It's important to keep in mind that there is a temperature sensor on the
evaporator of both of these vehicles that will not alow the A/C to engage at
any temperature that would cause the system to freeze up. So your -25
scenario is a non issue. Even though the system is asking for the A/C to be
on the sensor won't let it below approx 32 F.
Another stupid button that for 99% of drivers will get left permanently in
I remember reading about this "turn on A/C when defrost is on" feature back
the 70's on GM vehicles, I think. It was stupid then and is still stupid
only time it significantly shortens the defog time is when the car is cold
coolant temp is low and the air temp is above freezing.
The controls need to be designed so that the A/C can be optionally switched
on during defog, by the driver. Then, the minority of drivers that would
any benefit can make use of it, and the rest of us can just wait until our
It could come on under certian conditions in cold weather if you're sitting
still and you get a slug of hot air from the engine box that is recirculated
into the HVAC opening which would cause the sensor to read high.
Ted: It'll never be made an end user feature since it would drive up
warranty and JD Power numbers for window fogging complaints when people
forget to turn it back on....
Hi Sam Man,
Thanks for your thoughts but I must diagree about the safety benefits
of having A/C during defrost.
Here in New England during times of fairly cool days and high outside
humidity, having the A/C on during defrost causes the OUTSIDE of my
windshield to fog up due to the drop in glass temperature.
Then I have to run the windshield wipers in order to see and/or add
enough heat to reheat the glass making the interior temperature rise
to the uncomfortable level. Overall, it's a ridiculous situation that
also wastes energy and drops fuel mileage.
I've see to A/C come on during defrost down to about 50 degrees F -
well below the temp needed to start outside frosting of the glass.
Since all HVAC functions are controlled by software in the control
module, I'm sure that it can be reprogrammed to stop this function.
However, it may well take the use of the dealer programming device and
someone who knows the software coding to do it.
Obviously, unplugging the compressor is an alternative but not the
ideal one. Once my warranty expires I may well add a switch.
It is the accepted best practice of the industry. Of course there are
always some conditions which could be compromised by the strategy. We have
discovered that this strategy benifits the majority of our customers...
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