Recharge, revive AC
The majority of times, AC failure can be traced back to no or low levels of refrigerant coolant. All vehicles leak some refrigerant through seals and microscopic pores in hoses. But the older the vehicle, the higher the rate of that seepage.
There is a simple way to check for leaks. “Because coolant has an oil base, any coolant leaks will collect dirt much the same as a hydraulic leak,” points out Darrell Woods, service manager for Kalvesta Implement, an AGCO dealership in southwest Kansas. Another way to detect leaks is by spraying hoses and couplings with soapy water. Tattletale bubbles pinpoint leaks.
Once a leak is detected, repair it prior to recharging the system. Most repairs involve replacing O-rings, seals, or hoses. But the evaporator or condenser could be leaking as well. Repairing leaks is crucial since low refrigerant fluid levels prevent an AC system's compressor from turning on. The compressor is being overridden by a low-pressure safety switch. That switch protects the compressor from possible damage caused by a lack of lubrication.
After repairing leaks, turn the AC on to check if the system's compressor engages. If not, determine if the fuse servicing that component is blown or if the wiring feeding the compressor has developed a short.
If the compressor's magnetic clutch is getting voltage but not engaging, the clutch is likely defective and needs to be replaced. If there is any evidence of leakage around the compressor shaft seal, that seal should be replaced. If the clutch works but fails to turn the compressor, the compressor has seized and needs to be replaced.
Not all AC problems are caused by low fluid levels. If you own an AC pressure gauge set, connect to the high and low service fittings on the system. Readings from those gauges can be paired up with a simplified diagnosis chart (shown below) to help track down potential problems. But be sure to refer to service manual specifications for normal system operating pressures and the total refrigerant capacity of the system.
If your AC causes your cab to smell like an old shoe, you need to clean out the system's evaporator coil. The problem is mold on the evaporator. Various chemicals can be sprayed on the evaporator directly or through the blower ducts or air intake. Be sure to check that the drainage tube is open and clean so condensation can be carried away.