A common truck air conditioning issue is when the air emitted is cool but not cold. Problems with the compressor and leaks in the air conditioning unit are also common.
If the air blowing from the vents is cool but not cold, first listen to make sure the condenser and radiator cooling fans are running. Visually inspect the condenser and remove debris that could be restricting air flow. Check for clogs in the cabin air filter. Use a manifold gauge set to check the air conditioning unit's pressure. Normal ranges for pressure are between approximately 40 and 220 psi. High pressure can be caused by excess refrigerant or interrupted airflow; low pressure can indicate a lack of refrigerant or a problem with the compressor.
When checking the compressor, first make sure that the compressor's clutch engages when the engine and air conditioner are on. If it does not engage, check the voltage leading into the compressor using a voltmeter. Voltage indicates a malfunctioning clutch. No voltage may be a sign of a bad cycling switch, blown fuse or not enough refrigerant.
Leaks are typical when the air conditioner has low pressure. Secure all fittings and inspect the compressor's hose manifolds, front seal, o-rings and pressure switches. Make sure the hoses are properly inserted into the fittings and the Schrader valves are up to standard. Ensure there are no pin holes in the condenser. Finally, use an ultraviolet light to check for oil draining from the evaporator.