ServiceNomad RV · Knowledge
RV AC Fan Runs But No Cooling
When your RV air conditioner's fan runs but no cold air comes out, the compressor has failed to start. This is one of the most common AC failures and is usually caused by a bad start capacitor, run capacitor, or a compressor that has mechanically seized. The good news is that diagnosing this issue is relatively straightforward, and in many cases, it's a repairable problem rather than requiring a full AC replacement.
Understanding the Compressor Start Sequence
RV AC units have two main components: the fan motor and the compressor motor. The fan is relatively easy to start and draws low amperage, so it almost always runs when the AC is turned on. The compressor, however, is a high-draw motor that requires a significant electrical 'kick' to start spinning. This kick is provided by the start capacitor, and once running, the run capacitor helps maintain smooth operation. If the start capacitor has failed, lost capacitance, or the compressor itself is mechanically tight, the compressor will not start. You'll hear the fan running but no change in sound when the thermostat calls for cooling. The compressor may hum for a moment and then go silent as its internal overload switch trips to prevent motor burnout.
Testing and Replacing Capacitors
Capacitors are the most common failure point because they're subjected to extreme heat cycles on the RV roof. A start capacitor that has lost capacitance cannot provide the starting boost, and a run capacitor that has swelled or leaked is equally problematic. A technician can test capacitors with a multimeter in capacitance mode—if readings are below the rated value (stamped on the capacitor), replacement is needed. Capacitor replacement is straightforward but dangerous if you don't know what you're doing—capacitors store electrical charge even after power is disconnected and must be properly discharged before handling. This is why most owners choose to have a technician perform this repair.
Compressor Mechanical Issues
If capacitors test good but the compressor still won't start, the compressor may be mechanically seized. This happens when internal bearings fail, refrigerant contamination causes scoring, or the compressor has been damaged by repeated attempts to start on low voltage. A seized compressor will draw very high locked-rotor amps (often 40-60 amps) but never start spinning. A technician will measure the current draw during a start attempt. If it's far above the rated locked-rotor amps and the compressor never starts, replacement is necessary. Unfortunately, compressor replacement is expensive and labor-intensive—sometimes it's more cost-effective to replace the entire AC unit.
Refrigerant Loss and Low Pressure Switches
In some cases, the compressor won't start because the system has lost refrigerant due to a leak. RV AC units have a low-pressure safety switch that prevents the compressor from running if refrigerant pressure is too low (to protect the compressor from running dry). If you have a slow leak, the system may have enough refrigerant to function for a while, then suddenly stop when pressure drops below the switch threshold. A technician can check refrigerant pressure with gauge manifolds. If pressure is low, they'll need to locate and repair the leak before recharging the system.
⚠️ Safety Notes
- Never attempt to discharge or handle capacitors without proper training—they can deliver a dangerous shock.
- Do not force a compressor to start by bypassing safety switches—this can cause motor burnout and fire.
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