How to Inspect an A/C Compressor

Before you pull the A/C compressor from a heavy-duty vehicle, take these 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, it’s probably due to 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’s 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?

Any resistance outside this range will prevent the clutch from engaging or will cause used circuits to open. If the answer is yes, continue to Step 4.

4. Is the compressor able to produce 350 psig or more?

If not, leave the compressor where it is. The system may have a low refrigerant charge because of a leak that needs repair, or a high-side blockage that limits refrigerant flow to the compressor.

At Red Dot, we see lots of compressors that are returned fully functional and therefore not warrantable. Get the diagnosis right. Perform a simple compressor function check before you pull the component from the vehicle.