Lighting Problems Caused by Common Body Control Module Corrosion
This case study is about a 1997 Buick LeSabre that has a few interior issues. The customer’s main problem is that overhead map lights sometimes work, sometimes kind of work and sometimes don’t work at all. The other annoyance is the warning chime, chimes whenever the door is open, even if all the lights are off and key is out of the ignition. The customer then showed me his issues with the car, like the chime dinging with the door open. We then went and sat inside the car to check out the lights, but the lights were working properly, which happens quite often when a vehicle is being dropped off for me to fix. It didn’t take long though, for the overhead map lights to go very dim. Good, I was glad to have verified the complaints with the customer before he left.
I figured I would tackle the lighting issue first. It look like a ground issue the way the light would go dim. I really didn’t feel like checking the chime issue out since that could be caused by quite a few components. It could be an ignition switch key sense problem, park or headlamp on signal problem, seat belt issue, BCM or even a wiring problem. Maybe if I am lucky both of my problems with be connected and if so, the lighting problem will be much easier and faster to chase.
So I went and got some wire diagrams for the interior lighting. I noticed that all the interior lights had the same power and grounds so I went back to the car to check all the lights out. I noticed that when the map lights were dim so were the rest of the lights. That helps eliminate a lot of buried wiring. I then proceeded to look the ground circuit over. It looked like we had a ground problem by the lights going dim. What would happen is you open the door and the light would be on and bright. Then when you shut the door they would fad out like they should, but if you would manually turn the overhead map lights on they might be bright, dim, or not even work. Looking at the wire diagram, I saw that the light have two grounds. One is BCM controlled and the other ground was used when the light were manually turned on. Well these lights seemed to work fine when being controlled by the BCM and only had issues when manually operated. Well this is going smooth so I went and did a voltage drop on both ground circuits with the lights on. To my amazement, my multimeter read .01 volts. Well, that throws my whole theory out the window. So I then checked the single power feed and I only had 2.7 volts. There’s my problem, so what was it. A corroded wire, bad connection, maybe even a problem at the fuse box? I then check my wire diagram to see where my 12 volt power supply came from and it came from the BCM. I didn’t think it came from there since the BCM had the ground circuit when it needed to control the light, but it did. I went to the BCM and checked my power feed to the light and I still had low voltage there also. I then went and printed the BCM power and ground circuit wire diagram so I knew which wires to check. The BCM had three 12 volt supplies and one ground. With the harness unplugged from the BCM I used a test light to check the three powers and ground and saw no issues. I used a test light for this since it would load the circuit unlike my multimeter. This LeSabre got a new BCM to fix the lighting problem and even the chime issue was cured.
I always try to take the easiest and/or fastest path to solve problems I encounter. I do this because sometimes when you are chasing multiple problems they end up being linked together, like this one.
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