[Huskymaniac]

Originally Posted by oab_au

If you look at what would happen. The ECU has to interrogate all the cars units, by request, they all can't just send their reports together. It has to be a bi-directional line.
The ECU calls all units in turn, and they reply.

Harvey.

I don't know why but I will respond to this. The line in question is only running between the TCU and ECU. It does not connect to the ABS, Cruise or climate control computers. Therefore, it does not need to be bi-directional. The SSM lines are the ones that network these OBD2 computers together and the SSM link is indeed bi-directional. Also, if the ECU only has one thing to say to the TCU, which is "what is your status?", and it wanted to know this status on a very regular basis, why would it need to say anything? All of the critical parameters for operation are sent on dedicated lines (MAF, TPS, RPM, the ALL IMPORTANT TORQUE CUT and others). The only thing missing from OBD1 TCUs is their ability to send error codes back to the ECU. So why not think it is possible that the TCU is only sending error codes over this line?

I think it would be helpful to present opinions and assumptions as such and facts as such so as not to confuse people and cloud the subject. Too many assumptions are framed as known facts.