My Cat 420E IT A/C has never worked properly since new. it only has 650 hours on it now , but has never kept the operator cool enough in the summer. it seems to not put out cold enough air. New filters, clean machine, ck pressures by dealer and myself. completely shut off the hot water flow, nothing seems to help. It works ok when it is cloudy and below 70 degrees outside, but the sun and higher temps just makes it impossible to sit in the cab. and been like this since new. What do i need to do ???