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Air not working in 99 Ranger when vehicle is not in motion

Hey everyone I am a Rural Mail Carrier and recently purchased a 99 Ranger to run my mail route on. This is the first car Ive bought with my own money and I’m rather fond of it. The first time I ran it on the route I noticed the AC would stop working completely before the route was done. It takes me about 7 hours to run the whole route. At first the AC will cut off for a couple of min. and then cut back in, but the intervals between coming back on will lengthen as the day wears on until the cold air never comes back. I live in Alabama and our summers are absolute hell, AC is not an option but a necessity. I have had a mechanic I trust and have used for years who used to be the head mechanic at a Ford dealership before going out on his own look at it several times and he cannot figure it out. He as replaced the radiator fan clutch, messed around with the electronics making sure there are no bad connections, and changed a switch I believe as well. He claims the compressor is fine as it runs cool air when I drive normally not on the route. Nothing has worked, and the hotter the weather is outside the faster the AC goes out. Someone please help me out before I die delivering advertisements!

The problem might be with the expansion valve at the evaporator. The expansion valve is what controls the refrigerant pressure into the evaporator so it’s converted from a liquid back into a gas. When driving down the road and the compressor is rotating faster it opens more. When driving slowly or when idling and the compressor rotates slower, the expansion valve closes slightly. If the expansion valve fails to close slightly when driving slowly or when idling when the compressor is rotating slower, the refrigerant pressure at the evaporator falls off and the AC stops cooling.

Tester

+1 to Tester’s comments.
It sounds like you need an automotive AC specialist.
Sincere best.

Carry a quart spray bottle filled with water and the next time the problem occurs spray the condenser down thoroughly while the engine is running and the AC is switched on. If the AC quickly comes back to life the system may be getting too hot. If so, an auxiliary fan on the condenser would probably get the system cooling again.

Strange question to ask here, does the fan speed inside the cab of the truck sound and feel like it is blowing the same amount of air when the temp drops back? If it is falling off maybe the evaporator inside the dash is actually freezing over? That is a long shot though. Our 93 Ranger looses fan dash speed under moderate acceleration, I am thinking vacuum leak on ours. Maybe from the 93 to the 99 more of the dash controls are covered with vacuum control and could be causing some issues. Extended idle and low throttle angle should be the best condition for vacuum leaks to be masked but you never know with vacuum leaks.
Where at in Al, I’m there also.