Air conditioner
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- Which unit is on the roof? Read the label on the shroud or the data plate.
- What is the complaint? Pick what the customer sees.
- Is the fan running and air moving? Set the thermostat to COOL and the fan to ON. Feel the air from the vents. Expect: Strong airflow from the vents
- Is the compressor humming or starting? Listen at the unit on the roof. Set the thermostat below room temp and wait up to 3 minutes: many units have a restart delay. Expect: Click, hum, then a steady run
- Check the start capacitor Power OFF and unplugged. The start device is near the compressor: a round can (PTCR) or a potential relay with a cap. Look for a bulged top or burnt wire, then meter the cap. Expect: A reading near the label on the cap
- Is there 120v at the compressor? Qualified person: meter the compressor feed while it should be running. Expect: About 108 to 132 V AC
- How does the coil look? Check the return air filter and the evaporator coil for dirt or ice. Then check the condenser coil outside. Expect: Clean coils, clean filter
- Meter the run capacitor Power off, unplugged. Discharge the cap with an insulated screwdriver across the terminals, pull the wires, and meter microfarads.
- Where is the 120v lost? Check the breaker, the shore power, and the transfer switch.
- Measure the delta-t Run it for 15 minutes, then measure the air going into the return and the closest supply vent. Expect: About 16 to 22 F difference
- Why the short cycles? Check for a frozen coil, low voltage, or a bad start device. Note how long it runs before it quits.
- Why is it freezing up? Ice on the coil means too little air or too little charge. Check the filter, the fan, and the charge. Expect: Clean filter, full airflow
- Why is the fan dead? Check 12V to the thermostat and the control board, then 120V at the fan motor. Expect: 12V control, 120V at the fan
- Airflow is restricted? Check the return filter, the duct, and the squirrel cage. Feel how much air moves. Expect: Strong airflow with a clean filter
- Ohm the compressor windings Pull the terminal cover. Ohm C to S, C to R, and S to R. Then each terminal to the copper line.
Causes and repairs
Control relay or board
The thermostat calls but the compressor never gets the signal.
Tools: multimeter
Meter the relay or board output to the compressor. Replace the failed control.
Replace the start device
A weak or bulged start cap or PTCR causes hard-start and short cycling.
Tools: multimeter, screwdriver
Note the voltage and the exact microfarad number on the label. Replace with the same rating, wire the same way, and retest.
Clean the coil and filter
A dirty filter or coil starves airflow and kills cooling.
Tools: coil cleaner, soft brush
Wash the filter, clean the evaporator and the condenser coil, clear the condensate drains, and retest.
Low charge or sealed-system fault
Clean coils, strong airflow, and still little or no cooling point to low refrigerant or a restriction. Roof units are hermetic: there is no service port from the factory.
Tools: gauges, leak detector
Before you call it: confirm delta-T is low (under 16 F) with a clean filter and coils and full fan speed. Feel the suction line at the compressor after 15 minutes: it should be cool and sweating. Warm or dry suction line with good airflow points to low charge. Roof units have no service ports, so a licensed tech has to add one, check pressures, find the leak and charge by the weight on the data plate.
Breaker trips
A tripped breaker points to a short, a locked compressor, or too much amp draw.
Tools: multimeter, clamp meter
Power off. Meter the compressor and fan for a short, then clamp the amps on a start. Look for a loose backstab connection.
Thermostat or control
The thermostat never calls for cool.
Tools: multimeter
Check the thermostat batteries and the set point. Meter its output. Replace the thermostat or the relay/board.
Low voltage at startup
Voltage that sags at the compressor makes it drop out.
Tools: multimeter
Check the pedestal voltage, the cord ends for heat, and every connection. A melted plug or a hot neutral is a common find.
Ac is cooling to spec
The delta-T is in range, so the AC is working.
Tools: none
Look at what the customer actually feels: sun load, duct balance, or the room was hot and needs time to pull down.
Fan motor or capacitor
A slow or stopped fan over-cools the coil and ices it.
Tools: multimeter
Meter the fan run capacitor, check the motor spins free, and replace the failed part.
Fan motor or relay
The controller sends the call but the fan stays dead.
Tools: multimeter
Meter the fan relay output, then the motor. Replace whichever will not carry the load.
Run capacitor weak
A weak run cap makes the compressor hum, run hot or trip.
Tools: Hand tools
Replace it with the same microfarad rating and the same or higher voltage. Mark the wires before you pull them. Retest and clamp the amps on start.
Start device or hard start
Windings and cap are good, but it won't start or it hums.
Tools: Clamp meter, multimeter
Check the start device (PTCR or potential relay with start cap). A PTCR reads low ohms when cold; open means replace it. Clamp the common wire during a start: amps way above the label RLA that never drop means the compressor is locking. A hard-start kit can help a marginal unit on a long cord or low pedestal.
Winding open: let it cool first
An open reading can be the internal overload, which opens when the compressor is hot.
Tools: Multimeter
Let the compressor cool for an hour or two and ohm again. Still open: the compressor is bad. Rooftop units are usually replaced, not repaired.
Grounded compressor
A winding reads to ground, so the compressor is shorted.
Don't keep resetting the breaker. The compressor has failed: replace the rooftop unit.
Keep going on air conditioning
Learn it
Prove it
- AC hums and trips
- AC runs but barely cools
- AC ices up and barely blows
- AC kicks off on hot afternoons
- AC does nothing, thermostat is blank