[Huskymaniac]

Originally Posted by TomsSVX

Yes Key on is when the vehicle does an overall system readiness test. But system status is monitored throughout the ignition cycles and during "drive cycles" Drive cycles don't always have anything to do with key on but also relate to fuel used, heat cycles, hours running, and mileage. So it is an interactive process throughout the course of driving the vehicle.

In more modern vehicles you will find if an ecm is not programmed for the correct vin it will cause issue along the link. It is not something you can tamper with easily. Without all the manufacturers info about what is being broadcasted on the link, I find it "extremely unlikely" (not saying impossible in fear of a toddler's rage) that you will be able to reproduce any signal for the engine ECU to receive that will satisfy the communication.

Finding a good OBDII ECM writer who can take out the request from any TCM will likely be the best route

Tom

From what I can tell from the pinouts and wiring diagrams, there is only one additional diagnostic line between the ECU and TCU on the OBD2 cars. It goes to pin 80 on the ECU and is labled "AT diagnosis input signal". It could be poorly named but, if it isn't, it implies that this is a one-way signal from the TCU to the ECU.

Is it possible that the early OBD2 cars which still had SSMI didn't have the ECU-TCU diagnostic signals that were required of later OBD2 cars? In other words, was there a transition period where not all of the inter-computer networking signals were standardized or yet required to meet a new standard?