So my Coleman Mach air conditioner model 49003A879 decided to turn into a rain cloud inside my RV. I'm in Myrtle Beach, with super humid conditions (90s with crazy humidity) and getting pea-sized water drops falling from the ceiling vents at maybe 2 drops per minute, which doesn't sound like much but it's making puddles on my floor.
I've been keeping the main ceiling vents mostly closed to push more air to the back of the coach. Climbed up on the roof expecting to find some fancy drain pump or tubing system, but honestly don't see much of anything obvious.
Is this just how these Coleman units work, or am I missing something? Really hoping this isn't a 'deal with it' situation, but wondering if my closed vents are screwing up the airflow or if there's actually a drainage problem I need to fix.
Coleman Mach AC uses gravity drainage through base weep holes - no pump or tubing. Dripping likely from closed vents disrupting airflow, gasket seal deterioration, or blocked drain path. Try running with vents open first.
Your Coleman Mach air conditioner is dripping condensation through the ceiling vents because you're forcing too much humid air through the ducted system with the main vents closed. This model relies on gravity drainage through that 1/2" gap you observed - the drain pan directs condensation to exit through this gap rather than using tubing or pumps. The solution is to open your main ceiling vents and adjust your cooling strategy, though you should also verify the drainage system is functioning properly.
Your Coleman Mach unit is designed as a basic gravity-drain system, which explains why you didn't find drain pans or tubing on the roof. The condensation forms on the evaporator coil inside the unit and is supposed to drip down and exit through that gap you observed between the unit and roof.
The real culprit here is likely your cooling strategy. By closing the main ceiling vents to force air through the ducting to the rear, you're creating several problems. First, you're creating airflow imbalances that can cause the coil to ice up or run less efficiently, potentially leading to condensation drainage issues. Second, humid Myrtle Beach air is being forced through metal ductwork that may not be properly insulated, causing additional condensation inside the ducts themselves.
In 90-degree, high-humidity conditions like you're experiencing, this model can produce significant condensation. When the airflow is restricted by closed vents, this moisture has nowhere to go except back into your coach through whatever opening it can find, including those slider vents.
The timing you describe - random drops every 30 seconds or so - is classic for condensation overflow rather than a leak. Rain leaks are typically more consistent, while condensation varies with humidity levels and system cycling.
WARNING: Disconnect shore power or turn off the circuit breaker for your air conditioning unit before performing any electrical or roof inspections. Water dripping near electrical components creates serious shock and fire hazards - ensure the area is dry and power is disconnected before any work.
The good news is you probably don't need any parts or tools for this fix. However, if you want to verify everything is working correctly, gather these items:
If you discover any actual drainage issues during your inspection, you might need:
CAUTION: Always disconnect power to the air conditioning unit before performing any inspections or work around the unit. Exercise extreme caution when working on the roof - ensure stable footing and consider having someone spot you.
Start by opening all your main ceiling vents fully. This is counterintuitive when you want air in the back, but it's essential for proper system operation. Run the unit on high fan speed initially to help clear any accumulated moisture from the ducts. You should notice the dripping stop within 30-60 minutes once proper airflow is restored.
Next, inspect the roof gap you mentioned more carefully and check for any clogged condensate drains. Inspect the gap area to ensure it's clear of debris and appears to drain properly, and verify that any drainage channels or outlets are not blocked. The water should flow out smoothly without pooling. If water sits in the gap, you may have a slight sagging issue or debris blocking the flow path.
For better rear cooling without closing vents, try these alternatives: Install a small 12V fan in the rear area to improve air circulation, or partially close some front vents while keeping rear vents fully open. You can also run the air conditioner in cycles rather than continuously - cool the coach down with full airflow, then switch to fan-only mode to circulate air to the rear.
If you still see occasional dripping after making these changes, it's likely normal operation in extreme humidity. You can install a small drip tray under the interior vents as a temporary measure, but the root solution is managing airflow properly.
Contact a qualified RV technician if you discover actual roof seal damage during your inspection, or if the dripping continues after restoring proper airflow for several days. A professional should also handle any electrical issues with the unit, such as fan motor problems that might be reducing airflow.
You'll definitely want expert help if you find the unit is tilting or sagging, which would prevent proper drainage through that gap system. This could indicate roof structural issues that require immediate attention. Similarly, if you suspect your ductwork needs better insulation to prevent condensation, a technician can assess and upgrade your system properly.
Expect to pay $150-300 for a diagnostic visit and minor repairs, or $400-800 if significant ductwork or sealing work is needed. However, based on your description and the fact that you can see the normal drainage gap, this is most likely a simple airflow management issue you can resolve yourself by opening those vents and adjusting your cooling strategy.
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