Originally Posted by
blindsquirrel
The speed signal is generated by the sensor in the tailshaft housing, and sends the signal to the TCM. The TCM puts that speed data onto the high-speed GMLAN bus, where the ECM has access to it. The ECM then puts the speed data into a different format and sends it out over the low-speed class2 bus and also via C1 pin 21 for use by the modules that are not GMLAN-equipped, like the radio, ECAS, IPC, ABS, etc.
You are assuming a lot of things that will get you into trouble, diagnostics is best approached by assuming nothing. First thing I would do is make sure the aftermarket speedo isn't using an aftermarket add-on speed sensor on the driveshaft, like a lot of them use. If this were an unmodified 2005 Envoy brought into the dealership I do think it would probably be safe to assume that the ECM and TCM have working communication because of the way the GMLAN wiring is arranged, but in a swap vehicle with a modified wiring harness? No way, Jose.
The E40 and T42 use high-speed GMLAN (CANBUS) for all data to the diagnostic port; the low-speed class2 line is ONLY for the non-GMLAN modules in the original vehicle; which, being a swap with all those modules removed, you do not use and do not need, because there are no non-GMLAN modules present with only a E40 and a T42.