Question regarding J1850 protocol

[PSWired]

I have a 2002 6.0L engine in my 1995 truck. One project that I'm investigating is building a module to sit on the class 2 serial bus that will display pertinent engine information on a small LCD or VFD mounted in my dash. Information such as change engine oil, reduced power mode, low oil, low coolant, the number of any DTCs present, and some engine data like cooling fan status, transmission temperature, etc. I am considering using the M68HC12 for this since I am already familiar with the HC11 and the HC12 can be had with an onboard J1850 transciever. I was looking through the protocol description and functionality for the GM implementation of the VPW communication protocol and was wondering about how this is implemented in GM's instrument clusters. Is the cluster continually requesting individual PIDs from the PCM, or is the PCM continually broadcasting the data the cluster needs without being requested?

Surely I will have plennnntttyyyy more questions if this project gets off the ground, I'm just trying to assess the viability of even starting this project...

[Blacky]

The PCM broadcasts some* information periodically, although I doubt it would be everything you want.

* i.e. Coolant temp, fuel gauge, gear lever position etc.

eg. You must explicitly request DTCs - they are not broadcast. The MIL in the cluster is illuminated directly from the PCM - not via a class-2 message. If the comms failed then there would be no way to illuminate the MIL :)

The broadcast messages are not well documented. Ross (aka GMPX) and I went through them a few years back and only managed to figure out about half of them.

The broadcast messages "sort of" follow the messages defined in SAE J2178-4 in the SAE HS-3000 1999 edition.

Regards
Paul

[PSWired]

Thanks as usual for the quick reply. I did some poking around over at the *gasp* HPtuners forum and there seems to be a lot of VPW protocol information for me to look at there.

Would you happen to know the broadcast procedure or message structure for the error conditions such as reduced engine power, low oil, change oil, etc? I would rather just listen for these events rather than continually poll the PCM for those conditions.

[PSWired]

Oh, and one other random question that's on my mind. Why are there two data links from the PCM to the BCM in the trucks? One seems to be a dedicated serial line from the PCM to the BCM for whatever purpose. Is this a redundancy feature in case some module hoses up the main data bus?

[Blacky]

Thanks as usual for the quick reply. I did some poking around over at the *gasp* HPtuners forum and there seems to be a lot of VPW protocol information for me to look at there.

Seen the bottom of this page? 8)
http://www.hptuners.com/vpw.htm

Paul

[joecar]

Seen the bottom of this page? 8)
http://www.hptuners.com/vpw.htm

Paul

He he he, Paul you wag, that's very cool.
:lol: :lol: :lol:
8) 8) 8)

[GMPX]

Oh, and one other random question that's on my mind. Why are there two data links from the PCM to the BCM in the trucks? One seems to be a dedicated serial line from the PCM to the BCM for whatever purpose. Is this a redundancy feature in case some module hoses up the main data bus?

They usually have a dedicated line from the PCM - BCM simply to isolate any other module on the bus from killing the 'important' modules from communicating, usually the BCM will isolate the secondary data line until it is finished communicating with the PCM.
Eg, lets say a door module was shorting the bus to GND then if there was no isolation you could not start the car, with the secondary data line isolated then the PCM and BCM can still do the VATS communications etc.

On the GTO the BCM isolates the Cluster, HVAC, Radio, Seats, ABS etc until the BCM and PIM have finished communicating, of course all this happens faster than you notice.

Cheers,
Ross