Potentiometer Puzzle - TIG Pedal
Following your post of the schematic: Ah HAH. I think you may be getting thrown off by measuring the resistance to ground when two pots, and connections to Ground and +5V are involved (even though you no doubt had power off). Instead, with the pedal disconnected from the welder, attach a power supply (say +5V, or a 9V battery) across the power pins, and then measure the voltage coming out of the "variable signal". 

I expect that with the rear pot adjusted to max, the pedal pot should adjust smoothly from zero volts to max.

By the way, is red really ground, with black +5V? That seems a little suspect.