Electric Air compressor

Question

I have a 4HP 12 gallon pneumatic air compressor that is not blowing out air. The compressor motor is still working. Cannot make out the model because air compressor is old and labels are either barely readable or faded out. It is an Air America Type model. Could you please advise to where I should start to find problem. Also, no familiar with connection compressor pump. Compressor was given to me in this condition. The pump has an auto and off selection with 4 terminals: end terminal connections are L1 and L2 and the middle terminals are T1 and T2.