Wondering why your system has a low refrigerant charge? It could be because of the leak that needs repair, or a high-side blockage that limits refrigerant flow to the compressor. Diagnose those problems first, and if not resolved, then perform a simple compressor function check.
Before you pull the compressor from a vehicle, take these four steps to confirm that it really needs replacing:
1) Is the compressor rotation smooth?
With the vehicle off, turn the compressor shaft with a 14-mm socket. If you feel grinding or hanging as you rotate the shaft, there may be broken components within the compressor. If the rotation is smooth, move on to Step 2.
2) Is the coil getting more than 11.5 volts?
Take a reading with the engine running and the clutch engaged. If there is insufficient voltage, get to work on that. Otherwise, move on to Step 3.
3) Is the coil resistance between 2.8 and 4.4 ohms for a 12 vdc system and 11.2 to 17.6 ohms for a 24 vdc system?
Any resistance outside these ranges will prevent the clutch from engaging or will cause used circuits to open. If the resistance measures correctly, continue to Step 4.
4) Is the compressor able to produce 350 psig or more?
The compressor should be able to build over 350 psig on the high side with the condenser airflow restricted, and also able to pull the suction side down to 5 psig when the TXV is closed. If needed, the technician can close the TSXV by chilling the charge head using a can of dust-off held upside-down. This will temporarily freeze the TXV charge head and cause it to close. If the compressor is operating correctly, the suction pressure will drop below 5 psig.
