What Causes a Car AC Compressor to Lock Up?

by Chris Weis

    Air conditioning can help defog interior glass year round.

    new car on white snow image by Ivan Arhipov from Fotolia.com

    Air-conditioner compressors work a lot like car engines. Pistons move in cylinders, or rotors spin in housings, to compress and expel gases. Any of these moving parts can fail and cause the compressor to stop turning. The drive belt might squeal loudly as it is dragged across the halted compressor pulley. The alarming ruckus continues until the engine is stopped or the compressor clutch is disengaged. This sudden may be due to several causes, some of which are preventable.

    A Dry Rub

    It is common knowledge that a car engine needs oil for lubrication of the moving parts. The same is true for air-conditioner compressors. A lack of lubrication can allow parts to fuse from the excess heat and friction. Refrigerant and oil may escape slowly through a small leak somewhere in the system. Low refrigerant levels reduce system performance. Adding refrigerant alone can restore performance, but neglecting to replace the lost oil may cause the compressor to run dry and lock up. A system may be low on oil if it needed refrigerant added occasionally over the years.

    Running Hot and Cold

    An air-conditioner compressor needs to be cooled, the same way a car engine does. Just as an overheated car engine may suffer internal damage, so might a compressor. The refrigerant that provides cooling for passengers grants the same benefits to the compressor. A system with low refrigerant may perform tolerably, at times, while the underfed compressor is intermittently overheated. Continued operation of such a system can cause damage.

    Laws and Nature

    It is possible, in some air conditioner systems, to introduce liquid refrigerant into the the compressor intake manifold. Speeding the charging process might be a natural inclination, but turning the container upside down to hasten delivery can lock up a compressor. The frigid fluid can bend valves, and stop moving parts in their tracks. Federal laws apply to refrigerant venting, and related safety precautions are extensive. Be aware of all laws and recommendations before attempting a repair or service of an air conditioner. It's best to take it to a professional for testing and service.

    Murphy's Law

    Anything that can go wrong, can do so in the manufacturing or rebuilding process of compressors. A tiny piece that is out of place, or some slight irregularity in a casting could cause the eventual destruction of a compressor. It's possible that errant bits of some decaying part elsewhere in the system might clog a filter, and starve the compressor of oil. Worse yet, errant shrapnel could get inside the compressor and jam the works. Some mechanical failings are not preventable, even with the best of efforts.


    • Car Care Illustrated 2nd Edition, William Flerx et al

    Photo Credits

    • new car on white snow image by Ivan Arhipov from Fotolia.com