Just a stab in the dark.
Is the current set by a potentiometer?
If one end has gone open it could be pulled high or low causing the
welder to think you want full current.
If you're confident of not electrocuting yourself, try to measure the
voltage on the wiper of the potentiometer as you wind it up and down and
see if it sweeps up and down.
I'm not familiar with high end welders and can't believe how much
electronics there are inside that one!