A resistance value of 0.7 Ohms, in itself, is OK. It is quite a low value, and would correspond to a high power setting out of the 6 available, maybe even the highest ( but this is not what you are getting, the welder does not melt even thin steel ).
However even though a winding resistance is 0.7, they should not all be like that! It does not happen very often, but the primary could be burnt, but I guess in that case I would expect Zero or open-circuit. Given the history - switch was missing - there are many possible explanations: - incorrect wiring on new switch, incompatible new switch, old switch caused transformer failure.
I would start by labelling all the wires with very clear and secure labels. Then make notes and photos of where those wires connect at the switch. Then disconnect all of the transformer primary wires, and measure again.
If there are 6 power steps, there should be a total of 7 wires. One will be the "start", probably emerging from the flange of the wire bobbin very close to the iron core of the transformer. There will then a large winding - a lot of wire, and then 6 tappings all emerging quite close together. The "long" winding may be 0.7 Ohms, and then five sections of winding, each perhaps 0.2 Ohms, for a total end-to-end resistance of 1.7 Ohms.
On a multi-tap transformer the highest primary resistance corresponds to the largest number of turns of wire, and the lowest voltage output from the secondary. So the tapping with maximum resistance connects to the lowest power switch position - i.e. number 1 of the six positions.
However even though a winding resistance is 0.7, they should not all be like that! It does not happen very often, but the primary could be burnt, but I guess in that case I would expect Zero or open-circuit. Given the history - switch was missing - there are many possible explanations: - incorrect wiring on new switch, incompatible new switch, old switch caused transformer failure.
I would start by labelling all the wires with very clear and secure labels. Then make notes and photos of where those wires connect at the switch. Then disconnect all of the transformer primary wires, and measure again.
If there are 6 power steps, there should be a total of 7 wires. One will be the "start", probably emerging from the flange of the wire bobbin very close to the iron core of the transformer. There will then a large winding - a lot of wire, and then 6 tappings all emerging quite close together. The "long" winding may be 0.7 Ohms, and then five sections of winding, each perhaps 0.2 Ohms, for a total end-to-end resistance of 1.7 Ohms.
On a multi-tap transformer the highest primary resistance corresponds to the largest number of turns of wire, and the lowest voltage output from the secondary. So the tapping with maximum resistance connects to the lowest power switch position - i.e. number 1 of the six positions.