Third brake light issue, need help

so i own a malibu 06, and i need to pass NC inspections, to take to court due to a ticket, too try and get the ticket dropped, and also to avoid more in the future. I have tried replacing the light, and that was not the issue. They traced a low voltage issue back to the body control module, and with MUCH reluctance i had it replaced…and it did not fix the issue. apparently the part where it attached too the wire that goes too the third brake light has low voltage, but somehow replacing the bcm did not fix it so i am utterly stumped. does anyone have ANY suggestions i can work with before i get another diagnosis, maybe to cut down on the time that it takes them too find an issue?

Who is “they”?

And how did “they” trace this low voltage problem to the BCM? I.e. what was the method of testing components?

I don’t think the BCM is in the brake light circuit. There are two parts to the brake light switch, one for the left and right and one for the center light, so if the left and right brake lights are working, it could be either the switch, the wiring between the switch and the light or the light and ground. If the center light is in the trunk lid, look at the wiring between the body and the trunk lid.

they would be a trustworthy dealership. they used some device that would test the electricity by touching it to the wire. they touched it to the light, and it showed a very low voltage, so they went to the bcm and touched it too the part where the bcm went to the wire that went too the light, and it showed a low voltage. i hoped it would be a wiring issue before that, but if it shows low voltage where the wires connect to that, i figured it would be the bcm…but now i do not know

After looking at some service data it does show the middle lamp is indeed connected to the BCM. The lamp is LEDs. The lamp may be defective and could be loading down the output of the BCM. Have the shop check the voltage output of the BCM connection while the lamp is disconnected from it and then see if they get 12 volts that way. If that doesn’t help then I suggest they make sure input power to the BCM is ok.

the idea bout checking the input power may be of help. they did unhook the lamp and the voltage on the bcm was not 12 somehow. but if the input voltage was low, wouldn’t all the electric stuff be bogged down?

Since the lamp is LEDs the supply voltage may not be your typical 12 volt power. I don’t know for sure. I would assume it is supposed to be 12 volts. It may be a good idea to see if the lamp works ok by tieing it to power directly. But if the supply voltage is supposed to be say 5 volts, then applying 12 volts would damage the light.

From what you say it sounds like the shop already checked the BCM output without the load on it. If the voltage was around 5 volts on the pin then perhaps that may be what the lamp requires. I would check the resistance to ground through the lamp. If there is a low resistance seen then the lamp may be bad.

If the lamp circuit checks out ok then something has to be wrong with the BCM circuitry. The info I am looking at shows a pin going to a load resistor for the brake circuit that is located in the engine compartment and a brake position pot. I wouldn’t think they would cause this trouble but those circuits may need to be checked out along with any power inputs to the BCM.

when we switched out the lamp, we found that it had the same issue, and we did cross the wire on the lamp too see if it worked, and it shined bright. and when you say something may be wrong with the circuitry, do you mean that the issue could be further in then the bcm, like whatever sends signals too it. ( getting close to my limit of information on cars hehheh) i will take a copy of this stuff when i get the car diagnosed next week, and maybe it will speed things up. i am wondering how much it would cost to fix if the issue is farther in the car thou.

Since replacing the BCM and light didn’t help fix the trouble then I would want to check out the rest of the circuitry related to the BCM and the brake light system.

If GM puts a computer between a lightbulb and a switch, then they are off my list for my next vehicle purchase. That is the dumbest thing I have ever heard of.

It does seem absurd to me also Keith. I can only hope that it is so the BCM can monitor it and give a warning to the driver if the circuit has trouble.

There are vehicles that do monitor the left and right brake lights (Honda) and that can cause the loss of those lights, but the center brake light is a direct connect to the switch, just in case.

Something the OP could look into is to see if the center light is actually required by law. It wasn’t too many years ago that these lights almost did not exist. In the 60’s the only cars that had center brake lights were really old cars that only had this one light.


Sometimes states forget to update their laws.

Have you checked to see if the center brake light has a good ground to the receptacle? I’ve only repaired a couple of center brake lights and both problems were caused by bad grounds.

They have already determined that voltage isn’t coming from the BCM to turn on the lamp and the line isn’t shorted to ground.