If your rooftop RV AC is humming but not starting, clicking and cutting out, or running but barely cooling — a failed run capacitor is the most likely culprit. It’s one of the most common hardware failures in any AC unit, and in an RV it’s even more common because the units deal with extreme heat and vibration.
What Is a Capacitor?
A capacitor is a small cylindrical component that stores and releases an electrical charge to help motors start and run. RV AC units typically have two: a start capacitor (gives the compressor motor an initial jolt to get it spinning) and a run capacitor (keeps the compressor and fan motor running efficiently).
When a capacitor weakens or fails, the motor can’t start properly — or starts but runs inefficiently and overheats.
Symptoms of a Failing Capacitor
- AC unit hums for a few seconds then shuts off
- Compressor tries to start, clicks, then gives up
- Unit runs but doesn’t cool (compressor is on but weak)
- Fan runs but compressor doesn’t start at all
- Visible bulging or leaking on the capacitor body
Quick field test: When your AC clicks on, listen carefully. If you hear the fan start but then a struggling hum or a click-and-cutout from the compressor, the run capacitor is the first thing to check.
Can You Test a Capacitor?
Yes, with a multimeter that has a capacitance (microfarad) testing function. The capacitor has a rating printed on it (e.g., 45/5 MFD). Set your meter to capacitance, discharge the capacitor first (important — capacitors hold a charge even when power is off), and measure across the terminals. If the reading is more than 10% below the rated value, the capacitor is weak and should be replaced.
Safety warning: Capacitors can hold a dangerous electrical charge even after the unit is powered off. Before touching or testing a capacitor, discharge it by placing an insulated screwdriver across the terminals. If you’re not comfortable doing this, call a tech.
What Does Replacement Cost?
The capacitor itself is usually $15–$40 depending on the rating and brand. Replacement involves removing the outer shroud from the rooftop unit (usually 4 bolts), disconnecting and swapping the capacitor, and reassembling. In total it’s a 30–60 minute job for someone experienced.
The real cost is usually labor for getting on the roof and handling the unit properly. A mobile tech visit for a capacitor replacement typically runs $100–$200 all-in depending on your location.
Will Any Capacitor Work?
No — you need the correct MFD (microfarad) rating and voltage rating. Using an under- or over-rated capacitor can cause the motor to run hot and fail prematurely. Match the ratings on the original capacitor exactly, or within manufacturer specs.
Pro tip from Thomas: If your RV AC is more than 8–10 years old and the capacitor just failed, it’s worth having the full unit inspected. Capacitors often fail first, but the compressor and fan motors may also be nearing the end of their life. Know what you’re working with before investing in repairs vs. replacement.
Need a Capacitor Replaced?
Thomas can come to your location, test the full AC system, and swap the capacitor if that’s what’s needed — no unnecessary repairs. Serving South Mississippi within 1.5 hours of Saucier.
Request AC Service