Originally Posted by lanerd
So exactly what is the difference between an OBD connector and a J1939 Diagnostic Plug for ECM communications besides physical characteristics?
This is from a long time ago so if I am wrong sorry.
I believe diesels use a communication system similar (if not the same) as computer network. things like shifters, gas pedals communicate on the network and when you plug into a j1939 plug your device joins the network and can see the communication between ECM and device.
ODB2 is more like, the reader probes the ECM with predetermined codes like "Hey is there a O2 sensor 1" the ECM responds "Yes" reader then asks "What is the value of O2 Sensor 1" ECM responds "2.2"