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Discussion Starter · #1 · (Edited)
I usually leave the A/C at 70 AUTO front & rear. The front drain tube always drips water but I've never seen the rear one drip. I was about to crawl under the car to see if the rear drain was clogged when I decided to first manually crank the rear A/C to LO max, and VOILA, the rear drain started dripping! I set it back to 70 AUTO and then it stopped.

I'm glad the drain wasn't clogged but I'm perplexed by this behavior. I feel like the rear A/C system should be dripping out condensation at 70 AUTO on a hot, muggy day, like the front A/C always does.
 

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The front system gets first crack at the humid air coming into the cabin. It makes a certain amount of sense that the front evaporator will remove moisture down to the temperature you select at the front control, give or take.



TL/ DR:

Air passing through actually draws down to 100% RH at the surface temp of the evaporator, a temp that's below the temperature of the air that's actually passing into the cabin. By the time the rear system gets to work on it, the bulk of the moisture has already been removed.

It's only when you select a rear temp that's below the surface temp at the front evaporator that significant moisture will actually condense at the rear evaporator. Note that the drain tubes themselves are not at cabin temp all the way to the lower ends end. Fir the last several inches, they will be closer to under-the-car temps, so minor moisture amounts may evaporate again in the pipe before exiting the tube. The drains also come from the bottom of the condenser airboxes, so there's only gravity plus the few inches WC of pressure in the box (depends on fan speed...) to cause moisture to discharge.
 
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I'm not sure exactly where the rear drain is located on the Pilot but usually it has 2 wet areas, about 18" apart under the engine/firewall. The Traverse has the rear drain in the rear of the cabin, so I get a drip at the engine and a much smaller drip at the passenger rear mudflap.
 
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