The best answer I can give is look at the continental imports link that I included in the posting. The author (Steve B.) gives some enlightening info there. From what I gather from it the amp accounts for the powerplant operating status (real time) and compensates (refines) the vacuum output signal to the modulator to provide optimal transmission shift points. From what I saw yesterday the amp seems to be functioning properly and I need to move on to pressure adjustments at the modulator valve. If you connect the vac gauge and do the check as mentioned you can see the interrelation of the vac and pressures. Just reading the info and not doing the check was not enough for me to get it

I have never dealt with these diesel machines before but they are peculiar machines ('85 300CDT).