I had a call walk in wasn't cooling. when I arrived at site, found evap coil. frozen. Defrosted,checked T-stat in cooler box, that was set lower than 30*. Box temp was 48*,and I could not get it below 46* before pizzaria employee went into it. So I set low pressure controll to have unit shut off at 15 psig and turn on at 30 psig, on a r-12 system. I believe unit was never shutting off the way T-stat was set, so I set it like this. It has a defrost clock that was working. My boss said this wasn't a good practice, but didnt go int detail why. I'm really green, only been in field fo 3 1/2 weeks. I would like opinions of exp. techs on how I might have done things better.