RV air conditioner not cooling: The troubleshooting guide

Start here: match your symptom to its fault

Runs, blows air, but the air is not cold

The largest group. The temperature drop is what separates them. A drop inside the maker's range while the RV still will not cool is a heat-gain or air-distribution problem: the unit is doing its job and the vehicle is gaining more heat than the unit removes. A drop below the range points at the coil or the sealed system.

Nothing happens at all

No fan, no hum. That is a supply or control problem, not a cooling one. Check the fuse or circuit breaker at the panel, then the selector switch or thermostat that is supposed to be calling for cool. Nothing in the refrigeration side can make a unit completely dead, and Dometic's own service guidance says the same: check the generator or landline supply, the line sizing, and the fuse or breaker before calling anyone.

Cold for a while, then warms up

Classically a coil that has iced over. Once ice covers the evaporator, air cannot pass and the cooling stops even though the unit is still running. Restricted airflow is what causes it, so the filter, the return-air path and the divider are the places to look. Airxcel notes that restricted airflow over the evaporator can also produce a temperature drop greater than normal, which reads like a healthy unit and is not.

Ice visible on the coil

The same cause, further along. Airxcel states that a clogged filter will produce a loss of air volume and may eventually cause an icing-up of the cooling coil. Airxcel also publishes a maintenance note for its ceiling assemblies: check the filters and either clean or replace them, and do it often in heavy use.

Weak on high fan and colder on low

This is the signature of air leaking from the supply side back into the return side, which is the divider fault described below. On low fan there is less air to leak, so the vent feels colder.

The compressor cannot be heard running

If the fan runs and the compressor never starts, the unit is not cooling because nothing is moving heat. Airxcel lists low voltage, a capacitor that is shorted, weak or open, a start relay with open contacts, and a mechanically stuck compressor as the ranked causes, in that order. All of them are technician work.

The cooling sequence: where your unit is stopping

Every roof unit follows the same steps. Deciding which one has failed is most of the diagnosis, because the steps after a failure never happen.

1. Power and the control call

Line voltage arrives at the unit and the thermostat or selector switch asks for cooling. Dometic's installation guidance specifies a 120 VAC supply on 12 AWG copper, protected by a time-delay fuse or a HACR circuit breaker, and the thermostat on a mechanical unit covers roughly 65°F to 90°F at the air box inlet.

2. The fan moves air

The blower pulls air through the return, across the evaporator coil and out of the supply. If the fan is not moving enough air, nothing downstream works properly and the coil eventually ices.

3. The compressor starts

The compressor is what actually moves heat. Airxcel notes it is a hermetic compressor, sealed and welded, and therefore not internally serviceable. Dometic's operating instructions add one behaviour worth knowing: after shutting a unit down by the switch or the thermostat, wait two to three minutes before turning it back on, so the refrigerant pressures equalise and the compressor can start.

4. Heat leaves through the condenser

The condenser on the roof rejects the heat the evaporator collected. It needs outside air moving across it and it needs to be clean. Airxcel lists a dirty condenser coil among the causes of a compressor that overheats or trips its breaker.

5. The air that arrives is cold

If steps one to four happen, this follows. Measuring the drop is how you confirm it, and it is the reading that tells you which half of the problem you are in.

The temperature drop across the coil

This is the one measurement that does the most work. It needs no tools beyond a thermometer.

What the maker publishes

Airxcel is the manufacturer that publishes a figure this page could verify from the maker's own copy. Furrion's rooftop manual publishes no temperature-differential figure at all, and offers symptom-based troubleshooting instead. If your unit is a Furrion, work from the method below rather than from a maker's number, and treat the general range as guidance rather than a specification for your unit.

How to take the reading

Run the unit on high fan for at least half an hour so the coil reaches a steady state, on a day at or above about 80°F. Read the return air at the intake, ideally at the filter, and the supply air in the discharge right at the grille rather than across the room. Subtract. Write both numbers down: "the air feels cold" is not a measurement, and a shop cannot work from it.

What the results mean

Inside the range and the RV is still hot. The unit is working and the problem is heat gain or air distribution: sun on the glass, a slide-out with no shade, ducting leaking into the roof cavity, or a unit undersized for the vehicle. Airxcel's guidance for high outdoor temperatures is to shade the RV, use window shades, keep the doors and windows shut, avoid heat-producing appliances, and start the unit early in the day so it gets a head start rather than trying to catch up at noon.

Below the range. The coil is not getting cold enough. Airflow comes first because it is the common cause and the one you can fix. If airflow is clean and the reading is still low, the sealed system is the next step and that is technician work.

Above 22°F. Counter-intuitive but documented: Airxcel states that a difference greater than 22°F is possible in warm dry weather, and that restricted airflow over the evaporator can also push the number up while the unit is actually losing capacity. A big number is not automatically a good one.

Airflow: filters, coils and the divider

Almost everything an owner can fix lives here, and it is where the manuals put the emphasis.

The filter

Airxcel states plainly what a clogged filter does: it produces a loss of air volume and may eventually cause the cooling coil to ice up. Its maintenance guidance for the ceiling assembly is to check the filters and either clean or replace them, and to expect that to be frequent during heavy use. This is the cheapest item on the page and the most common cause.

The evaporator coil

The face of the coil collects lint and dirt, and a coated coil restricts the airflow through it. Airxcel's instruction for a soiled coil is that the unit comes off the roof and the coil is cleaned with a proper coil cleaner and water. A vacuum and a soft brush handles the surface; anything more thorough is off-unit work.

The divider between the return and the supply

This is the fault owners rarely check. The air box separates the cold supply air from the warm return air, and if the divider has moved, torn or was never sealed, cold air is pulled straight back into the intake. The unit then reheats its own output, the vent air feels barely cool, and the coil can sit below freezing and ice up.

Dometic's own AirCommand installation instructions show what the part is meant to do and how it is fitted: a polystyrene baffle is cut to the height between the ceiling and the steel base of the unit, with a self-adhesive foam strip on three sides, and it is cut from the side without the foam so the foam seals against the surfaces. Any gap left at that baffle produces the same symptom as a failed divider. When you have the ceiling grille down, this is the thing to look at.

How much return air the unit needs

Furrion publishes a requirement worth checking on any brand, because a return that has been blocked by a curtain, a cushion or a storage bin is a real cause of poor cooling: the return-air opening must have at least 80 square inches of free area including the filter.

The compressor, and where owner work stops

This part exists so you can understand what a technician will do and tell whether the answer makes sense, not so you can do it yourself.

Running amps against the nameplate

The rating plate gives a compressor rated load amps figure, and Dometic's own specification tables publish the comparable numbers for their units, for example 12.5 to 13 amps full load cooling with locked rotor amps of 63 on the 13,500 BTU AirCommand models. The measured current is not fixed, because compressor current rises with outdoor temperature, and Airxcel publishes the correction: the reading should equal the rated figure at 95°F outdoor, and rise by about 0.7 amps for every 5°F above that, falling the same way below it. Airxcel labels its own chart approximate, and so should anyone quoting it.

Why the manuals will not let you test the rest yourself

Every document behind this page says the same thing in different words. Airxcel's notice reads that these instructions are for qualified individuals specially trained and experienced in this equipment, and that persons not qualified shall not service it, adding that service personnel are required by some states to be licensed. Dometic prints that the unit must be serviced by qualified service personnel only, and that this product must be installed by a qualified service technician. Furrion requires that installation and repair be carried out only by qualified personnel familiar with the risks and the relevant regulations, and that any work in the refrigerant system, including recharging, be done by a certified service technician.

A capacitor is the specific reason that warning is not boilerplate. It holds a charge after the power is off, and the manuals treat that as a hazard rather than a curiosity. If you take one thing from this guide, take this: disconnect the power, and do not put your hands inside the control box.

Refrigerant: what the documentation supports

You will read online that these units are sealed and cannot be recharged, so a unit that has lost its charge is scrap. The documentation supports part of that and not the rest.

What is documented

The compressor is hermetic: Airxcel states it is completely sealed and welded together and therefore not internally field serviceable. Airxcel also prints, in capitals, that if you have not been trained in sealed-system refrigeration repair you should not attempt to break into the system. Dometic and Furrion both require qualified or certified personnel for work in the refrigerant circuit, and Furrion adds that the refrigerant must never be released into the air. Manufacturers do publish the charge a unit holds, which tells you the sealed system is a specified, measured thing rather than a black box: Dometic lists 18 oz for one AirCommand model and 24 oz for the larger one, in R410A.

What this page will not tell you

The field procedures for diagnosing a low charge, and the charging procedure itself, are in service documentation this page could not read from the manufacturer's own copy when it was written. Procedures that appear only on third-party mirror sites are not published here, because a mirror can vanish and it is somebody else's copy. If you want the details, ask a technician who has the current service manual, or ask Dometic directly through their documents database. What you should not do is let anyone top the system up without finding out where the charge went, because refrigerant does not leave a sealed circuit on its own.

What you can and cannot conclude from this page

Two things are deliberately absent, and both are gaps in the documentation rather than gaps in the research.

Only one manufacturer publishes a temperature drop that could be verified from the maker's own copy, so the range here is Airxcel's and is labelled as such. And no United States rooftop air conditioner document found for this page publishes an outdoor temperature above which the unit is not expected to cool. If your unit is losing ground on a 105°F day, that is consistent with the documentation rather than proof of a fault, and the manufacturer's own advice in that situation is about reducing heat gain, not about repairing anything.

Sources

• Airxcel / Coleman-Mach, Operation, Maintenance and Trouble Shooting Guide, 1976A455, for the 16 to 22°F temperature differential, the conditions it is measured under, the note that a greater drop can mean restricted airflow, the compressor amp correction chart, the clogged-filter effect, the condenser-coil cause and the heat-gain guidance.
• Airxcel / Coleman-Mach, Service Manuals for Mach 8 Roof Top Heat Pumps, 1976-665, for the hermetic compressor statement, the instruction not to break into a sealed system without training, the qualified-persons-only notices and the capacitor hazard.
• Furrion 8K Rooftop Air Conditioner with Electronic Control, CCD-0008666, for the 80 square inch return-air requirement, the absence of a published temperature differential, and the certified-technician requirement for refrigerant work.
• Dometic AirCommand Rooftop Air Conditioner, AC-1361 and AC-1511, installation, operation and maintenance manual, for the polystyrene baffle detail, the refrigerant charge, the full-load and locked-rotor amp figures, the two to three minute restart delay and the qualified-personnel requirement.
• Dometic Air Distribution Box installation and operating instructions, for the 120 VAC supply requirement, the thermostat range at the air box inlet, and the pre-service checks.