Which is the ground wire and which carries 120V on an old 240V welder that needs a new plug at the end of the power cord if the wire colors under the outer insulation are white green and black?
If you only have 3 wires you don't have the fourth wire for
120V. Since this is a 240V only device it doesn't need it. Black
and white are your hots and green is ground.
As always, if you are in doubt about what to do, the best advice
anyone should give you is to call a licensed electrician to advise
what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact,
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
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SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.