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Your Auto Inspector
Your Auto Inspector, Assistant Service Manager
Category: Car
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Experience:  Tires Plus
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Why would my brake lights attached to my tail lights not ...

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Why would my brake lights attached to my tail lights not work, but the one upper brake light on my Cab does work? I have checked the bulbs on both tail lights and they are fine, just not working when I hit the brakes. I have also checked the fuses and they are fine.

Sounds to me like you have a bad brake light bulb. Those bulbs have dual filaments in them, so they can work as the taillight fine, but not light for the brake light function.

Because the 3rd brake light is working, you know that you have a signal from you stoplight switch.

You can check to make sure you are recieving signal to the socket by using a test light. Remove the brake light bulb, ground the test light clip to the chassis, and probe the brake light connector. Have someone push the brake pedal to see if you are getting a signal. If you are then you just need bulbs. If not, let me know and I can provide you with detailed wiring schematics so that you may trace down the problem.

Please let me know if there is anything else I can do to assist you.

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Customer: replied 9 years ago.
I have checked and replaced the bulbs and they are still not working as noted in my question, so that is not the answer.

Have you verified that you are getting signal to the wiring harness going into the connector. This will let you know if you have a wiring problem, or a problem inside the bulb connector.

You can check this by probing the brake light signal wire. See if you get a voltage signal when someone steps on the brake. You can use a test light again for this.

Please verify brake signal in the wire before the connector and let me know the outcome.

Customer: replied 9 years ago.
I have done what you suggested, but still have not solved my problem. My idea is that potentially there is a short, or a disconnect somewhere in the wiring between the spot where the wires "split from the Cab light to the two brake lights, if you can give me an idea where this junction is , I will hit the accept key for your payment. Thanks...

Here is the wiring diagram you requested. It will probably be a damaged signal wire because the tail lamps, stop lamps, turn signals, and license plate bulbs all share a ground together.


Here are the harness connectors so that you can find the correct wire:



Here is the ground distribution diagram:



The signal wire from your brake pedal switch goes into the bussing block under the bed. It is component #8 in this diagram. This is where you need to check for signal.