Bottom Line
Keeping your cool?
Don't grab for an A/C quick-charge kit before you know what's really wrong with your air conditioning system

By Jeff Barker
Land Line contributor


Summer is definitely here, and many of us are already out there enjoying the nice, warm weather during our travels. Unfortunately, some are turning on the air conditioners and not feeling the cold air rush from the vents.

It may be tempting to leap to the assumption that the system is low on R-134a refrigerant and that stopping at an auto parts store to grab a quick-charge kit will do the trick. But there’s a lot more to a cab air conditioning system than that.

Before you blow money on something that won’t do you any good, you need to see what’s really going on with the system.

Is there airflow?
Is the system blowing air out of the dash vents or any other vents at all? In order to work, the air conditioning system will first need air flowing freely through the evaporator coil. If air isn’t coming out anywhere, the blower fan may not be running or a blower wheel – also referred to as a “squirrel cage” – may be broken.

If a little bit of air is coming out, an evaporator or blower wheel may be clogged. Also, most newer trucks built after 2000 have cabin air filters designed to catch dirt, pet hair and other debris that you wouldn’t want caught in your evaporator core. Also, loose ductwork inside the dash can hinder air movement.

Hot or cold air?
Is plenty of air coming out of the vents, but it’s not cold? If the system is not blowing cold air, you will need to check a few basic items before jumping to the conclusion that it’s something more serious.

First, with the engine running and the air conditioning turned on, examine whether the compressor clutch hub is turning.

If it is, it’s possible that a heater control valve is open and letting hot water flow through the heater core. If you have manual shut-off valves in the heater hoses that are open, turn them off and see what happens. If you still have no cold air, the likely problem is with the refrigeration portion of the system. It needs to be diagnosed and repaired by a reputable mechanic with the right equipment.

Otherwise, the problem could be as simple as a bad electrical connection in the circuit to the compressor drive clutch. Look at the wiring that goes between the compressor and the coil, which is mounted behind the pulley on the compressor clutch. Did an improperly secured pulley run through it? If so, repair that wire and see what happens. Also, a blown fuse or tripped circuit breaker may need to be replaced before the system will work again.

Another common problem is the connection to the binary switch, often mounted to dryer assemblies on the frame rail near the steer tires, which is a high-spray area. Many Freightliner Century, Columbia and Coronado trucks – as well as other makes – have the dryers mounted there, and the connection gets dirty. Unplugging and cleaning the connection with electrical contact cleaner spray will often cure that problem. Wrapping the connection in electrical tape afterward is a helpful preventive measure to keep moisture out.

If the electrical connections check out OK, the system may be low on refrigerant because of a leak that needs to be repaired. When the system is low on refrigerant, it will keep the circuit through the binary switch open to prevent damage to the compressor.

Although truck manufacturers have come a long way in working with their parts suppliers to develop more reliable air conditioning systems, the most common refrigerant leak is often caused by a seal failure on the compressor input shaft because of heat. This is especially true on the newer engines with higher under-hood temperatures.

A failed compressor clutch hub bearing will also create a lot of heat and cook that seal, causing the system to lose its charge. At that point, a new compressor and dryer need to be installed by a qualified mechanic before the system can be evacuated and recharged, which requires special equipment.

Stop or go?
If your A/C system cools only while traveling at high speeds but not in traffic, it may be because of the engine fan clutch not kicking on. The binary switch triggers a relay for the fan clutch solenoid to engage the engine fan when discharge pressures reach a certain point.

Does the system cool at idle but not at higher engine rpms? That can be caused by a loose compressor drive belt or a low refrigerant level.

Leaky lines
Refrigerant leaks in the lines on newer air conditioning systems have become quite rare over the years. Manufacturers have begun mounting them more securely to prevent problems like broken fittings and sections being rubbed through.

Still, you should check them to be sure that they are still properly secured. An oily residue on a line, especially near a crimped hose connection, is probably the source of a refrigerant leak.

Also, if your evaporator drain tubes in either the cab or sleeper unit have an oily residue, a leaking evaporator coil might need to be replaced. LL

Editor’s note: This article is for information purposes only. If you’re not sure about performing the work yourself, it’s advisable to seek the help of a competent professional.

Jeff Barker is an OOIDA member and a former certified diesel mechanic. He can be reached at