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I agree with @Cadillac Jim , the FSM information would support the theory that the brake lights should have the ability to illuminate with ignition off.
This would be a safety feature in IMO....... BUT ------\/
Lets assume that the BCM controls the circuit. This means the BCM is controlling the ground circuit which would require a relay. It still needs a signal from someplace, I'm thinking that would be the brake switch. See my previous post...
Since the CHMSL is getting 14v (the engine must be running or a battery charger is connected) it still needs a path to ground. Usually the CHMSL is the first in the circuit to illuminate. SO, if 14v is present supply a redundant ground and see if the lamp illuminates. Is the "BRAKE" relay working/clicking? You can usually "feel" them to verify.
@Cadillac Jim there should be a relay prior to the CHMSL is it ID'd in the manual you have?
The Northstar uses a very powerful ignition system that requires very high tension silicone plug wires, and installation of the plug wires in such a way that they do not cross under the beauty cover. There is a routing diagram for the plug wires in the FSM; we can post it here if you like. Cheaper wires will not last more than a few months. Your safest bet is OEM, which you are putting on your car as soon as they arrive.
Congratulations on your success, and on getting your fine car into a fully trouble-free, zero-code state.
As BBF says, you are more than welcome to browse questions and help out. I got started that way myself.
Looking at the 2007 FSM, power to the stop/turn lamps and the CHMSL are through the BCM, which has power to those circuits through three separate fuses that is always ON if the corresponding fuse is present and good.
Since the lights are powered through a module, firmware in the module may be used to change the logic from simple switch/relay connections. The BCM has signals for key in ignition and the key position, so it may not light the CHMSL with the key off and brake depressed if it is so programmed. But it would seem to me that safety dictates that all three of the stop lights always turn on when the brake is depressed, in case the key is accidentally turned off while the car is being driven, or the key is turned off with the transmission not in Park.
If the BCM is in sleep state, it will wake up if any door is opened, any other module puts anything on the car's network the key is put in the ignition, the headlights or parking lights are ON, or you press any button on the fob. If it's in the sleep state when you press the brake, the lights may come on or may not but I think that they will because the brake switch will activate the EBCM and such and put signals on the network, which will wake up the BCM. There may or may not be a noticeable delay when you do this but if there is it will be under half a second.
Seems to be a issue with GM in general lately.
They changed the circuit so that they only work with the key on.
I have seen many newer cars with multiple symptoms
from no brake lights, brake lights always on, no third, only third. You name it.
But as soon as the key is turned off lights go out.
If you are getting at least 12v to one of the wires and ground to the other then likely the lamp itself is your failure
I have a few questions that I'm hoping to get some help on. I recently purchased a very low mileage (25,000 miles) 2006 Cadillac DTS that was very well maintained. I have put 9000 miles on it with no troubles other than the stabilizer light will come on occasionally and then go off. Last week when the temperatures here in Virginia hit around 100 degrees, I was making a turn onto the highway and the steering wheel seemed to shudder and I lost control of the vehicle for maybe 5 to 10 seconds. The car also seemed to shut down. I suspect that it may have been a computer failure. Then the car became normal again and I have not encountered the problem since. None of the error lights came on the dash. Yesterday I called Cadillac inquiring about any recalls that may not have been repaired on this vehicle. They told me that there were four (4) recalls on my vehicle but only one (for the ignition key/lock) would be attended to. They said that the other three are "closed". THe local Cadillac dealer who is going to do the ignition key fix said that would not have caused the steering/shut down problem that I had encountered. So my questions are if any of you have heard of this type of problem before and do you have any advice on how to troubleshoot it? I suspect that heat may have played a part in it, since it was such a ht day when it happened.