Shorting leads on centre tapped transformer
Shorting the centre tap to either end of the winding is effectively adding shorted turns to the transformer. 

You should connect to 1 &amp; 2 or 2 &amp; 3 and leave the 3 or 1 pin respectively OPEN. 

If this was a power transformer connected to the mains, the results of your experiment might be a lot more dramatic.