Hacking GM’s CD Changer

Hey all. I'm working on a project now, trying to reverse-engineer the protocol that GM/Delphi uses to communicate between their head units and external CD changers. Specifically, those that use the Class 2 bus to do this. Anyone know of anyone else working on this problem? I'd hate to waste my time repeating work that someone else has already done. Thanks.

what exactly are you trying to accomplish. There are several companies that offer interfaces for those vehicles that utilize the Class 2 digital language. Jazzy

My question would be what exactly are you trying to do?Certified Security Specialist

Always check info with a digital multimeter.

I promise to be good.

Tell Darwin I sent you.

I've been sick lately, sorry I won't be on much.

I'm sure you're going to recommend that AAI-GM12 or the GM12-AUX, but they don't do what I want. Why do you care what I want to do with it?

crazywingman wrote: I'm sure you're going to recommend that AAI-GM12 or the GM12-AUX, but they don't do what I want. Why do you care what I want to do with it?

Because it matters in terms of any advise we might give you as to an adapter.  Are you trying to control a factory changer with an afteremarket deck?  Are you trying to add a changer to an OEM head?  Or are you simply playing around?  If you are merely trying to crack GM's DBII protocol, this is not the forum for that.  Do a Google search I'm sure there is information out there.  Or do the right thing and pay GM for it.

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Some people actually answer the questions that others ask, rather than pussy-footing around, assuming the other guy doesn't know what he's talking about. Seems like I'm probably not going to find that here. Nevertheless, I'll try to go along here anyway... I am attempting to crack this protocol so I can control hardware of my own design. Perhaps you should have assumed that I would try a Google search. Trust me, the information is very much NOT out there. I have been in contact with several people at GM. They do not offer the information I want. No one is able to tell me who to ask to buy the info. I've tried Helm, GM's official information distributor. I've tried the dealer, as per GM Support's request. I've tried Delphi, who either reply that only the dealer has this info, or that their techs are too busy to answer requests for technical data. I even tried talking to PAC and PIE to find out how they made the AAI-GM12 and GM12-AUX. You know what they told me? They reverse-engineered the protocol themselves because GM does not sell the spec. And, of course, they were unwilling to send me what they had. I already have copies of SAE J1850 and J2178, which define the electrical and logical protocol upon which GM has based their "Class 2". Unfortunately, no entertainment or radio messages are defined in these documents. I've now bought a J1850VPW-to-RS232 adapter and am snooping the bus. With any luck, I'll be able to learn something. So, I decided I'd post on a few BBS's that seemed full of people who might be thinking about doing similar projects. If this is the wrong forum, please tell me which forum would be the correct one. And next time, don't assume that the person you're talking to hasn't already done background research. It's really annoying to have to wade through the same information I could get out of Google.