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Hot air from vent when climate ctrl is OFF

SL-Class (R230) 2003 -- 2012: Discussion on the SL500, SL550, SL600

Hot air from vent when climate ctrl is OFF

Old 07-20-2018, 01:27 PM
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Camry 2.0, CLK320, SL350, SL63
Hot air from vent when climate ctrl is OFF

Hi there. I got a 2003 SL350.


- When climate control is off (zero bottom pressed. No AC and no fan) , there is a little bit warm air coming out from vent and foot well.

- I can feel the temperature increases when engine temp increases.

- The amount of air increases as I speed up. But this amount is still less than the air flow at lowest fan speed if climate control is on.

- This happens to both driver side and passenger side. Hot air leaks out from both front vents and foot well vents regardless the air flow mode I set before turning off climate control. Maybe hot air also comes out from the vent right underneath windshield but I haven't tried to observe it.

I think somehow there is a small open path from engine compartment to air vent inside the cabinet and thus as car moves outside airs naturally flow into cabinet. Has anyone experienced such problem?

My AC’s air flow mode is working fine. Meaning that when AC is on, cold air does come out from the vent specified by the direction knob. Hence I think step motor or regulators are working okay. I suspect that there is a sealing mechanism for these regulating flaps when AC is off and mine has failed to do so? Or maybe there is an extra set of flap that is supposed to block air flow when AC is off and is not sealing properly? Any comment is highly appreciated.


Last edited by a32amg; 07-20-2018 at 01:31 PM. Reason: Change formatting for better readability
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Old 07-23-2018, 11:20 AM
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2003 sl 500
warm air on floor

On my 2003 sl500 I have noticed the same thing. Mostly floor air a bit warm.. I do not notice this at present. More noticeable in cooler weather. Hot here now. But A/C and heater work very well. No warm air output with A/C on. System problem shows when "0" ( red light)( system totally off) is illuminated. I have noticed this with top down also. I am not sure this is a real malfunction though.

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Old 07-24-2018, 06:40 PM
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2005 SL500
I would say it might happen just because of the excess heat coming from the engine. Even with the AC/system set to "0" the air still can go through, since the vents are not vacuum sealed. it will be noticeable both in summer and winter for same reason - engine is hot and air flows.

I experienced same thing, but can't say the heat was to a level for me to raise questions. It's normal for air to circulate from engine bay inside, it just happens to be hot all the time. A good reason for me is that there is hot air only, as you said - no matter what temperature you set it to. If the system would be working even you press "0" then there would have been cold air coming out when you set to cold.
Also a good reason for me is that when you turn on the AC in the summer, even if the temperature is set to ~60 degrees, first second or so the air coming out is still hot. always in all the cars.

Food for thought
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Old 07-25-2018, 10:51 AM
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CURRENT: 2011 SL550, 2010 ML350, FORMER: 2007 CLK550 Cabriolet, 2002 C240, 2001 ML320, 1984 300TD
I agree with Ghostty. I think it's just hot "ambient" air from around the car being drawn into the ventilation system. Note that all cars do draw some outside air into the cabin, even with the climate control off or in RECIRC mode. This is a safety requirement to keep passengers supplied with some fresh air.
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