Not long ago, or so it seems, air conditioning service remained as one of the last segments of automotive repair in which things didn't seem to change much, and working on one car maker's system was virtually the same as working on the next.
Eventually, environmental concerns put the EPA on a mission to regulate the service community's practices. At about the same time, automakers embarked on a quest to make their systems more efficient, and less costly to manufacture. The end result is a marked change in the way A/C service should be approached.
Pressures And Temperatures
The old way of checking a system for proper charge involved taking high and low-side pressure gauge readings, perhaps looking through a sight glass, and reading outlet temperatures from the dash registers. Unfortunately, the traditional ways are simply no longer viable.
When compared to those of yesteryear, today's systems are much more sensitive to the amount of actual refrigerant charge, due to smaller capacities. Back in the old days, R-12 systems often held 3 lbs. or more of refrigerant. Today's systems, thanks to cost reduction and efficiency goals, have about half that capacity. This results in a smaller margin of error with respect to the actual refrigerant charge. The old days of topping off (where legal) a system to get temperatures back in line are far behind us.
Dash outlet temperatures are also not reliable enough as a means of checking system performance, because a system can be partially low on refrigerant before it effects outlet temperature. Although cooling performance may seem fine, the loss of charge can effect lubrication throughout the system, especially within the compressor.
So what's the bottom line? The only way to properly charge a system these days is to remove the refrigerant, properly evacuate the system and then re-charge the system with the exact charge specified. It's the only reliable way to ensure the refrigerant charge is on target, and guessing just won't cut it.
Alternative Refrigerant Hazards
Although the threat of cross-contamination (mixing of refrigerants) is nothing new, there's a legal loophole of sorts that makes the threat loom larger for R-134a systems.
Basically, EPA requirements addressing unique fittings and dedicated labels for alternative refrigerants apply to R-12 systems, because that's how the regulations were originally drafted. So without the teeth of a law protecting R-134a systems, it's even more likely that you'll encounter some cross-contamination.
A refrigerant identifier is your best ally when it comes to a contaminated system, but there's no absolute guarantee that it will accurately sniff-out whatever wild concoction might be present. Plus, it'd be one thing if the refrigerant inside consisted of just one alternative, but you may run into a system with a bizarre mix of different things thrown in from several top-offs. If you run into a car with a questionable service history and strange cooling symptoms, it might be best to steer clear of the threat it poses to your equipment.
Maybe you've been tasked with more than one customer who complains of a musty smell coming out of the discharge vents. At a minimum this is annoying, but these odors can also trigger serious allergies. These smells are the result of water collecting in the bottom of the evaporator housing, which then combines with dirt from the air in creating mold.
While it's true that the water should drain from the evaporator housing through a drain tube, this drain tube often plugs and creates a backup of water in the evaporator housing. In severe cases, this water might even leak out into the interior. Note that systems with good drainage can still house enough condensation to promote mold growth.
To fight the mold problem effectively, you need to kill it where it lives and breeds. As a routine check, take a look to make sure the evaporator drain tube flows properly. With the A/C system on, you should see a discharge of water below the car. If not, inspect the tube and correct any problems that could inhibit water flow.
Tips to prepare for the switch from R-134a to HFO-1234yf refrigerants.
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