Question: I have a 1999 Ford Crown Victoria ex police car with the 4.6 engine. The air conditioner is fully charged and has no leaks. The system will run cold for about twenty minutes and then start blowing hot air. Shut it off for awhile and it will blow cold again. I tried a new high pressure switch but the problem remains. Any Ideas what the problem could be?
Answer: That's sounds like an evaporator core that's freezing. If the core freezes, the ice build up pevents air from flowing through the fins of the evaporator. This reduces the amount of heat that is removed from the air. Turning the a/c off allows the evaporator to thaw, which restores cold vent temperature until the core freezes again.
The first thing to do is verify that the compressor clutch is engaged and the compressor hub is spinning when this problem occurs. This would help to rule out clutch or other elecrical faults.
If you have a manifold gauge set, attach to the low side and check pressure. If pressure is below 22 psi, and the compressor is operating, then suspect a bad low pressure cycling clutch switch. The low pressure cycling switch is there to maintain evaporator pressure to a point slightly above freezing to prevent evaporator icing.
The high pressure switch shuts the compressor down if pressure goes above design limit (420 psi). Instead of the high pressure switch, your problem sounds more like the low pressure cycling switch.
The cycling switch on the 1999 Ford Crown Victoria is located on the accumulator.
The other posibilty is ice formation in the system. This most often happens when the system is recharged without first evacuating. Moisture in the system forms into ice which accumulates near the orifice. A gauge set should be used to determine if the low pressure cycling switch is working as desinged, or if ice formation on the inside of the system is causing problems.