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Neither the trunk nor the fuel door release actuators will open electrically when you push the buttons. The trunk & the fuel door open just fine manually. The button switches to open the trunk & fuel door are integrated into the headlight module and the buttons are just inputs to the BCM.
I started my diagnosing at the power wires for these switches. I got battery voltage at the wires and they both pulled to ground when I hit either switch. The switches are good.
My next move was to test right at the BCM. Each release actuator is controlled by the BCM with a relay for each. The relays are ground side switched through the BCM. I tested for voltage for those relays at the BCM and I got battery voltage there but neither would pull to ground with a button pressed. The BCM is probably fine, it’s probably just not getting either signal.
In looking at the diagram, it shows the signals get sent to serial data lines through the instrument cluster and then on to the BCM. I know the switch signals are at least getting to the instrument cluster because the signal voltage comes from the cluster. Whether or not it’s getting past the cluster is beyond me.
I got confused by the boxes on top of the diagram saying the data lines go to the DLC. I lost confidence I could follow through at that point and stopped my diagnosing. I realize now I probably should have just looked at the other side of the box and continued chasing the wires to the BCM as shown on the diagram. I also realized I should have went home and grabbed my Modis to see what kind of bidirectional controls I had for actuator testing. If I could have opened the trunk with the scan tool, that would have narrowed down possible faults to the cluster or a bad purple wire between the cluster and the DLC.
I did take a minute to look at the wires on the DLC and found the purple wire listed on the diagram indeed gets to the DLC. So I experimented with the purple wire at the DLC by looking for a voltage signal whenever I pressed the trunk or fuel door buttons. With or without pushing a button, I had the same 20ish millivolts at the purple wire on the DLC.
Shouldn’t I have seen a voltage increase on the DLC when I was pushing buttons? Have a gander at my diagram and tell me what you think.
Thanks!
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I’m not clear if you repaired the issue or not….but….
The serial data line is only used to communicate between modules. For example for the fuel door and storage compartment to open via compartment switches some enable critical has to in place for that function to take place. ….vehicle at 0 mph, shift selector in park, engine not running… ect.
Remember a module that is overlooking a system that is not working to the way its programmed will not send signal to another module via the data line to ground a relay. The relay is not grounded by the data line…the module that controls functions grounds the relay.
Reread the circuit description to see what criteria has to be met and see if all other function is communicating.
I can’t remember what data system was used on a 99 Cadillac. GM usually used the newer system a few years ahead of the other car lines on the Cadillac …so I’m guessing you have the Class 2 system…. I’ll try to explain both…
Usually on a GM class 2 system voltage will be lower if all modules are not communicating.
General Motors vehicles typically use either UART or Class 2 serial data. UART is a 5–volt circuit. When the controller is using UART, it will pull the voltage low to communicate. Class 2 is a 7–volt circuit. When a controller is using Class 2, it will pull the voltage high to communicate. Other communications systems that may be found include: The Class 2 serial data bus has no master control module for its operation. Each electronic control module connected on the Class 2 data bus transmits and receives messages composed of square wave digital signals that may vary in length. These signals are typically 6 Volt +/- 1 volt when measured with a DVM set on the 1 msec record feature. When no communication occurs, Class 2 data line voltage is zero. Class 2 communication differs from other types of serial data used on GM vehicles such as UART and SBI. The Class 2 data bus is toggled from 0 to 7 volts. Class 2 serial data also uses arbitration and priority to control data messages. All of this combined allows Class 2 communication between controllers to be faster and more complex.
JTF, I feel like you did a horrible job of actually reading what I posted. At any rate, I learned about the role a stupid button called the valet switch that resides in the glove box.
I feel like you did a horrible job of actually reading what I posted
I thought your diagnosis was very good…as for as the valet switch that resides in the glove box….I learned that the hard way also…. once upon a time…
I’ve read many of your post; you’re a very good technician; don’t cut yourself short…..You have very interesting post which I have learned from.
One thing I’ve learned…the hard way; always understand how the system/circuit functions before diagnosis.
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