Automotive A/C. How do I know if refrigerant low?

Most every vehicle I’ve owned in recent memory (of those that -had- A/C ) had a little window to view the level of freon or refrigerant. Question is though… um, what am I looking for ? What are the sign or signs that something is wrong? Is the little window only there for a real mechanic to see if, in test conditions, if the coolant dye is moving? Am I supposed to see cloudy stuff in the little eye, or empty blackness, or swirling streams of stuff or what?

Specific to most recent case:
I have a '99 Ford F150, and I suspect that the A/C just isn’t up to snuff (and being a '99, it is out of warranty) is there something that I can look at to confirm suspicions?

Further, is adding refrigerant all that hard, as long as I get the right kind?


Apparently, an easy check for your AC unit is to measure the temperature of the air coming out. If it’s 40F, it’s fine.

Other than that, I’ve no clue as to the mystery that is AC, other than I’d not be able to survive without it.

230# irish decendants are not meant to be in heat over 75F. Ireland is a COLD WET ISLAND, and that’s what I’m bred for… :smiley:


You can get an R-134 charge kit that includes a guage at any auto supply place like checker or pep boys. Unfortunately checking for 40ºf air is not reliable since newer AC systems don’t get as icy cold as the old R-12 systems did.

For R-12 systems with a sight glass, it will be clear when the system is empty or full (but not in between). If the system is partially full, it will have bubbles, which sometimes make it appear cloudy.

For R-134a systems with a sight glass (including systems that have been converted from R_12 to R-134a), the sight glass is useless. Ignore it.

For systems without a sight glass, the only way to be sure it’s full but not overfull is to empty the system and measure how much refrigerant is put into it. Practically speaking, one can use A/C pressure gauges and/or measure outlet air temperature, as mentioned above. Generally, outlet air temp of 35-45 F is considered good. It can vary with ambient temperature and humidity.