Modding Windshield Defrost Mode

[Jerseyglock]

Not sure if anyone noticed this. Corolla's windshield defrost mode will engage the ac compressor and your mpg will go down. Any mods to manually disable / prevent ac compressor to kick-in. Thanks

[djb383]

The a/c compressor engages on virtually all cars in the windshield defrost/defog mode. If the outside temp is below X degrees, the a/c compressor will not engage.

[Jerseyglock]

With my Honda CR-V, there is a way to disengage a/c compressor (till you disconnect / reconnect battery then resets to factory default) when turn to defrost mode.
As for my 05 Subaru Legacy GT (my sleeper), default is engage a/c compressor (a/c light on) and you can manually turn off a/c button.

Originally Posted by djb383

The a/c compressor engages on virtually all cars in the windshield defrost/defog mode. If the outside temp is below X degrees, the a/c compressor will not engage.

[djb383]

If you need deFROST, (remove ice on the outside of windshield) the outside temp is probably cold enough that the a/c compressor will not engage. However, if it's warm/humid enough to need deFOG, (remove condensation on the inside of windshield) the a/c compressor must engage in order to dehumidify/evaporate the condensation. I wouldn't think you would need the air selector in the winshield position for more than a minute or two, so whether or not the a/c compressor is engaged is kind of a moot point.

[Jerseyglock]

djb383,
How will I verify it "If you need deFROST, (remove ice on the outside of windshield) the outside temp is probably cold enough that the a/c compressor will not engage"

The reason I said this is that, I had setting on defrost mode when I first got the vehicle and I was getting around 31mpg (hwy). On the hunch, I decided to change it back to vent mode and mpg jump to 35mpg (hwy). This tells me that a/c compressor is engaged at defrost mode.

[hungt1999]

Went to the dealer to pick up my BRAND NEW Corolla an hour ago!

The temperature here today is near freezing point and I noticed that turning the knob to the windshield defrost mode did not automatically trigger the AC. You have to manually push the AC button to turn it ON.

[djb383]

There's your answer and use the windshield air selection position only when needed.

[jdhog]

There is a really long thread on this in this forum. Even with the AC button turned off, the defrost/defog position overides it and turns it on (even though the light isn't on). the thread is titled 'so the A/C is on all winter'

The way I've learned to deal with it when needing to defog is to have the vent position in the defog mode but to turn the fan off and just let the incoming airflow direct onto the windshield, thereby hopefully not engaging the AC - I also try to keep the inside of the windshield very clean, even though you have to be a yogi to twist your arm around and wipe it.

[djb383]

...........and hungt1999 just confirmed the compressor is OFF when outside temp is low enough.

[hungt1999]

When I turned the knob to the defrost mode, I think it did not triggger the AC because the AC light was OFF

Someone said the AC would turn ON even if the light does not show

I will confirm that tonite on my way driving home. Usually it is easy to know if the AC compressor is working or not by the sound and an increase in the RPM at idle

[jdhog]

Originally Posted by djb383

...........and hungt1999 just confirmed the compressor is OFF when outside temp is low enough.

If so that's good to know, and/or can you just turn the heat up to prevent the compressor running I wonder. Every time I've had my vents in defog, I get home and watch a trickle of water dripping on my driveway indicating the AC was on. adding: but the outside temp was warm

[pwain]

Ohhh no, another A/C compresor topic.


There was one about a month ago on the same topic that spanned around 5 pages. Pretty good info if you are concerned.

[hungt1999]

Upon a closer look, I confirm now that jdhod above is right, if you turn the knob to defrost mode, I can hear the AC compressor engaging and a slight increase in RPM to about 950 RPM from about 800 even if the AC light did not turn ON. This was tested today in Montreal and we had snow today so the outside temperature was near the freezing point.

Can anyone else confirm this behaviour of the defrost mode?

[jdhog]

uh-oh.....I guess it's a moisture removal thing

FYI - there is a mod for the Tacoma, It's just a simple notch removal from the bezel and the vent seletor switch has 1 extra hidden position which turns OFF the AC but keeps the vents at the winshield. I don't know if it can be done on the trix though, I doubt it.

[Jerseyglock]

Found the link. So, the A/C is on all winter ???