Why Your Rooftop AC Stops Cooling in a Southern California Summer

Put a thermometer in the return air opening and a second one in a supply register, run the unit on high cool for fifteen minutes, and subtract. That difference is the temperature split, and it is the single number that tells you whether the refrigeration circuit is doing its job. A healthy rooftop RV air conditioner produces an 18 to 22 degree split. Anything in that band means the unit is removing heat correctly, whatever the cabin temperature happens to be.

The split is diagnostic because it separates two completely different complaints that owners describe with identical words. A unit producing a 20 degree split in a coach that is still 88 degrees inside is working properly and losing to heat gain. A unit producing an 8 degree split is genuinely failing. Both owners say the air conditioner is not cooling. Only one of them has an air conditioner problem.

A split above about 25 degrees is its own warning sign. That usually means airflow across the evaporator has dropped so far that the coil is over-chilling the small amount of air passing it, which is a restriction problem heading toward a frozen coil, not a sign of excellent performance.

Restricted airflow is the leading cause by a wide margin and it is the cheapest to fix. The return filter is usually a piece of foam or a thin mesh in the ceiling assembly, and after two seasons in the inland valley it is packed with dust to the point that it is functionally a solid panel. Owners who have never removed the interior shroud are often unaware there is a filter at all.

Behind the filter is the evaporator coil, and that is where the real restriction accumulates. Fine dust passes the filter, hits the cold wet coil surface, and cements itself into the fins as a felt-like mat. From below the coil looks fine because you are seeing the outer face. From above with the shroud off you can see that the fin pack is loaded. A coil in that state cannot exchange heat and cannot pass air, and the unit responds by freezing over, which restricts airflow further and drives the whole thing into a spiral.

The other airflow failure is on the coach side. A ducted roof unit pushes air through a plenum and a duct run, and if the duct has separated at a joint or the plenum divider between return and supply has come loose, the unit recirculates its own cold supply air straight back into its return. The split at the register looks acceptable and the coach never cools, because the air conditioner is efficiently cooling a two foot loop of ductwork.

The fan motor run capacitor is the most common electrical failure in a rooftop unit, and heat is what kills it. The capacitor sits in the control box on the roof, where the ambient in a Palm Desert July is well above 140 degrees, and electrolytic components have a service life that halves with every ten degree rise. A capacitor rated for a decade in a temperate climate does not last a decade in Indio.

The symptoms are specific enough to be recognizable. A fan that hums but does not spin, or one that starts only after you nudge the blade through the vent with a stick. A compressor that clicks and then trips on internal overload after a few seconds. A unit that runs fine at night when the roof is cool and refuses to start on a hot afternoon. That last pattern is nearly diagnostic on its own.

A capacitor that has failed visibly, meaning bulged, split, or leaking, is easy to identify. A capacitor that has drifted out of tolerance while looking perfectly normal is not, and that is the more common failure. It has to be measured against its rated microfarad value. Many units use a dual run capacitor serving both fan and compressor, so one failing section produces symptoms that seem to implicate the other motor entirely.

A rooftop air conditioner is the largest single load on a coach and it is the most sensitive to supply voltage. Compressor motors draw enormously more current on startup than while running, and if the voltage sags during that inrush, the motor cannot come up to speed and trips its overload. Below roughly 105 volts at the unit, a compressor will struggle. Below 100 it usually will not start at all, and repeated failed starts damage the windings.

The voltage measurement that matters is at the roof, not at the pedestal and not at the shore power inlet. A 50 foot 30 amp extension cord of marginal gauge, plugged into a 25 foot factory cord, feeding a coach with an aging inlet, can lose eight or ten volts across the run under load. The pedestal reads a perfectly healthy 118 volts. The air conditioner sees 104 and cannot start. Both readings are correct and only one of them describes what the compressor experiences.

Park voltage in Southern California in August is the other half of the problem. A crowded campground in Temecula or Palm Desert on a 108 degree afternoon with every rig running two air conditioners can pull a park distribution system down into the low 100s. This is why owners report a unit that works everywhere except one specific park, and why a hard start capacitor kit sometimes rescues an installation that is otherwise marginal.

The condenser is the outdoor coil, the one under the shroud whose job is to reject the heat the evaporator collected. It only works if outside air can pass freely through the fins, and it lives on a roof, exposed to whatever the last thousand miles put into the air. Units that spend time in Barstow, Victorville, or the Coachella Valley pack their condenser fins with fine grit. Units that park under pines in Idyllwild or Big Bear pack them with needles and pollen.

A loaded condenser raises head pressure, which raises compressor current draw and cuts capacity. The unit runs continuously, the split drops into the low teens, and the compressor spends its life closer to its overload trip point than it was designed to. Eventually it trips on a hot afternoon and the owner is told the compressor is bad, when the compressor was healthy and being suffocated.

Cleaning it correctly means getting the shroud off, which most owners will not want to do, and washing the coil from the inside out with a fin-safe cleaner and low pressure so the debris exits the way it came in. Blasting a condenser coil from the outside with a pressure washer drives the packed material deeper into the fin pack and flattens the fins, which permanently reduces the surface area available for heat rejection.

Air conditioner capacity is a heat removal rate, and it competes against heat gain. A 15,000 BTU rooftop unit in a 35 foot fifth wheel in Indio in August is losing that competition, and no repair changes that. The physics are straightforward: the unit removes roughly a fixed amount of heat per hour, and the coach gains heat through a thin roof, single or dual pane windows, and a slide with an uninsulated floor, at a rate governed by the difference between inside and outside.

If the outside air is 118 degrees, a properly functioning unit producing a 20 degree split against an 88 degree return air is delivering 68 degree air into the coach and is doing exactly what it was built to do. The interior sitting at 88 does not mean the air conditioner is broken. It means one unit cannot overcome that heat gain, and pretending otherwise leads to owners buying replacement air conditioners that perform identically.

What actually shifts the balance is reducing the gain. Reflective window covers on the sun side, a light-colored or coated roof, an awning deployed over the largest window wall, and running the unit continuously from early morning rather than trying to pull the coach down from 105 at four in the afternoon. A second rooftop unit is the real fix on a large coach used in the Coachella Valley, and that is a decision about the coach, not a repair.

On a ducted coach the air conditioner discharges into a plenum that feeds ductwork running the length of the ceiling. That ductwork is frequently a channel formed between the roof structure and the interior ceiling panel, taped and sealed at the factory in a manner that varies enormously by builder. Tape lets go. Panels shift. A duct run over a slide opening can be crushed when the slide seal was serviced by somebody who did not know what was above it.

The signature is uneven cooling that follows geography rather than time. The front registers blow cold and the bedroom register at the far end blows warm regardless of how long the unit has run. Or the register nearest the unit blows hard and every other one is weak. Owners frequently interpret this as an undersized air conditioner, which sends them toward the most expensive possible answer.

Diagnosing it means measuring temperature and airflow at every register and comparing, then getting above the ceiling panel to trace where the loss is. The repair is usually inexpensive once found. Finding it is the work, and it is exactly the kind of thing that justifies a diagnostic hour rather than a parts guess.

Every rooftop air conditioner sits on a foam gasket compressed between the unit base and the roof around a fourteen inch square opening. Four bolts through the interior ceiling assembly pull the unit down onto that gasket, and the correct compression is roughly half the original gasket thickness. Under-tightened and it leaks. Over-tightened and the gasket takes a permanent set, crushes, and then leaks anyway once the foam gives up.

These gaskets fail on a schedule in Southern California. Inland valley thermal cycling works the compression back and forth every single day, and low desert heat degrades the foam directly. A gasket that was correct at delivery is frequently done at six to eight years in Riverside, Corona, or Temecula and considerably sooner in Indio or Palm Desert.

The symptom rarely appears during the failure. It appears with the first real rain, or during monsoon flash rain in the low desert, as a stain on the ceiling panel at a corner of the air conditioner shroud. Because that opening is a fourteen inch hole cut through the roof structure, water entering there reaches the ceiling framing and the wall cavity fast. Anyone servicing an air conditioner should be looking at that gasket while the unit is accessible, since the labor to get to it is already spent.