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3rd brake light works - rears don't - what to check?

3rd brake light works - rears don't - what to check?

Postby Teikas Dad [OP] » Oct 06 2015, 11:17pm

My brake lights on the rear of the box (2000 GMC Sierra 1500) don't work. The third/center mount brake light on the cab works. The fuse is good, the bulbs in the tail lights are good. I took a bulb out and tested the socket with a test light with the brake pedal down - nothing.

Where's a good spot to start troubleshooting for this problem? Are there any plugs in the wiring harness that I can unplug and check for power coming through?
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Re: 3rd brake light works - rears don't - what to check?

Postby philmonger » Oct 06 2015, 11:29pm

Gotta be a connector or a ground issue. You gotta climb underneath and find the tailight wires. Look for a connector and check voltages.Ground problems can be a pain. If they are grounded through the body, rust and corrosion on the ground straps would be a good place to start.....good luck!
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Re: 3rd brake light works - rears don't - what to check?

Postby hornnumb2 » Oct 06 2015, 11:49pm

Does 2000 still have the circuit board that the bulbs pop into? If so you can get the boards at the autoparts. Very common in those taillights. If not check the connector behind the bumper, there is a box that it plugs into and gets corroded.
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Re: 3rd brake light works - rears don't - what to check?

Postby Teikas Dad [OP] » Oct 07 2015, 12:05am

hornnumb2 wrote:Does 2000 still have the circuit board that the bulbs pop into? If so you can get the boards at the autoparts. Very common in those taillights. If not check the connector behind the bumper, there is a box that it plugs into and gets corroded.



No circuit board there. However, I do recall seeing a connection box behind the bumper when I put on a new trailer plug last year. I'll check that first, thanks.
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Re: 3rd brake light works - rears don't - what to check?

Postby Teikas Dad [OP] » Oct 08 2015, 2:25am

OK, I feel like a fool.....

I got a chance to troubleshoot the brake lights this morning. I put a 2X4 between the seat and brake pedal to keep it depressed. The third/center mount brake light works with no problem. I started checking the 7 pin trailer plug and shorted one of the turn signal lugs. I went looking for the fuse but couldn't find it in the dash fuse box. While I was checking the fuse box I double checked the fuse labeled BRAKE and made sure the fuse was good - it was. But, I couldn't find the signal light fuse..... I opened up the under hood fuse box and found that's where the signal light fuses are. While I was searching for the signal light fuse I discovered that there's another BRAKE fuse in the under hood fuse box. I pulled that fuse and found it was blown. Replaced it and my brake lights are working. I can't find a fuse for the center mounted brake light so I don't understand how that one can be on when the rears aren't.

Anyway - the moral of the story is look for more than one fuse on your Chevys and GMCs.
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Firing on 8 cylinders [L1]
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Re: 3rd brake light works - rears don't - what to check?

Postby olred05 » Oct 08 2015, 12:22pm

Teikas Dad wrote:OK, I feel like a fool.....

I got a chance to troubleshoot the brake lights this morning. I put a 2X4 between the seat and brake pedal to keep it depressed. The third/center mount brake light works with no problem. I started checking the 7 pin trailer plug and shorted one of the turn signal lugs. I went looking for the fuse but couldn't find it in the dash fuse box. While I was checking the fuse box I double checked the fuse labeled BRAKE and made sure the fuse was good - it was. But, I couldn't find the signal light fuse..... I opened up the under hood fuse box and found that's where the signal light fuses are. While I was searching for the signal light fuse I discovered that there's another BRAKE fuse in the under hood fuse box. I pulled that fuse and found it was blown. Replaced it and my brake lights are working. I can't find a fuse for the center mounted brake light so I don't understand how that one can be on when the rears aren't.

Anyway - the moral of the story is look for more than one fuse on your Chevys and GMCs.


Third brake light is on a fuse with some other stuff I forget what..

Don't ask me how I know that.

They do this so that if a fuse blows you don't loose all your brake lights. Good thinking actually.
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