To troubleshoot an AC compressor, first check for indicators of an electrical problem. If the unit is completely powered off, check the wall socket to ensure there is a power supply.
Disconnect the power to the unit, and remove the terminal cover to check whether the wires and terminals are damaged. Check for continuity or resistance between the three terminals of the unit using a basic multimeter. If the resistance is high and the unit is hot, the internal overload is open. Wait for it to cool down and close. If the resistance is high and the unit is cool to the touch, the compressor motor is faulty and needs replacement.
In the case of excessive noise within the compressor during operation, check for loose pulleys, coolers and clamps, and tighten them as necessary. The noise may also come from a lack of enough oil in the crankcase or friction between the piston and the valve plate. Replenish the oil levels, and inspect for foreign matter on top of the piston. Replace the crankcase if it is defective.
Excessive belt wear indicates that the belt or pulley is out of alignment. Realign the pulley with the compressor flywheel, and adjust the tension on the belt. Once done, restart the unit to check if the problem is solved. If the problem persists, seek professional assistance.