What does 12 volt LED mean? LEDs operate at voltegas around 2.8 to 4 volts, not 12 volts. If it means the LEDs have a resistor built in, so they operate from 12 volts, then you should be fine, and the transformer should not shut down, as the current would be a low lower than the halogen lamps drew.So I suspect these are NOT 12 volt LEDs, and they need a series resistor which is missing, so the current is much higher than it should be. Probably means the LEDs are cooked already.Another possibility is that the 12 volts for the halogens is AC, not DC, and the LEDs require DC.So you need to look at the specs on the LEDs, and also check the 12 volt supply. I suspect it is AC. .