Here's why you should consider a service station:
* Finding the Leak: Low refrigerant is a symptom, not the problem. The real issue is likely a leak somewhere in the system. A service station has the equipment to locate and repair the leak. Simply adding refrigerant will just result in the leak draining it again quickly.
* System Pressure and Diagnosis: A properly functioning AC system needs to be at the correct pressure. Adding too much refrigerant can damage the compressor. A mechanic has gauges to measure pressure and identify other potential issues.
* Vacuuming the System: Before adding refrigerant, the system must be vacuumed to remove air and moisture. This prevents further damage to the compressor. You'll need a vacuum pump to do this properly.
* Safety: Refrigerant handling requires precautions. Improper handling can expose you to harmful chemicals.
While you *can* buy refrigerant at an auto parts store, it's usually best to take your 1999 Ford Ranger to a qualified mechanic or AC specialist to diagnose the problem and fix it correctly. The cost of a proper diagnosis and repair is likely to be less than the cost of potentially damaging the compressor by attempting a DIY repair without the proper knowledge and tools.