Inspect the Wye-Delta assembly. Make sure that all connections are tight, and that there are no signs of arching or burn marks on the contactors, burnt marks on the jumper bus-bar or contacts.
With the machine OFF, measure the resistance across the coil between connectors A1 and A2. The resistance should read 50-100 Ohms. If the reading is open, then the coil is defective.
Power-up the machine and make sure that only the Wye contactor is energized when no spindle speed is commanded. If both contactors are closed at the same time, one of the contactors may be stuck (welded shut). The vector drive sees this condition as a short.