[Skip to Content]

Don’t live with A/C that doesn’t keep the cab cool

By on July 1, 2015
Homer-KeepYourCool

When summer temperatures start climbing, a long drive in an enclosed space would be unbearable if not for the modern convenience of air conditioning. The sun may blaze down heat, but with just a flick of the wrist, the A/C gets turned up and a comfortable chill enters the air. At least it should. If the A/C is on full blast, but the cab still feels warm, it’s time to investigate. This scenario is all too common and can be a result of many faults in your A/C system.

Is there a leak?

Thirty years ago, truck A/C systems contained a refrigerant called R12 — best known as Freon — that was
highly effective due to its ability to correlate pressure and temperature. This correlation is critical because manufacturers change the pressure in the system in two places: the compressor and the expansion device. If the temperature did not move with the pressure, your A/C system would not cool very well. But R12 has one major flaw: it contains CFCs, which have been shown to harm the atmosphere and cause severe health problems. So the government passed legislation to eliminate the use of CFCs in mobile A/C systems. R12 was replaced by the refrigerant R134a, which is safer for the atmosphere. But the molecules that make up R134a are 20 times smaller than the molecules that make up R12. This makes R134a susceptible to leaking.

You may notice that the A/C gradually starts to lose effectiveness through the course of the trip, or that the system that cooled just fine last season now struggles to keep up to counteract the heat of the day. So let’s say you determine that your truck’s A/C system is low on refrigerant. Just adding more refrigerant to the system is not the answer. It may temporarily revive your A/C to produce cold air, but the better solution is to have the A/C system checked for leaks.

The A/C compressor can be severely damaged each time the R134a leaks out because the compressor needs that cold refrigerant for its own cooling. To add insult to injury, when a leak causes R134a to escape, precious oil needed for lubrication of the compressor dribbles out right along with it. Avoid costly repairs by staying aware of any decline in performance of your truck’s A/C system. Then do not delay to get it checked for leaks and get the leaks repaired.

Ditch dirty filters

Most modern vehicles use filtration systems to help maintain the air quality inside a truck’s cab and I cannot overstress the importance of clean cabin filters for your A/C system. These filters must be changed once they become dirty. Doing this in a timely manner means that the driver breathes clean air in the cab, but it also is important for the truck because it allows air to flow freely across the A/C coils under the dash.

When the cabin air filter gets dirty and restricted, the A/C system will freeze up, leading to potential compressor damage. These filters must be checked at least once a year and replaced or serviced according to the truck manufacturers’ recommendations. Some filters can be cleaned, but most are made of paper and must be replaced when dirty.

Update technology

All modern truck A/C systems rely on complex technology to create a steady, comfortable temperature inside the cab. Sensors throughout the truck measure key information — such as ambient temperature, duct temperature, the temperature of the A/C coils under the dash, the pressure in the A/C system and more — so the A/C computer(s) can adjust as needed.

These computer-controlled A/C systems are very sensitive and may require recalibrating and/or program updates to ensure they operate effectively. Technicians must hook computers up to the system to determine if it is operating correctly and to make or monitor adjustments.

Keeping the cab cool when the sun is blazing can be challenging if you don’t pay attention. But a little thought to maintenance should give you years of cool summer days — at least while you’re inside the cab of your truck.


Homer Hogg, Technical Development Manager for TA and Petro, has worked as a truck technician for more than 30 years. He is ASE Master-certified, a Daimler Certified Trainer and a member of the Nashville Auto Diesel College Hall of Fame

Homer Hogg’s “Maintenance Matters” airs on the Dave Nemo Show (Road Dog Trucking, SiriusXM 146), 8 a.m. ET, the first and third Thursday of each month.

About Road King

For the professional Driver

Leave a Reply

Your email address will not be published. Required fields are marked *