A three phase compressor will have three terminals. T1 – T2 – T3 Remove the wiring from the three terminals before testing. Set your multimeter to read ohms. Check the compressor for resistance to ground. With the wiring removed from the compressor terminals, test the resistance of each terminal to ground. Ensure that your lead is making a solid connection to ground without any interference from dirt, paint, rust or any other debris. If no grounds are present you should read infinite (OL) resistance. If grounds are present the compressor will need to be replaced. Check the compressor for open windings. With the wiring still removed from the compressor terminals, test the resistance between pairs of terminals (T1-T2, T1-T3, T2-T3. Ensure that your lead is making a solid connection to each terminal. If an infinite (OL) reading is obtained between any pair of terminals then there is an open winding in the compressor. Due to the presence of internal overloads, an open winding does not necessarily indicate a bad compressor. Internal overloads protect the compressor by opening the winding in the case of high amperage or overheating. If the compressor is hot to the touch you must allow it to cool down and before testing again. If the compressor is cool and continues to read infinite resistance then the compressor must be replaced. Check the compressor for shorted windings. With the wiring still removed from the compressor terminals, test the resistance between pairs of terminals (T1-T2, T1-T3, T2-T3). Ensure that your lead is making a solid connection to each terminal. The resistance between each of the three pairs of terminals should read the same. If there is a large variance between readings then the compressor most likely has an internal short. Reconnect the wiring to the compressor terminals. CAUTION – 3 phase scroll and screw compressors must be checked for proper rotation after wiring is completed.