I have a 2002 Astra Bertone convertible which has a six button steering wheel control. It worked with the original Blaupunkt 520 radio, however I have since replaced that with a touchscreen (Android Auto/ Car Play) HUD which is "compatible" with original steering wheel controls.
All good so far.
BUT.
The steering wheel control lines are labelled GND, MRQ, SDA, and SCL, which makes me think that is I2C, and they are at a logic level of 5.01 volts (unsure what MRQ is, and nearly impossible to find info on Uncle Google).
The HUD has GND, KEY1, and KEY2, which are for a resistance based steering wheel control. Should be fairly doable to use an arduino to convert between the two, however when I hook an arduino up to the GND, SDA, and SCL lines with I2C scanner code, it cannot find an address.
Do I need to connect the 5v source between the two?
Could the issue be different I2C speeds, and if so, how do I work out the vehicle I2C speed, having not been able to find documentation?
How would you progress?
Could it be some other communication protocol? Could it be the fact that I don't know what MRQ is (any ideas on what that could be - maybe a master request?)?
Can you post links to the technical information on the switches. It may also be in the radio schematic. I do not have time to check but here are two links you need to check:
I have scoured high and low for that, but cannot find it.
I have the physical copy of that manual, it does not contain relevant information as to the communication protocol, or the wiring of those switches.
There has to be some reverse engineering done, and I'm very bad at that. I'm guessing that the MRQ line is a high/low "enable" line. I might play around with that and see what happens.
The key might lie in the radio schematic—you’ll need to determine what’s happening there. Resistor trees were commonly used in cars and were typically limited to about five buttons, which could be managed with just two or three wires. Keep in mind that ground may not necessarily be a separate wire.
There are six buttons, four wires with labels literally on them saying GND, MRQ, SDA, and SCL. I feel like that's not likely to be a resistor tree. Plus pressing the buttons does not change the output voltage, which I would think it would if the buttons had resistors on them? Though I don't know much about that, maybe the voltage isn't supposed to change from a resistor tree?
I also thought about CAN bus, but why would the wires be labelled SDA and SCL?
But maybe what's worse is that there is now only ONE wire going to the steering wheel controls...
Time to stick my DMM on that and see what it's doing!! Is it possible for a resistor tree to use only one output like that?
Yes, it will need either a power source or ground as the second wire, but only one wire would need to return. I expect the voltage will change as you press the buttons. If it’s not powered, try measuring the resistance (ohms) between the positive and negative terminals, as I’m unsure how it’s powered.
