The 2015 Audi Q3 radio generally turns on during vehicle startup if the radio was previously in use, and plays music. I believe the complete set of behaviors is:
- When the vehicle is started, the radio is set to the station and volume it was set to when the vehicle was last turned off.
- If the radio was muted (by pressing the power button briefly) at the time the vehicle was turned off, the mute is forgotten and the radio is turned on again at the previous volume.
- If the entire multimedia unit is turned off (by pressing and holding the power button) the unit remains off. However, this requires the operator to remember to completely turn off the unit before turning off the vehicle.
The desired behavior is that the vehicle never play music on vehicle start. The vehicle should play music only if the operator explicitly requests it.
Secondary goals are modify the vehicle as little as possible and avoid permanent modifications to the vehicle.
This seems difficult to attack by modifying the behavior of the multimedia unit. Aftermarket units exist, but I think they're a bit of an "order something that might serve as a replacement unit off eBay and cross your fingers" kind of gamble.
It doesn't seem like there's a robust open source head unit ecosystem (just "OpenAuto", which isn't exactly a head unit platform?) and I'd rather not put one in the vehicle anyway even if one existed.
I paid $9 for a service manual for the vehicle on eBay. Is this legal? Why can't you just download the manual? If this is illegal, I definitely did it hypothetically.
I think the audio signal travels from the radio unit over a "MOST bus" to a "Digital sound package control unit" in the sidewall of the trunk.
If we could proxy the MOST bus, that would probably cover things, and we'd just need a couple of MOST connectors. But this seems very hard, and I can't find anyone doing DIY MOST bus proxying or much documentation on what the MOST bus protocol looks like, so this feels un-promising.
From the control unit, the audio signal goes to the speakers over a 38-pin connector:
This seems more practical to proxy. There are 9 speaker-wire pairs leaving this bus and we should be able to splice into them. The connector itself (TE 1534531-1) has a generally available male half ($12 on Digikey).
The female half is trickier. I think it's actually three parts:
- a housing part (TE 1534184-1);
- a 16-pin connector (1534579-1)?; and
- and the TE site says this is also a mating part, but how do we get to 38 pins?
I also don't know what hardware I'm supposed to crimp and insert into those connectors (probably this?) or what crimping tool to use ("te msqs crimp tool" finds some tools starting at around $150).
Fortunately you can buy the whole assembly with everything pre-crimped on AliExpress for a comparatively modest $29.
So it seems theoretically practical to build a connector-to-connector speaker proxy and shove it into the sidewall next to the control unit.
Some open questions:
- I haven't actually pulled the car apart yet, so: is the unit actually where I expect it to be, can I physically proxy a device in the middle of the connection, etc?
- Where can I get power in the sidewall cavity?
- Can I tie all the "-" sides of the speakers together and connect them to ground, or do they need to remain electrically separate?
- Can I disable a speaker by running the "-" side through a 2N3904 transistor?
- How much power flows through the speaker wires? Is running them across a PCB trace going to create issues?
- How do I run a cable (Cat6?) to the front console without getting in the way of regular operation of the vehicle?
- Is a ~3M run of Cat6 going to run into RF/voltage drop problems when used for logic-level signaling?