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Two of the most common RV AC problems

When an RV air conditioner hums but won't start, or runs but barely cools, two problems cause a big share of the calls we get: a failed capacitor or a frozen evaporator coil. One is a cheap part; the other usually needs no part at all. Here's how to tell them apart.

Signs of a bad capacitor

Capacitors give the compressor and fan motor the extra push they need to start, and keep them running efficiently. RV air conditioners use a run capacitor (often a dual capacitor serving both the compressor and the fan) and some also have a start capacitor for the compressor. Common signs one has failed:

• The AC hums or clicks but the compressor doesn't start.
• The fan doesn't spin, or spins slowly, while you hear the compressor.
• The compressor starts, then shuts off after a few seconds.
• The breaker or generator trips as the AC tries to start.
• The capacitor looks swollen, cracked or is leaking.

How to check a capacitor safely

Safety first: capacitors can hold a dangerous charge even after the AC is off. Turn off shore power, the generator and the inverter, and discharge the capacitor before touching the terminals. If you aren't comfortable doing that, have an RV tech do this step.

1. Remove the shroud and locate the capacitor (usually in the control box area of the rooftop unit).
2. Photograph the wiring before disconnecting anything.
3. Read the rating printed on the capacitor: microfarads (MFD or µF), voltage, and the tolerance (for example ±5%).
4. Test it with a multimeter that has a capacitance setting. A reading outside the printed tolerance means it should be replaced.
5. Replace it with one of the same MFD rating and the same or higher voltage rating.

Finding the right capacitor

• Coleman-Mach capacitors, including the 35 MFD 370V compressor run capacitor 1499-5681 and start capacitors such as the Mach I / Mach III start capacitor 1497-0471.
• Not sure which one you need? The Coleman-Mach run capacitor variety pack 1499-5001 covers six common values.
• Dometic capacitors, including the run capacitor 3311563.000 for 15,000 BTU units and the FreshJet 5 run/fan capacitor 4450037547.
• Furrion capacitors.

If your compressor struggles to start on a generator even with a good capacitor, a soft start may be the better fix. See our post on RV AC soft starts and generators.

Signs of a frozen evaporator coil

• Air comes out of the ceiling vents weakly, or barely cool, even though the AC is running.
• You can see frost or ice on the coil behind the filter.
• Water drips from the ceiling assembly after the AC shuts off.

Dometic's manuals list two main causes: very cold output at a low fan speed, and low outdoor temperatures. In practice, the usual culprits are a dirty filter and closed vents, both of which cut airflow over the coil.

How to fix a frozen coil

1. Switch the AC to Fan Only and let it run until the ice is completely gone. Don't chip at the ice.
2. Clean or replace the return-air filter. Dometic says to clean it at least every two weeks of use and never to run the AC without it.
3. Open the vents. Dometic says to keep at least two distribution vents open in cool mode.
4. Set the thermostat a little warmer and run the fan on high, especially on cool nights.

If the coil keeps freezing with a clean filter and open vents, a failed freeze sensor, a weak blower or a refrigerant problem may be to blame, and it's time to call a tech. Replacement Dometic filters, Furrion filters and Dometic freeze sensors are in stock.

Quick diagnosis

For more troubleshooting by brand, including thermostat error codes, see RV Guidance: RV Air Conditioners and Furnaces.