Hi all. I'm doing a bit of hacking with my car's audio system, and I
have a few questions.
The short version is: I'm looking for all information related to
GM's/Delco's use of the Class 2 data bus to control CD Changer-Head
The long version goes something like this...
I drive a 2004 GMC Canyon, and I'm interested in doing some fun things
with the data that gets passed around the communications bus. My first
goal is to hook a small computer to the CD Changer interface of the
radio. With this, I'd like to be able to play MP3s in my car, while
displaying ID3 tag info on the head unit, and controlling the playlist
with the head unit's buttons.
Now, before too many people get started with alternate solutions, let
me say that the purpose of this project is not necessarily just to be
able to play MP3s in my car, but rather to have fun hacking the serial
bus and doing an interesting project. So, I'm not into iPods or
pre-boxed auxiliary adapters.
Here's the data I have so far, in hopes to lure others who are
interested in similar projects to colaborate with me:
To start with, the head unit in my car is a Delphi Delco Electronics,
part no. 15104155. Just Googling for that part number brings up the
which is a supposedly backward compatible head unit (according to the
spec sheet they publish on that site).
I contacted the dealer at the start of my project, and was able to get
the pinouts for both the main head unit-car connect, and the head
unit-cd changer connection. If people want those, I'd be happy to post
them here - the dealer didn't say anything about them being copyright
restricted or anything. The important parts are that it is a 12-pin
connector, and the only pin in use that is not labeled as an audio
signal is labeled as "Class 2 Serial Data".
So, I searched around the 'net for a while and finally figured out that
"Class 2" is GM's implementation of SAE's J1850, in VPW mode, with part
of SAE's J2178. I have obtained copies of each of these specs, but
they are proprietary, and not given away for free, so I'm afraid I
won't be able to post them here. In any case, they don't list anything
about audio device controls, other than the suggested range of
addresses for such devices.
Once I knew which kind of bus they used, I bought myself a VIA LDV100P1
from B&B Electronics. This little module can convert data from
J1850-VPW to RS-232 and back. So, I've written a small terminal
program, and I've been snooping my car's bus for interesting data.
What I was hoping to find was that the CD player, even though built
into the head unit, was passing information back and forth to the head
unit's display over the Class 2 bus. No such luck, unfortunately. In
fact, the only data I have seen sent to the audio range of addresses is
targetted at the Audible Warnings devices.
My next thoughts center around buying a compatible CD changer to
install, and then snooping the communication on the bus (as data would
absolutely have to be there for an external module). The only problem
with this strategy is that I have been unable to find a compatible CD
changer. The closest I've come is USASpec's SCD-G7i, but they say that
there is a software incompatibility with the 2004 Canyon. I find this
a bit suspect, as the G7i is supposed to work with the 2005 Canyon, as
well as the '04 and '05 Colorado, and the head unit I talked about
Googling for above says it's compatible with all 4 models. But, it's
hard to say that the company that developed the instrument is wrong
about it's capabilities, you know? ;) Also, I think I remember
reading somewhere that the 2005 Canyon actually uses a Bose system, not
As any of you have tried will know, my phone calls and emails to places
that should know this information have turned up absolutely nothing.
No one at GM seems to have access to the data I want. Delphi has given
me two replies: 1. GM should have that data, and 2. their engineers
don't have enough time to reply to my request. Helm, Inc., GM's
official literature distributor, claims to have nothing that details CD
changer communication. The Chilton's manuals don't even touch audio
equipment. My last resort was contacting PIE and PAC to find out how
they got the documentation they needed to create their auxiliary
adapter modules. They both replied that they had to reverse engineer
the communication themselves, and could not release the data that they
So, that's most of what I know for now. Any pointers for other people
to contact about this issue, or other websites to read would be great.
And, if I've left out important details, please ask - my data is a bit
spread out and sometimes hard to compile, but it's probably here.
(P.S. My appologies for the repost to those who are also on
rec.audio.cars. I had this over there for a few days, but didn't get a
response, so I thought I would probe interest in this group.)