1992 GMC Van NO UPSHIFT FROM FIRST GEAR

Shifting is electronic, no TV cable or vacuum modulator. I was told by the dealer that there might be a PCM/TCM located on or around the passenger side kick panel and that Chevy/GM has had problems with them but I couldn't find it. Need further diagnostic procedures if you have any or any other suggestions before I condemn the trans, thanks.

This has been the transmission from hell; I followed everything you gave me and finally condemned the tranny. The customer (fleet) pulled me a shifting tranny out of one of their junk vans that had recently had the engine blown. This tranny came out of a late model 1992 GMC Vandura with a different pin/11-wire configuration than the van I've put it in, an early model (12/91) with a 12-wire configuration. The customer sent me the male plug that goes in the tranny to adapt to the existing wiring harness along with a wiring schematic (attached). I installed the tranny & plug, road tested it and got it to shift into high gear; even got it on interstate and initially into overdrive but after I turned around to head back the speedometer started wandering between 45-55 mph and then all of a sudden will drop down below 35 mph and the tranny shifts into a lower gear. It then becomes erratic as the speedometer wanders all over the dial and the tranny tries to shift accordingly. I swapped the rear output speed sensor with the input speed sensor to see if the problem would follow the swap and it made no difference at all. I stripped the wiring harness attached to the tranny and found the two wrapped/twisted wire leads going into the input speed sensor to have the outer insulation stripped and touching so I replaced the plug with a like used plug an wiring I had from the wiring harness the customer sent me but this made no difference. I searched and ensured this vehicle doesn't have a PCM (also confirmed by the dealer with the VIN #) then pulled the ECM out and ohmed out the wiring between the tranny plug and the plugs going into the computer. There are two plugs going into the ECM; one 16 pin & the other 12 pin. According to the schematic the customer supplied me, I get continuity from pin E on the tranny plug at three different locations on the 16-pin ECM plug; I also get continuity from pin L on the tranny plug at five different locations, three on the 16-pin ECM plug and two on the 12-pin ECM plug. I'm not sure now that the diagram the customer sent me is accurate; the wiring color doesn't seem to be matching up like I thought it did and I'm at a stopping point. Need any and all assistance you can give me and also need an accurate schematic.

Can anyone supply me with the wiring schematic(s) that show the wiring from the transmission harness plug to the ECM? This truck does not have a PCM; confirmed that at the dealer with the VIN #.

Thanks for the reply & info; the email is correct and I'll be waiting for them.