I had the compressor in my pickup replaced 5 years ago and the A/C worked fine until this spring. Then it started working intermittently: it will blow cold air for awhile, and then blow warm. I thought it was low on R-12 freon. I took it to a local shop and the retrofitted it for the newer freon, including a new dryer, and filled it with R-134. But I still have the problem. It blows cold for 10-15 minutes, then barely cool at all for 10-15 minutes, the blows cooler again for awhile.
I had a similar problem with a 1990 Ford Aerostar. The clutch was defective on the fan and while the engine wouldn’t overheat, the fan clutch slipped enough so that insufficient air was pulled through the condenser.
The independent shop that does my work did the following:
Connected the gauges to the air conditioning system.
Noted that when the air conditioning quit putting out cold air that the
pressures went way up
Took a big fan in the shop and directed the air through the grille of the
Aerostar. The pressures on the gauges went down to the normal range and
the air conditioning began cooling.
If your Ranger has an electric cooling fan, this doesn’t apply, although you should check to see if it is coming on as it should. At any rate, have the shop connect the gauges and check the pressures when the system is functioning correctly and then watch to see what happens when the systems ceases to blow cold air. This might help pinpoint the problem.