* Low refrigerant: This is the most common cause. When the engine is under load, the compressor needs more power, and if refrigerant is low, it might not be able to maintain pressure and cooling. A leak somewhere in the system is the likely culprit for low refrigerant. This requires professional diagnosis and repair. Don't try to add refrigerant yourself without knowing the cause of the leak; you could damage the system.
* Weak compressor clutch: The compressor clutch engages the compressor to the engine. If it's weak or failing, it might disengage under load because it can't handle the increased demand. This will require either a clutch replacement or potentially a whole compressor replacement.
* Failing compressor: A failing compressor may struggle to generate enough pressure under load. Similar to a weak clutch, this needs professional diagnosis and likely replacement.
* Electrical issues: Problems with the wiring, fuses, relays, or the AC pressure switch can cause the compressor to cycle on and off erratically or fail to engage under load. A thorough electrical check is needed to diagnose this.
* Restricted airflow: Although less likely to cause it to *completely* stop, a severely clogged condenser (the radiator-like component in front of the radiator) can restrict airflow and cause the system to overheat and shut down under load. Clean the condenser carefully with a proper cleaner. Don't use high-pressure water.
* Low system pressure: This could be due to a leak, as mentioned above, or a problem with the expansion valve or orifice tube, restricting refrigerant flow.
Troubleshooting Steps (Some require professional tools):
1. Check the refrigerant level: This requires AC gauges. A qualified mechanic will check the pressure and temperature of the refrigerant to determine if it's low. Low refrigerant should be investigated for leaks *before* adding more.
2. Inspect the compressor clutch: While the engine is running, listen for any unusual noises from the compressor. A clicking sound might indicate a weak clutch. Visually check for any signs of damage.
3. Check fuses and relays: Examine the fuses and relays associated with the AC system. Replace any that are blown. This is a simple check but often overlooked.
4. Clean the condenser: Carefully clean the condenser to ensure proper airflow.
5. Check for leaks: A mechanic can use a leak detection dye or electronic leak detector to pinpoint leaks in the system.
Do not attempt to recharge the AC system yourself unless you have the proper tools and experience. Improper charging can damage the system and void any warranty.
In short, taking your 2002 Ford Ranger to a qualified automotive AC technician is the best approach. They have the tools and expertise to diagnose the problem accurately and perform the necessary repairs. Describing the symptom (AC stops blowing cold when the engine is under load) will help them focus their diagnosis.
