alllllll about the gmt-900 audio systems.....

I know im a dork for thinking this stuff is so cool, but I coudlnt help myself....I kept checking ACDelco's online Tech data base every week from January on, waiting for tech docs to be published about the GMT-900 audio/electrical systems. FINALLY they posted their basic description, and I just kinda weaved in my previous understanding of how the GM entertainment systems worked and came up with my own "educated interpritation" of how everything in the -900's works. Its pretty cool.... id consider it "evolutionary" of the 2003-2006 data bus/electrical systems/entertainment systems. Again, big thanks to THE main guy, Sperry, for getting me so interested in this stuff in the first place.

bunch of people on one of my other forums, the Diesel Place, were asking me what the deal was with the GMT-900s so I did a lot of reading thru tech docs and wrote up this big long description/explanation...cause as I said before, im a dork and I love reading/talking about this stuff. Here is what I wrote there and thought maybe some of you guys here would be curious too.....

Here are a couple things I wrote up on anothjer forum, I just copied and pasted them here:

The Big Whole crazy deal with the GMT-900's underneath the skin:

theres a couple different systems. UQ3/UQ5, which is base model stereo, powered by an internal amp in the head unit, just like before in the 2000-2006 GMT-800's, so thats completely unchanged, however as like everything else in the 2007+ GMT-900's, everything communicates on the low speed GMLAN data bus.

No more Class II 

Then theres uplevel systems which are reminiscent and based on the -800 systems, but still different in a couple ways. Then theres the new blow your face off 5.1 surround sound system in the 2007 Escalade. The Denali gets a system thats sort of "inbetween" the Escalade and Tahoe Bose system. Its not 5.1, but its better thant he Tahoe system.

As for the Denali and Tahoe 2007 Bose systems:

Speakers are the same, tweeters are the same, Amp is in the same location (center console) as is the sub. Basically identical speaker setup and config as the -800 Bose systmes. The Door speakers are in the same locations. Front a-pillar tweeters are in the same location. Rear d-pillar twiddlers are in the same location. And they are wired as before too. In parallel with the rear door speakers in the Tahoe/Burban/non-lux systems and wired to separate "midrange" channels on the amplifeir in the LUXURY systems. IM still trying to figure out whether the amps on the Tahoe/non lux vehicles communicate on the data bus. It seems there are two different non-lux Bose systems now, just to amke things more complicated!!! UQA/UQ7 and just UQ7 I think. I think one might be for DVD/RSE equipped vehicles and one is for non-RSE vehicles?? One communicates on the GMLAN bus and the other only has an "amplifier control wire". Im thinking its a simple gain control from the head unit? So it operates on the same therory as GMT-800 lux amps...fixed output from the head unit and volume is varied in the amp...only its not done thru data bus (like on Lux amps), its done witha simple gain control wire. The old 1995-2002 radios used a system like this I think... I know they had an "amp gain control wire". Whether the audio left the head unit at a fixed level on those old 1995-2002 radios, I dont know, I dont have any personal experience witht hem... But basically there are two non-lux systems now. One with a data bus amp and one with a non-data bus amp. Both amps are the same audio quality, just that one is controlled by GMLAN.

2007 tahoe UQA/UQ7: data bus amp, operates the same way that the 2003-2006 Lux amp does, but is not as high audio quality and does not have an internal DSP/spacious/driver/etc settings that the 04-06 Luxa mps do.

2007 tahoe UQ7: non data bus amp, uses a "amp control wire" to vary volume level, as the head unit sends audio to this amp at a fixed level. No sound processing circuits, so basically a 2003-2006 non-Lux amp, except instead of varying volume in the head unit, it varies volume in the amp via the control wire.

2007 Denali: same speakers as 2007 Tahoe, data bus amp, operates same way as 2003-2006 LUX amps do, has internal DSP, so basically just an updated/revised 2003-2006 Lux amp that went to GMLAN language school. However, it does have an additional center speaker (in the middle of the dash) and the internal DSP supports Bose "centerpoint" surround encoding, basically its Bose's version of Dolby ProLogic II. It takes stereo 2 channel audio from the head unit and uses an algorithm in its DSP to make "simulated" 5.1 sound. The 2007 Denali does NOT have the capability to send surround audio or 5.1 audio from the head unit directly to the amp as discrete channels. The only 5.1 you'll get in a 2007 Denali is what can be generated in the amp. I generally dont like these algorithms...they make it sound wierd...so you can select just normal 2 channel stereo on the Denali's and it will sound pretty much like a 2003-2006 Denali/Escalade Lux system sounds.

Now for the big bad daddy of them all, the Escalade 5.1 surround sound amp.
center speaker, bigger sub, much more powerful amp, Dolby Digital and DTS decoding, fully databus controlled. Head unit has discrete 5.1 audio signals coming from it, so if you play a DVD that has Dolby D or DTS surroudn sound, you'll hear it just like in the movie theater, with the proper discrete sounds coming from each speaker, and all dialouge coming from the center speaker. Im 99% sure the AC3/DTS decoding is all done in the head unit and sent as discrete channels to the amp...as far as I know the amp doesnt actually have an internal decoder chip other than the standard DSP chip that does the Bose Centerpoint and other Bose stuff. I cant wait to hear it. Im looking over pinouts for the Z75 (Bose 5.1 Dolby D/DTS amp) amplifier and it says soemthing about a rear subwoofer....so I think the Bose 5.1 systems have a monstor sub in the rear as opposed to the cetner console sub on all the other Bose systems. The center speaker is located smack in the middle of the dash, firing up at the windshield, just as in the Denali.......

Basically thats it for the audio systems....ill post more on the rest of the electrical system/data bus stuff in a minute.

Alright everyone....heres "part 2" of my big long write up thing. I just love this stuff so much that I could have written 3 pages more and still not been done............but everyone reading would have been asleep by then?

Sperry correct me if I made any big goofs...

As far as swapping 2007 GMT-900 components into the 2003-2006 GMT-800's? Wont work directly...different databus. Class II (SAE J1850) is a single wire serial databus set up in a star configuration, bus floats at 5 volts, and I think the average data rate is 10.4kbps or something liek that. The bus floats at 5 volts, and then drops to zero volts when it wants to send data. It pulses between 0 and 5 volts, 0 volts representing a "0" in computer code, and 5 votls representing a "1" in compter code. GMT-900 trucks/SUV's are switching over to a 'variation' of CAN (SAE J1939) called GMLAN (gm local area network). Now of course because its GM and htey need to make things complicated, it is not directly compatible with standard/universal J1939 (which was used on the 2001-2005 duramax's). "Standard J1939" as used on the 01-05 Dmax's was nice because it had sufficent data rate for real timne transfering of info between the ECM/TCM/FICM/GPCM (eng control module, trans control mod, fuel inj control mod, glow plug control mod), 250kbps, and it is used everywhere. Everything from Semi's to industrial machinery uses J1939...its nice having the same technology used everywhere. GM changed to the new smarty-pants Bosch A35 32bit ECM for the 2006 Dmax's and for whatever reason abandonded CAN/J1939 for their own revision/variation of CAN (controller area network) called GMLAN. I dont know why....there was nothing wrong with CAN. I read that GMLAN has something like the same transparency carrier and therefore is compatible with CAN on a protocol-level, but is not directly compatible....I dunno, Im not an electrical engineer so I dont know the specifics.? So for the 2006 Duramax's, and all of the coming GMT-900 powertrains, they all use HS GMLAN (High Speed GMLAN, 500kbps) which works like an ethernet network of sorts....it allows full duplex communication over two wires, a high and low wire. Theyre terminated with a 120ohm resistor to act as a bridge between the high and low wires....I dont know why or the reason, I think I remember reading that tho. It can send/receive simualtaniously because while one wire is being pulled high, the other is pulled low, allowing lots of fast transmissions. Also, its similar to standard CAN in that messages ont eh bus have identifiers...like messages are sent to every module, and the module looks at the identifier (basically it receives a "letter" in the "mail" and looks at the "recipients address" to see if the message is for it or someone else) to determine whether the message is for it or not. This way, widespread messages can be sent out to everything at once, rather than Class II which had to transmit messages individually. The messages can be more complex too, and because they are sent to everything, messages are transmitted much more efficently....on Class II if a message was sent and the databus was congested, the message got sent back to the transmitting module, and the module would have to wait until the bus was clear, then resend the message. You'll notice this if you installed a lux amp in your 2003+ Avy, and simualtaniously put the windows up, turn up the volume via steering wheel cotnrols, and shut the truck off. I bet the volume wont actually increase until like a second or so later, when everything? is finished being powered down by you turning the key off. My truck does this....I hit the volume control and turn the key off, volume doesnt change, then like a second afgter the engine is off, it all of a sudden changes the volume.....


As you remember in 2003 when the GM trucks/SUV's got the huge electrical system overhaul, every single body control module was redesigned and talks on the common Class II data bus. This was a good move in 2003, because the trucks/SUV's were getting more complicated and it really makes thigns easy when everything is in communication with eachother. Everything is interfaced/integrated and therefore works more smoothly, wiring is cut down hugely, things are more reliable, and best of all, every module has bi-directional control and full data read out when you plug in a Tech 2 (the dealer handheld diagnostics computer/gameboy looking thing). For example say your horn doesnt work...in the "olden" days you'd crawl under the dash witha test light, multimeter, and spend 300 hours tracking down a loose wire or broken switch somewhere. With the 2003+ trucks, you plug in the Tech 2, and in 3 seconds it can tell you the status of the horn switch, relay, triggers, wiring, and even run a diagnostics test on the horn circuit. If the rear window doesnt work, you can press some buttons on the Tech 2 and take over control of the window motor directly, all from the Tech 2. Every single electric circuit has its own diagnostic trouble code (DTC)...even the smallest little things like if one of the driver side power seat control buttons has a short in it, there is a specific DTC just for that switch (Bxxxx) so troubleshooting time is reduced to nthing. I love it..... things like the Bose Lux amplifier are all on the data bus. You can press a couple buttons on the Tech 2 and pull up the full real time data display of the amp, it shows realtime volume settings, chime volume, bass, trebel, status, speaker diagnostic checks...everything! And theres about 20 other modules on the data bus as well...memory seat module, airbag module, onstar box, radio, climate control, window/door control module, instrument cluster etc...

In the 1999/2000-2002 GMT-800 trucks/SUV's there were only like 3 modules in the body that used Class II, then 2 other modules used the clunky UART (universal asynchronus resource transfer....I THINK thats what it stands for, im not 100% sure, but for some reason that acronym pops up in my head, could be wrong) data bus, and the radio and CD players used the 600-year-old E&C (entertainment&comfort) data bus. Basically it was a hodgepodge mess and the increasing number of electrical goodies was increasing, so Class II was widespread implimented in 2003.


The GMT-900/2007+ bodies are dumping Class II? and its getting replaced by LS GMLAN (low speed GMLAN). This was understandable, because Class II was really kinda getting long in the tooth...the 2003-2006 vehicles were really stretching the limits of the data bus's bandwidth, and the 2007 vehicles have even more electrical goodies and computers that need to be integrted. In comes Low speed GMLAN... it directly takes the place of Class II in controlling all of the modules in the body. (radio/windows/instrument cluster etc) It is a form of SW CAN (single wire CAN), so basically its wired same as Class II, sends electrical pulses over a single wire to each module, bus floats at 7 volts (instead of 5 like class 2) and the modules interperate these voltage pulses as 1's or 0s to make up the data signal. LS GMLAN is speedier than Class 2, with an average data rate of 33.3kbps. This is good because its fast enough to support a couple real time things like vehicle speed sensor data. Class 2 was too slow for real time VSS transfer, so the speedomter had to ahve a dedicated wire going to it directly from the speed sensors/ECM. LS GMLAN is fast enough to just do all that over data, further cutting down on wiring and making things nicer and more integrated. but otherwise in function and operation it does the same thing as Class 2, just faster.

High speed and Low speed GMLAN are not directly compatible (but then again J1939/CAN and Class II werent compatible either) tho. The BCM (body control module) has to serve as a gateway "translator" for HS and LS GMLAN devices, so for like the ECM (hi speed device) to talk to the instrument cluster and tell it how fast the truck is moving (low speed), the BCM will have to do translating between the two.

I havent personalyl had a chance to crawl all over and take apart any GMT-900 vehicles yet (im trying to convince my dad to trade in his 03 Tahoe for an 07 just for that reason? ), but once I do ill learn and figure out a lot more about how they work and stuff.....once I dig around in them and get to play with them, I might look into building some sort of "standalone" data bus/controller for fun that would allow GMT-900 GMLAN-speaking electronics/goodies to work in 2003-2006 GMT-800 trucks and SUV's. On the same note....Im already testing out a similar standalone controller setup for the 1999/2000-2002 trucks/SUV's that will allow y'all with 1999-2002 trucks to run the 2003+ instrument clusters, driver info centers, navigation radios, Bose amps etc.? It works on the bench, but I have to find a friend close by with a 1999-2002 truck for me to test it in.....................

check back later for more boring pages of info that only I find interesting ?

-Ben