None of the three bulbs of the third brake light are working (in reverse or when pressing brake pedal)?

Hi,

The CHMSL gets power via its own relay and fuse. I attached a pic below showing the wiring. Is this the fuse you checked? It is located in the under-hood fuse box.

If you checked it and the fuse is good, confirm there is power to and from it when the brake pedal is pressed. Here is a link you may find helpful:

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Let me know. Also, confirm there is power to and from the fuse.

Take care,

Joe

See pic below.

Hi,

Hang in there. We'll figure this out.

I worked backward through the wiring schematics. Believe it or not, the wiring schematic for the exterior lighting is six pages long. Ugh!!!

Okay, here is what I need you to do. I need you to remove the brake light relay. The relay is a simple 4-pin relay. Where the relay plugs in, I need you to test each of the 4 pins for power. Two of them should have power and two won't. If that is the case, then either we have a ground issue, or the relay is bad.

To test ground, check for continuity in the two pins that don't have power.

Here is how this works. The relay has a primary and secondary side. When you press the brakes, a coil signal (low voltage) is sent from the body control module (BCM) to the primary side of the coil. When this happens, a ground is provided through the primary side automatically in this case, the primary side is completed and an electromagnetic coil energizes, causing the secondary side of the relay (a switch) to close and send power to the light.

The primary side gets power as mentioned, from the BCM. The secondary originates in the under-hood fuse box. It is fuse 2 and a 175-amp fuse. If that failed, that may explain why you lost power to other fuses.

So, if with the relay out you do have power to two of the four pins, we are golden and the likely problem is the relay, but still could be a ground. If you have power to both, and you have continuity to ground, the relay is the issue. If no ground, that is where we go next.

If there is no power, check the main fuse I mentioned in the under-hood fuse box.

Just FYI: when you turn the relay over, you will see numbers 85, 86, 87, and 30. Pin 86 should be the low voltage signal from the BCM. Pin 85 is the ground for the primary side. Pin 30 should be the power supply to the secondary side. And pin 87 is what sends power to the light.

Pin 30 should have 12v always and pin 86 will only have power with the brake depressed.

Here is a link that may be easier to understand. I likely made this confusing. Sorry. LOL The link also explains how to test the relay circuit.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

I attached three pics below, so you see where the power comes from and where it goes. Note, I had to cut out approximately four pages to reach the end of each side. LOL The last pic is the BCM.

Take care and let me know what you find.

Joe

See pics below.

Hi,

If nothing in the under-hood fuse box has power, did you check the battery fuse box which is mounted at the top of the battery?

See pic 1 below. That is the battery fuse box which is powered directly from the battery. From there, power gets distributed to different locations. If you have no power at the under-hood fuse box, check to make sure fuse 2 (X2 in pic 2) is good and has power at both ends. Fuse 7 (X7 in pic 2) powers the right instrument panel fuse box.

Let me know.

Joe

See pics below.