Good morning Max, you should have 120 VAC to inverter and variable DC voltage out to compressor. Voltage from inverter to compressor is generally not measurable as it varies greatly depending on the current status of the thermostats. It sounds as though the voltage TO the inverter is the culprit. Most often that voltage comes directly from the main HV control board, are you able to find and check that? Kelly
Also, all voltage checks need to be performed with all loads connected...
You should have 120 VAC to the inverter then the inverter changes it to DC voltage to compressor, so if you don' t have 120 VAC TO inverter, check the maion control. Kelly
Frigidaire tech says faulty compressor as well. 2-5 VDC at compressor should be running compessor, provided the condenser fan is running. However, he did say that if the compressor ohms out ok, it probably isn't bad. But given that you replaced the inverter, he speculates that it is. I am trying to come up with a manual for it...It is either a EI26/EW26 or a EI23/EW23. Do oyu know the cubic feet of this unit?