As mentioned above, there are already some threads already about it. There is also somewhere mentioned that the A/C sometimes is working (when on Auto) even when the "A/C On" light is not lit up.One month and one thousand miles in, I hear what I think is the click of the A/C clutch engaging even with the A/C off. Anyone else?
This article will appear in the December 2011 Honda Service News.
Here is the individual article:
A/C Comes On By Itself? Could Be the Fog Risk Minimization Logic
Currently Applies To: ’11–12 Odyssey with climate control
Got a vehicle in your shop because the A/C compressor comes on even if the climate control unit display
says it’s off? It’s really noticeable if the temperature was set to LO or to a cool temperature.
There might not be anything wrong here; it just might be the climate control system’s fog risk
minimization logic at work. If its sensors detect that windshield fogging is likely, the system automatically
takes the needed action, including running the A/C, to help maintain good visibility.
This little known feature determines the likelihood of windshield fogging based on the difference between
the calculated windshield glass temperature and the dewpoint inside the vehicle—we call it the fog
The fog margin is determined using these sensor inputs:
• Outside air temperature sensor
• In-vehicle temperature sensor
• Humidity sensor
• Vehicle speed from B-CAN data
• Climate control unit logic
If the fog margin is in the range of 2–6° F (1–3° C) (this spec varies by model), fog risk minimization has
priority over automatic and selected climate control settings. The A/C compressor runs, even if the climate
control unit display reads A/C OFF. It doesn’t run, though, if the climate control system is turned off.
You can read the fog margin value using the climate control unit’s input display mode. It’s listed as
sensor d and reads in °C. For info on how to access it, enter CLIMATE CONTROL in ISIS, scroll down to
How to Troubleshoot the Climate Control System, and go to Displaying Sensor Inputs at the Climate
Control Unit. If the fog margin reading is in the range of 1–3° C, the fog risk minimization logic is at work.
If you still think you need to troubleshoot a suspect vehicle, first, make sure the climate control system is
working right. Hook up your HDS PC and MVCI, and go to the PGM-FI Data List. Look at the A/C SWITCH
and A/C CLUTCH signals while duplicating the conditions that caused your customer’s complaint
(assuming you can). If both signals read ON (turn green) just before the compressor and fans cycle on,
you’re looking good. But if you see something different, then it’s troubleshooting time.