Automotive Service Excellence (ASE) Air Conditioner Certification Practice Exam

Foam in the sight glass of an A/C system typically indicates that the refrigerant charge is low. When the refrigerant level is inadequate, it can cause improper vaporization and lead to the formation of foam instead of a consistent flow of liquid refrigerant. This foam occurs because the low refrigerant charge prevents the system from achieving the necessary pressure for efficient operation, leading to a turbulent mix of vapor and liquid.

In a properly charged system, the sight glass should display a clear liquid with no signs of foaming. The presence of foam signals that the system isn't functioning as designed and may require a refrigerant charge to restore optimal performance. If the charge were too high, one would expect to see continuous liquid rather than foam, and other conditions like air entrapment or a malfunctioning expansion valve would present different symptoms in the sight glass.