Auto A/C Terms - Glossary
Condensation

When looking at the auto A/C system, condensation actually occurs in a couple of ways. Condensation is when a vapor or gas is cooled so that it changes state to become a liquid. In the A/C system, that occurs all the time in the condenser, because vapor refrigerant is being cooled. When that occurs, it changes state back to liquid. However, there is also condensation that occurs at the evaporator. The typical sign of a good working A/C system is when you see the drip of condensation from behind the passenger side front tire.

What is happening is that the moisture or humidity in the air is being condensed. Understand that the auto A/C system performs several different functions. One of them is to cool the air entering the vehicle. Another function is that it will dehumidify that air.

On hot humid days, it is a fact that the humidity makes it actually feel warmer than it really is. In order to provide the best passenger comfort, the auto A/C system will remove the humidity from the air as it is entering the vehicle.

That is accomplished because the evaporator is very cold. The liquid refrigerant in the evaporator makes it cold. When the hot outside air full of moisture contacts that cold evaporator, the sudden change in temperature causes the moisture or humidity to condense back to water. The evaporator drain is routed behind the passenger side front tire to drain that condensation.

The condensation causes no other problem unless it is allowed to freeze on the face of the evaporator. Understand that the temperature of the evaporator is regulated by the flow or refrigerant into the evaporator and the compressor suction pulling vapor off the top of the evaporator. If the compressor is allowed to create more suction than is required, the evaporator pressure will be reduced. Doing that automatically reduces the evaporator temperature. If the temperature falls below the freezing point, that condensation could freeze and restrict air flow into the vehicle.

When that happens, it is common that the air flow from the dash vents is seriously reduced. Shutting the A/C system off for a short period of time to allow the frozen condensation time to thaw will restore air flow. However, the problem lies in the switch that is controlling the compressor operation. The sensing bulb is malfunctioning or it is not properly secured and is not sensing the proper temperature. This problem is more common on vehicles using temperature sensing cycling switches as opposed to those using pressure cycling switches. Also know that in many cases, the temperature sensing switch utilizes a sensing bulb that is attached to the evaporator, pressed in between the evaporators cooling fins.