I'm pretty new to RV maintenance and could really use some guidance. I have a 2012 King Aire 4584 and recently tried replacing the run capacitor on my rear A/C unit because it was only running for about two minutes at a time and the compressor was getting really hot.
Now I'm in a worse situation - the unit won't start at all, even after I put the old capacitor back in. I'm wondering if there are some basic things I should be checking that I might have missed? Could I have blown a fuse or tripped something during the replacement? The compressor was pretty hot when I was working on it.
I'm hoping there might be some kind of reset button or other troubleshooting steps I should try before calling a repair tech. Any advice would be really appreciated!
When your RV A/C won't start after a capacitor change, check for blown fuses, thermal switch reset, proper wiring connections, and overheated compressor lockout. The compressor may need time to cool down before restarting.
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SAFETY WARNING: Before attempting any repairs, always disconnect power at the A/C unit's dedicated breaker in the main electrical panel and remove the thermostat's power wire connection. Capacitors can hold dangerous charges even when power is off - discharge them safely using a proper capacitor discharge tool or by carefully connecting a high-value resistor (20,000+ ohm, 2+ watt rating) across the terminals with insulated tools. Do not work on refrigerant systems unless you are EPA certified. Use proper electrical safety equipment and procedures throughout.
Your King Aire's rear A/C unit likely has a thermal protection switch that tripped when the compressor overheated during those two-minute run cycles. Even though you've correctly identified and replaced the run capacitor (which was smart thinking given the symptoms), the unit won't restart because this safety switch is still open and preventing the compressor from receiving power. The fact that your compressor was running very hot before the capacitor change confirms this - those short cycles were the system's way of protecting itself from damage, and now it's locked out completely until the thermal switch resets or is manually reset.
Unfortunately, simply swapping capacitors back and forth won't solve this issue because the underlying problem isn't just the capacitor anymore. When a compressor overheats repeatedly, it can cause the internal thermal overload protector to stay open even after the unit cools down. Your King Aire likely uses a Dometic or Coleman Mach A/C system, and these units have multiple safety mechanisms that can prevent restart after an overheating event.
The good news is that this is often fixable without replacing the entire compressor, but it requires a systematic approach to reset the thermal protection and address what caused the original overheating. The bad news is that there might be additional issues beyond just the capacitor that caused the overheating in the first place, such as low refrigerant, a dirty condenser coil, or a failing compressor that's drawing too much current.
The thermal protection system in your rear A/C unit is designed to prevent catastrophic compressor failure by shutting down the system when temperatures exceed safe operating limits. When you were experiencing those two-minute run cycles, the compressor was likely overheating due to a failing run capacitor that couldn't provide adequate starting torque. This forced the compressor to work harder, draw more amperage, and generate excessive heat.
Here are common causes of this cascading failure in RV A/C systems (listed in order of likelihood and ease of diagnosis):
The thermal protection in your system likely consists of both an external overload protector (usually mounted on or near the compressor) and internal thermal protection built into the compressor windings. When either of these trips, the compressor won't start until the protection resets. Reset mechanisms vary by manufacturer - some reset automatically when they cool down, others require manual reset, and many are non-resettable and must be replaced when tripped. Consult your specific unit's service manual to determine the correct reset procedure.
Additionally, your King Aire's A/C system probably has a control board that monitors various parameters and can lock out the system if it detects problems. These boards often have their own protection circuits that can prevent restart after certain fault conditions, even if the mechanical thermal protection has reset.
Before diving into repairs, you'll need to safely diagnose what's preventing your A/C from starting and determine if there are underlying issues that caused the original overheating. Start by ensuring you have the right tools and safety equipment, as you'll be working with electrical components and potentially refrigerant systems.
Essential tools and materials you'll need:
Safety is paramount when working on RV A/C systems. Always disconnect power at both the thermostat and the main electrical panel before opening the A/C unit. The capacitors can hold dangerous charges even when power is off, so discharge them safely using insulated pliers with a high-value resistor or a proper capacitor discharge tool. Never work on the refrigerant system unless you're EPA certified - this diagnosis focuses on electrical components.
Essential diagnostic steps (perform in this order):
Start your diagnosis by locating the A/C unit's main electrical panel, typically accessed through a cover on the roof or inside a ceiling-mounted return air grill. You'll need to identify the compressor, condenser fan motor, run capacitor (which you've already worked with), and any visible thermal protection devices. Take photos of all wire connections before disconnecting anything, as improper reconnection can cause additional problems.
Check the basics first: ensure the thermostat is calling for cooling, verify that 120V power is reaching the A/C unit, and confirm that the control board (if equipped) is receiving power and not showing any error codes or fault indicators. Many newer RV A/C systems have LED diagnostic lights that can indicate specific problems.
Begin your repair process by systematically checking and resetting the thermal protection systems in your A/C unit. The most common location for the external thermal overload protector is mounted directly on the compressor housing or attached to the compressor's electrical terminal box. Look for a small, round or rectangular device with two wire terminals - this is likely your thermal overload protector.
To reset the thermal protection:
Next, address the control board reset if your unit has electronic controls. Many RV A/C systems have a control board reset procedure that involves either pressing a reset button on the board or cycling power in a specific sequence. Check your King Aire owner's manual for the specific reset procedure, but a common method is to turn off power for 5 minutes, then restore power while holding down the thermostat's fan button for 10 seconds.
If the thermal protection appears to be reset but the unit still won't start, you'll need to check for other issues:
If you discover that the thermal overload protector needs replacement, this is typically a straightforward job. Purchase the correct part for your model (verify with dealer) and connect it using the same wire positions as the original. Make sure the new protector has the same temperature and amperage ratings as the original - this information is usually printed on the device itself.
For compressor winding tests, you should see resistance readings between terminals but infinite resistance from any terminal to the compressor case (ground). If you find a short to ground, the compressor has internal damage and needs professional service or replacement. Normal resistance readings vary by compressor size, but typically range from 1-10 ohms between terminals.
After completing any repairs and resets, restore power and attempt to start the system. Monitor the compressor temperature and run time carefully during the first few cycles. If it immediately overheats again or trips the thermal protection, there's likely an underlying issue such as low refrigerant or internal compressor damage that requires professional attention.
While many thermal protection resets and electrical repairs are within the scope of DIY RV maintenance, certain situations require professional HVAC technician expertise. If your troubleshooting reveals that the thermal protection resets properly but the compressor still won't start, or if it starts but immediately overheats again, you're likely dealing with refrigerant system issues or compressor internal damage that requires specialized equipment and EPA certification to address properly.
You should definitely call a professional if you encounter any of these conditions:
When selecting a repair technician for your King Aire, look for someone with specific RV A/C experience rather than just general HVAC knowledge. RV systems have unique challenges including vibration, space constraints, and different installation requirements compared to residential units. Many RV dealerships have certified technicians, or you can contact Dometic/Coleman directly for authorized service centers in your area.
Expect professional diagnostic fees to range from $75-150, with thermal overload protector replacement typically costing $100-200 including labor. If the compressor itself needs replacement, costs can range from $400-800 depending on your specific unit and local labor rates. However, many thermal protection issues can be resolved for much less, especially if the underlying cause was simply the failed capacitor you've already addressed.
Before calling for service, document all the troubleshooting you've already performed, including voltage readings, what components you've tested, and any observations about system behavior. This information helps the technician diagnose more efficiently and can reduce diagnostic time and costs. Also, gather your A/C unit's model and serial numbers, which are typically found on a label inside the unit or on the exterior housing.
Remember that intermittent overheating issues like you experienced can sometimes indicate that your A/C system is reaching the end of its service life. If your 2012 King Aire's rear A/C has been working hard for over a decade, it might be worth getting a professional assessment of the entire system's condition rather than just fixing the immediate problem. Sometimes investing in a new A/C unit provides better long-term value than repeatedly repairing an aging system, especially if multiple components are beginning to fail.
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