auto a/c sight glass diagnosis

Auto A/C Sight Glass diagnosis.


A sight glass is a transparent glass tube or window installed in a tank or boiler system that allows an observer to view what is happening inside the system. A/C sight glass diagnosis could give clues about what is happening inside the air conditioning system.

Where is the Sight Glass located in my vehicle ?

The refrigerant sight glass is normally located anywhere liquid flow exists and anywhere it can serve a purpose. When it is installed just prior to the expansion device, the technician can be assured that a solid column of liquid is reaching the device. The automotive sight glass is usually installed on top of the receiver drier as a way to help the a/c technician to read into the vehicle’s air conditioning system. 

Types of Auto Air Conditioning Sight glass ?

Sight glasses come in two basic styles: plain and with a moisture indicator. The plain sight glass is used to observe the refrigerant as it moves along the line. The sight glass with a moisture indicator can tell what the moisture content is in the system. It has a small element in it that changes color when moisture is present.

Understanding the Information given by the Sight Glass.

What you see in the sight glass can help you diagnose system malfunctions. It is strongly suggested that you do not use the sight glass as the only means of diagnosis. Also, do not use the sight glass to diagnose R134a systems.

If the sight glass is clear, one of three conditions can exist: the system is completely empty, the system is full, or the system is overcharged. If there is foam, the charge is low, but if there are only a couple of bubbles, it is probably Ok. few bubbles when the compressor cycles on is normal. Oil streaks in the sight glass usually indicate a low charge and that the compressor is pumping oil from its sump. If the sight glass is cloudy, the desiccant bag has probably burst.

If an R134a system has a sight glass, do not use it or try to perform diagnosis, as it is normal to observe bubbles in this type of system.



System Ok

System overcharged

System empties (No refrigerant)


System low on refrigerant

System contaminated with air


System may be low on refrigerant

Oil may streak as oil circulates


Desiccant breakdown?

Contaminated system

Do you need Assistance with your auto air conditioning system ?

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