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Okay, completely different powertrain from what we usu. discuss in this section. Maybe a mod will move this thread to the IDI section. The PSDs have an integrated PCM which controls both the engine and transmission. Your truck has NO electronic engine control (major reason people buy/keep IDI trucks...) It has a standalone trans control module, TCM, also known as TECA, with a different set of codes. I ***THINK*** the trans codes on an IDI can be obtained via a blink code (jumpering a connector and counting blinks of the OD-OFF light, or some such) But again, that's outside the territory of this section of the forum. The fellas in the IDI section (some of the same fellas...) will know more.
Still worth disconnecting the VSS and/or PSOM just for proof of concept.
Oh, that reminds me -- is your OD-OFF light blinking?