Connecting to 2002 Duramax ECU without the BCU?

I'm trying to connect to a 2002 duramax ECU with my mpvi pro. I ASSUMED that GM connected the OBD port straight to the ecu and there are two pins on the ECU labeled "class 2 serial data" so I figured one of those two had to be it... Wrong. HPT connects to it and acts like it loads parameters but everything is gibberish.
Upon further investigation I see that the wiring diagram for the truck shows the obd port connecting directly to the BCM through a different class 2 channel.

Am I screwed and am going to need to buy a BCM and install it as well?

edit-so I tried to read the ECU and it figured out what truck it is from, but failed the checksum.
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I am not understanding what your talking about. From what I read your saying the truck doesn't have a bcm? Does the truck run now? Normally when you get a vin like that the ecm is junk.

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Originally Posted by Road

I am not understanding what your talking about. From what I read your saying the truck doesn't have a bcm? Does the truck run now? Normally when you get a vin like that the ecm is junk.

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Sorry, I see now that it's confusing. I was avoiding a long story.

Think of it as a engine on a dyno. No truck involved, just the ECU, FICM, and a web of wires.

I'm actually trying to get the FICM to work in a stand alone application to fire a cummins 6.7 isb using a neweagle ecu and I haven't had much luck so I hooked up a dmax ECU and harness to the FICM with the hopes I could read OBD codes from it to try and figure out why it wasn't working. I logged the CAN data between the FICM and ECU but without definitions I don't know what it means.

My guess is your ecm is at fault. Either the checksum is disabled due to some kind of custom operating system or its corrupt. There is a few companies offering to repair them. You could try flashing it with GM TIS or license another file and write entire with hpt.

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Originally Posted by Road

My guess is your ecm is at fault. Either the checksum is disabled due to some kind of custom operating system or its corrupt. There is a few companies offering to repair them. You could try flashing it with GM TIS or license another file and write entire with hpt.

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Thank you for the advice.

So in theory a healthy ecu should have no problem communicating with the mpvi directly?

That's kind of what I am starting to think as well. I looked around further on alldata and see that the class 2 network that is used to communicate through the obd port pin is in a "star" configuration which I assume to mean that everything on the network is connected to each other at a single wire and each device also has it's own pull-up resistor to 7 volts. Each device then goes low to transmit. If that's true then I shouldn't be having any troubles unless there needs to be a little stronger pull up to 7 volts since I only have one device on the network. Also, if that's true I don't know why the ECU has two class 2 pins that both appear to have the same data coming out of them according to my scope. Alldata says one goes to the Allison TCM. BTW the signal looks clean on the scope, edges aren't super square but I think it's okay.

All these parts came from a running but rusted out truck I was told by the junkyard I bought them from.

Sounds like your mpvi is connected to it but it is not in a format that it can use for hpt to map out the address's. Just like you would see if you was reading a ecm tuned with EFI Live with switchable tuning (custom os).

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Originally Posted by Road

Sounds like your mpvi is connected to it but it is not in a format that it can use for hpt to map out the address's. Just like you would see if you was reading a ecm tuned with EFI Live with switchable tuning (custom os).

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Interesting. I feel like the chances of the truck it came from being tuned would be pretty slim but there's always a chance. Would a car part store scanner still work if it had a locked EFI live tune?

The HPT scanner doesn't work properly. Pulls up a goofy VIN and acts like it's connected but I don't get any PID values or DTC's. If it was CAN instead of class 2 I could manually decode the messages with labview but I'd never even heard of class 2 till starting this project and I don't know any way to read it. I can definitely see messages flying around on the scope when I click connect on the HPT scanner.

edit- so I hooked up a cheapo obd scanner to it and it reads some PID's. -40 coolant temp, -40 IAT, etc. I shorted the coolant temp wire to ground and it changed from -40 to +149C on the screen. Still no DTC's though which is impossible given the amount of sensors missing. Is there a canned tuner out there that would be disabling all DTC's?

I guess this is pointing toward my next step being to download a tune from the repository and trying to flash it? Is this going to take 2 or 4 credits?
What happens if I try to flash a cal I have unlocked for a 2001 gas truck? Brick?

Yes you will have to license the file you flash to it. I don't think any can tuner will be deleting any codes. Don't flash nothing but a dmax tune to it. Duramax are slow to set codes sometimes. With the vin being curupt I think it is already bricked.

Well that was a waste of two credits. The VCM Editor was able to successfully flash entire to the ECM, however the VIN is still nonsense and it's behaving the same way. Also, when I tried to read the calibration I had just flashed, I got the same errors as the same point in time as before.

Any other thoughts? I feel like someone else out there has to have tried to swap an LB7 into something without the TCM, GPCM, and BCM?
Thanks for the help!

I have a bench set up, you can send me the ecm if you like. I can try to set the vin and send it back to you. Those ECM's are well known to corrupt them self's, especially if it has been flashed by TIS. Send me a PM if you like.