakes are being applied. thanks for any suggestions.

 

Are you getting any voltage and a good ground at the socket? Check that first. Ground problems should be obvious: corrosion; or a broken wire.
If no voltage, maybe...
could be your brake light switch adjustment. The switch is operated by the brake pedal. Lay down and look up at the upper part of the brake pedal arm and you will see the switch. It is activated by a plunger type mechanism. When the pedal is pushed, it should allow the plunger to extend enough to activate the switch and complete the circuit.

You can pierce the wires with voltmeter probe to see if the switch is closing, letting the circuit be completed.

A cheap test probe like this can be bought at any auto parts store: https://tinyurl.com/y6qvoawg

It adjusts by disconnecting the wire harness plug, loosening the lock nut(washer) and turning it. You may need to back it out a little to allow the plunger to extend further to close the circuit.

 

Thanks for the response, if it is a ground issue or wire issue would it be possible that both lights would be out? and the running lights are working fine.

 

Have you checked the fuse? Assuming the fuse is ok, then unplug the brake petal switch and jump the harness side terminals with a piece of wire, this should complete the circuit as if you are applying the brakes and see if the lights work then.
Please do not pierce the insulation to check for voltage, use the electrical connectors, when you pierce insulation you make a small hole for water to get into the wire and may have corrosion problems down the line, no wrapping the part you pierced will not be as good as new.
It is unlikely to be a ground issue if the other lights work as the ground is shared.

 

I was referring to up under the dash at the switch. Guess I should have been more specific.

 

Hell for that just unplug the brake switch and check.

 

Well, there is almost no space up in there to get your hands. I have tried getting that connector loose on two different trucks and its a real pain. But yeah, either way.

 

Just bought a new switch, plugged it in and the brake lights still don't work. I'm thinking my next bet might be where the trailer harness is connected. Thanks for the help all.

 

The taillights and stop lights are on a different circuit. TAIL and STOP fuses. I guess you're going to wait to check the STOP fuse until after you've replaced a bunch of other perfectly good parts????

If the STOP fuse is blown then you have to check for a short. Those trailer harnesses are notorious for shorting out.

 

Ok here is a read basic run down of what to do, only go to the next step if you have checked after each step and the brake lights still do not work.

Step one, check if fuse is good, if you dont know how to do that then just a put a new fuse in, Also while your there check with a volt meter is you have 12V at the fuse. DC voltage. If you have 12V continue, if not then find the open before this point in the wiring.

Step two, unplug the brake switch and check if you have voltage at the connector, check both wires as I do not have a wiring diagram so dont know which wire is power. If you have 12V continue, if not then find the open before this point in the wiring.

Step three, check that the switch works correctly, with the brake switch uninstalled and in your hand see if you have continuety with the switch botton released, you should read OL when the switch is pushed in and 0.1 or so ohms when it is released. If you do read OL when pushed in and 0.1 or so ohms when released the switch is good.
(Yes I know in a perfect world it would be 0.00 ohms but this world is not perfect and everything has a little resistance)

Step four, reinstall brake switch and have someone step on the brakes and check if you have 12V at the plug for the brake light bulb, make sure the headlight switch is off so you dont get confused by running light voltage.

Step five, You should by now have found at between what two points you have 12V at and where you do not, you need to look between these points to determine where you lost the voltage and repair/replace whatever is broken between those two points.

Honestly based on your complaint I would say its likely to be the fuse or the switch, could it be something else yes, but those are the two most likely culprits.

 

joeyv141, Great overview. Regarding "check if you have 12V at the plug for the brake light bulb", how is this test performed? place one lead on the inside center of the socket and the other lead on the socket housing?

 

You would have someone step on the brakes while one lead is on a known good ground and one at a time touch the terminals in the socket(I dont know which is power so just check all three).
You should also verify that the turn and running lights work since on every vehicle I've seen they share a ground wire so you can eliminate said ground wire if the other lights work.