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How to Manually Engage an A/C Compressor

by Tom Price

If the air-conditioning compressor on a late model car does not engage, the system is probably low on refrigerant and the low-pressure switch prevents the compressor from turning on. This problem goes away by simply adding refrigerant. Opening a can of R134a into the system usually overrides the low-pressure switch and allows the air-conditioning compressor to engage normally. On older cars, the air-conditioning compressor has to be engaged manually to recharge the system. The procedure is simple and usually takes less than a minute.

Check the compressor oil level. Add more oil if necessary. Since different manufacturers have different procedures for checking compressor oil levels and filling compressors with oil, refer to the auto manufacturers shop manual for specific instructions.

Locate the single wire connector near the front of the compressor and unplug it.

Hook one end of the fused jumper wire to the compressor side of the wire connector.

Hook the other end of the jumper wire to the positive terminal of the battery. By supplying voltage to the compressor, the compressor will engage manually without even turning on the air-conditioning switch inside the car.

Tips

  • Manually engaging the compressor without sufficient lubricant can damage the compressor.
  • The compressor needs to be engaged for the system to recharge with refrigerant.
  • Route the jumper wire so that it does not interfere with any moving parts.

Items you will need

About the Author

Tom Price began writing in 1989 after earning his master's degree in English at the University of Notre Dame. He has served in positions ranging from features writer to the managing editor of the "Daily Herald." Before college, Price was a GM factory trained tech and owner/operator/truck-driving instructor. He is also a private pilot as well as a lifelong expert boater and woodworker.

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