Hi Guys,
This will be my last post here, as I no longer have a Tahoe. I thought I'd share the solution that took me a year to track down. Ever since I owned my Tahoe, I could not use the feature of having the interior lights come on when a door was opened, otherwise they would never turn off. Also, I would always have an instrument cluster message about a "rear door ajar" or "liftgate open". I finally started to dig into it when I was having battery drain issues. I had replaced the battery a year ago, but it only lasted 12 months before needing to be replaced. I discovered the root cause was making all these symptoms appear.
Until a month or two ago I didn't try too hard to diagnose it. The battery drain issue in particular forced me to address it. I will explain what is most likely going on if you are having a problem with the doors constantly being reported as being open, when they are in fact closed. There is super easy test to diagnose and verify the root cause. Underneath your dash is the BCM (Body Control Module):
You have to remove the lower dash panels to to access it. Here is what it looks like:
The BCM detects whether the doors are open or closed. If the BCM is faulty, no amount of shutting a door harder will cause the warning messages to go away. Here is how to detect if the BCM is faulty, or you have a door switch problem-
On the bottom of the BCM is a blue and purple plug:
Unplug the connector. If you still see messages on the instrument cluster about the rear doors or liftgate being open, then the BCM is absolutely faulty. There still could be problems with the door switches, but you know for sure the problem will not be remedied until the BCM is replaced. If you unplug it, and the problem goes away, then the problem lies with one of your switches.
Now for the theory, in case you are interested. There is a microcontroller in the BCM that controls and handles many functions. One of those functions is that it can measure voltages. For the door switches, when the door is closed, the switch is open (disconnected). The resistor at R1 pulls up, or raises the voltage, seen at the microcontroller for the door switch. So if the door is closed, the microcontroller effectively reads the voltage as Vcc:
You can check this by measuring the voltage seen at the door switch terminals.
If the door is open, the switch connects to ground, and the microcontroller reads a much lower voltage - much closer to ground. So when we disconnect the blue and purple harness, it should make all rear doors and liftgate appear closed, if the BCM is working properly. Even if the doors were open, when the purple connector is unplugged, the switches cannot ground the signal:
The circuit is a little more complicated, but that's the gist of it. I got irritated with my BCM so I removed the PCB and reverse engineered how it worked. When the BCM is faulty, it draws current on those signals, and thus measures a lower voltage than it should, and thinks the door is open. I have a theory this may happen with vehicles that have had a large number of door open-close cycles. The inductance of the traces and wires may damage those microcontroller pins over time, depending on how fast those switches close/open.
This will be my last post here, as I no longer have a Tahoe. I thought I'd share the solution that took me a year to track down. Ever since I owned my Tahoe, I could not use the feature of having the interior lights come on when a door was opened, otherwise they would never turn off. Also, I would always have an instrument cluster message about a "rear door ajar" or "liftgate open". I finally started to dig into it when I was having battery drain issues. I had replaced the battery a year ago, but it only lasted 12 months before needing to be replaced. I discovered the root cause was making all these symptoms appear.
Until a month or two ago I didn't try too hard to diagnose it. The battery drain issue in particular forced me to address it. I will explain what is most likely going on if you are having a problem with the doors constantly being reported as being open, when they are in fact closed. There is super easy test to diagnose and verify the root cause. Underneath your dash is the BCM (Body Control Module):
You have to remove the lower dash panels to to access it. Here is what it looks like:
The BCM detects whether the doors are open or closed. If the BCM is faulty, no amount of shutting a door harder will cause the warning messages to go away. Here is how to detect if the BCM is faulty, or you have a door switch problem-
On the bottom of the BCM is a blue and purple plug:
Unplug the connector. If you still see messages on the instrument cluster about the rear doors or liftgate being open, then the BCM is absolutely faulty. There still could be problems with the door switches, but you know for sure the problem will not be remedied until the BCM is replaced. If you unplug it, and the problem goes away, then the problem lies with one of your switches.
Now for the theory, in case you are interested. There is a microcontroller in the BCM that controls and handles many functions. One of those functions is that it can measure voltages. For the door switches, when the door is closed, the switch is open (disconnected). The resistor at R1 pulls up, or raises the voltage, seen at the microcontroller for the door switch. So if the door is closed, the microcontroller effectively reads the voltage as Vcc:
You can check this by measuring the voltage seen at the door switch terminals.
If the door is open, the switch connects to ground, and the microcontroller reads a much lower voltage - much closer to ground. So when we disconnect the blue and purple harness, it should make all rear doors and liftgate appear closed, if the BCM is working properly. Even if the doors were open, when the purple connector is unplugged, the switches cannot ground the signal:
The circuit is a little more complicated, but that's the gist of it. I got irritated with my BCM so I removed the PCB and reverse engineered how it worked. When the BCM is faulty, it draws current on those signals, and thus measures a lower voltage than it should, and thinks the door is open. I have a theory this may happen with vehicles that have had a large number of door open-close cycles. The inductance of the traces and wires may damage those microcontroller pins over time, depending on how fast those switches close/open.
