Hi Lar3ry,
Yes, I found a schematic in the electronics case at the back of the sprayer, and the connections to the pot are all labelled. Relevant photos here...
I'll try and run through the tests you prescribe. There is a microcontroller on the PCB. Could it be feasible that the pot is signalling the chip. Would a digital pot do the same thing?
Bit renascent to pull the pcb out for fear of breaking the sprayer in the middle of the season.
The sprayer is a John Deere 732, but I think the majority of the electronics is based on a TeeJet 844.