RV Air Conditioner Service & Replacement Guide (Dometic / Coleman-Mach)

RV Air Conditioner Service & Replacement Guide (Dometic / Coleman-Mach)

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RV Air Conditioner Service & Replacement Guide (Dometic / Coleman-Mach)

Your rooftop AC is one of the hardest-working systems on your rig. It runs for hours straight while you’re parked in a scorching Arizona campground, and it gets rained on, hailed on, and baked by the sun for years at a stretch. Whether you have a Dometic Brisk II, a Coleman-Mach 8, or an older unit that came with the coach, this guide walks you through everything from a simple filter cleaning all the way to swapping out the entire unit — with every part, tool, and step laid out clearly.

Required Parts & Tools

Step-by-Step Instructions

Step 1: Diagnosing Your RV AC Problem

Before climbing on the roof or ordering parts, take 15 minutes to systematically diagnose what is actually wrong with your AC. The most common failures fall into three categories: the unit runs but doesn’t cool well, the unit tries to start but trips a breaker or blows a fuse, or the unit is completely dead. For poor cooling, start by checking the return air filter — a clogged filter is responsible for more weak-cooling complaints than any other single issue. Remove the interior ceiling assembly (usually held by two or four thumbscrews) and hold the filter up to light. If you can’t see through it, that’s your problem. For units that trip breakers, the culprit is almost always a failed run capacitor — this is a cheap, 15-minute fix. For units that are completely dead, test for 120V power at the AC unit using a non-contact voltage tester before doing anything else. Also check the dedicated 15- or 20-amp breaker for that circuit in your main panel. If you have power at the unit but nothing happens, the problem is inside the unit. Also note: if your AC cools fine on shore power but not on a generator, your generator may not be producing enough clean power — test with a multimeter set to AC voltage at the generator outlet.

Step 2: Seasonal Maintenance: Cleaning Filters and Shroud

Every spring before your first trip, and again mid-season if you camp heavily, perform a basic AC service. Start on the interior: remove the ceiling return air grille/filter, wash the filter with warm water and mild dish soap, rinse thoroughly, and allow it to dry completely before reinstalling — never run the AC with a wet filter. Inspect the evaporator coil fins visible above the filter; if they’re coated in dust and debris, use a coil cleaning spray (like Nu-Calgon Evap-Fresh) applied per the can’s directions, then rinse with the water allowed by that product. On the exterior, climb safely to the roof with a rubber-soled shoes and a bucket of supplies. Remove the four corner bolts or screws securing the plastic shroud (the outer cover). Lift the shroud straight up and off. Inspect it for cracks — UV exposure causes shrouds to become brittle, and a cracked shroud lets rain inside the unit. Inside the shroud, use a garden hose on low pressure to rinse out the condenser coil fins from the inside out (spray from inside toward outside), dislodging road debris, cottonwood fluff, and insect nests. Use a soft brush to dislodge stubborn material. Straighten any badly bent condenser fins with a fin comb. Reinstall the shroud snugly and tighten all bolts.

Step 3: Capacitor Testing and Replacement

The run capacitor is a cylindrical component inside the AC’s electrical compartment that keeps the compressor and fan motors running smoothly. When it fails, the AC will typically hum for a few seconds and then trip the breaker, or will start but compressor and cooling won’t kick on. Before touching any capacitor, disconnect shore power or turn off the generator, then wait at least five minutes — capacitors store electrical charge and can shock you even after power is removed. Access the electrical compartment by removing the shroud and then locating the small metal box or cover with the wiring. The capacitor looks like a small silver or tan cylinder with two to four terminals labeled HERM (hermetic/compressor), FAN, and C (common). Using an insulated screwdriver, carefully short across the HERM-to-C terminals and then FAN-to-C terminals to discharge any stored energy. Using a multimeter with a capacitance setting (µF), measure the capacitor value and compare it to the rating stamped on the side. A reading more than 10% below the rated value means the capacitor has failed and should be replaced. Note the µF rating and voltage rating printed on the old capacitor (e.g., 35/5 µF at 370VAC). Purchase an exact or higher-voltage replacement, then reconnect the terminals exactly as they were — take a photo before disconnecting anything.

Step 4: Cleaning Evaporator and Condenser Coils

Dirty coils reduce cooling efficiency dramatically — even 10% coil blockage can cut BTU output by 20% or more. Cleaning coils twice a season is one of the best investments of time you can make for your AC’s lifespan. For the condenser coil (the large coil on the outside of the unit, visible once the shroud is removed), use a gentle water spray from the inside surface outward. This forces debris out the way it came in rather than pushing it deeper into the fins. For stubborn organic material like dried grass or insect nests, apply a dedicated coil cleaner foam, let it dwell for 5-10 minutes as directed, then rinse. Do not use a pressure washer — you will bend the delicate aluminum fins. For the evaporator coil (inside the ceiling assembly, above the air filter), the access is more limited. Use a no-rinse evaporator coil spray (Evap-Fresh or equivalent) applied generously; condensation during normal AC operation will rinse the solution into the drain pan. If the drain pan itself has algae or standing water, clean it with dilute bleach solution and ensure the drain hole is clear — a clogged drain pan overflows water into your ceiling. Check the drain hole by pouring a small cup of water into the pan and watching it drain.

Step 5: Thermostat Replacement

RV thermostats generally last many years, but they do fail — most commonly showing as the AC never shutting off, the AC never starting despite the unit being powered, or temperature readings that are obviously wrong. Before replacing the thermostat, verify it’s actually faulty by measuring the room temperature with a separate thermometer and comparing it to the thermostat’s display. To replace it, first identify whether you have a simple two-wire "dumb" thermostat or a multi-wire unit that also controls a furnace and a zone system. The wiring is color-coded and must be duplicated exactly on the new thermostat. Turn off power to the AC, remove the thermostat face from the wall (usually one screw at the bottom or snap-off cover), and photograph the wiring before removing any connectors. Purchase a direct replacement for your brand (Dometic SXT or Coleman-Mach Bluetooth, for example) or a compatible universal unit. The Dometic Single Zone LCD thermostat is compatible with most older Dometic units and installs with just the existing wiring — no cutting or splicing required. After installing the new thermostat, set the mode to COOL, set the temperature 5 degrees below the current room temperature, and verify the AC unit cycles on within 3 minutes.

Step 6: Installing a Soft-Start Kit

If your AC unit trips the breaker when running on a generator or struggles to start on lower-amperage pedestal power (especially 20-amp or weak 30-amp service), a soft-start capacitor kit is the answer. Products like the MicroAir EasyStart 368 or the SoftStartRV device limit the startup surge current from the typical 50-70 amps down to under 20 amps, letting a 2,000-watt generator or shared 30-amp pedestal handle the load. Installation is straightforward: turn off all power to the AC unit, locate the compressor wiring inside the electrical compartment, identify the HERM and C terminals on the existing run capacitor, and wire the soft-start device in parallel according to its specific instructions (the MicroAir has a labeled diagram in the box). Most installations take under 30 minutes and require only a screwdriver and wire stripper. After installation, test on your generator — the AC should start cleanly without a voltage dip or breaker trip. Note: a soft-start kit does not improve cooling performance; it only reduces startup amperage draw.

Step 7: Full AC Unit Replacement

When capacitor replacement and cleaning aren’t enough — compressor is seized, refrigerant has leaked, or the unit is simply over 15 years old — a full unit replacement is the most reliable fix. The physical swap is very manageable as a DIY project. First, measure the cutout dimensions in your roof (most are 14" x 14" or 14.5" x 14.5") and verify the replacement unit fits your mounting footprint. Turn off and unplug all power. On the roof, disconnect the existing unit’s power cable (typically entering the unit from below through the ceiling), remove the four to six mounting bolts that secure the unit to the roof, and have a helper inside ready to hold the interior ceiling assembly as the roof unit is lifted off. The new unit drops into the same opening. Critical: replace the foam roof gasket that seals the unit to the roof — this is what keeps rain out and is always included with a new unit but must be installed dry and clean. Torque the mounting bolts in an X-pattern to 35-40 in-lbs to compress the gasket evenly without over-crushing it. Reconnect the power cable, restore power, and test the unit through a full cooling cycle. Run it for at least 20 minutes and verify cold air is reaching the return vents inside the coach.

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