When the compressor is in a refrigeration system and in operation, if the suction-side and high-side pressures are almost the same, the valves might be bad. An ammeter will tell you if the electric drive motor is doing the work for which it is rated. Without proper compression in the compressor, the amperage reading will be very low. The problem could be a piston not pumping. When a compressor is operating properly, it should draw an amperage close to its rated amount on the data plate. The suction line should be cool to the touch and sweating. This cool gas is needed to help cool the compressor windings. The small liquid line should be warm to the touch . . . not hot. If the liquid line is very hot when in the cooling mode, there is a problem with the unit. Remember, refrigeration equipment and air conditioning equipment have design temperatures and conditions. In different areas of the world there will be different designs in the equipment. In a low humidity area, the suction line might not sweat. If a unit is designed to attain 74 degrees F. conditioned space with an ambient of 95 degrees F., and it is being checked on a day when the ambient is 80 degrees F., the equipment will accomplish the 74 degree F. without any problem. Even if there was an inefficient compressor with a possible bad valve, it wouldn’t show that easily until the unit was being operated under its design conditions. Conditions such as the evaporator coil being clean or the condenser coil blocked with grass cuttings will affect the operation of the entire system. Another thing that I learned a long time ago, know what the engineer wanted the equipment to do. If you don’t know what it is supposed to do, how can you repair it? As a service technician, you will see many applications of the refrigeration theory, from food processing to industrial manufacturing. That is why it is important that you know what the unit is supposed to do before trying to make a repair.