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Manually control a/c compressor with defrost on

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Posted by Bob Simon on April 14, 2015, 3:00 pm
2012 Prius C, Trim Level 3

I would like the ability to keep the windshield defrost button engaged for  
the duration of a trip while manually cycling the compressor on and off at  
will.  I presume the best way to do this involves overriding the signal to  
the compressor relay.  Please advise how this can be done or suggest a bett
er approach.

If the modification involves working with an IC, I will need the exact loca
tion of the device.  Please also include the part # and names of the signal
 pins I need to work with if you have them.

This will be my first mod to my car but I have steady hands and an adjustab
le power soldering pencil.

Bob Simon

Posted by Elmo P. Shagnasty on April 15, 2015, 10:48 am

ummm.....you're WAY overthinking this.

Don't worry about the windshield defrost button.  Just manually control  
the climate system.  Turn the temp to where you want it, turn the  
airflow to where you want it (in this case the windshield), and turn the  
AC button on and off at will.

You DID read your owner's manual, right?

Why did you immediately go to soldering onto a circuit board???

Posted by Bob Simon on April 15, 2015, 5:39 pm
 On Wednesday, April 15, 2015 at 5:48:58 AM UTC-5, Elmo P. Shagnasty wrote:







Elmo, thanks for the reply.  It is possible that there's a better way to do
 what I want but your suggestion does not do it.

There isn't an option to set the air flow to both head + windshield on my s
ystem.  Airflow toggles between head, head + feet, feet, feet + windshield.
  I've been doing something like what you suggest by setting the airflow to
 head then intermittently pressing the defrost button when the windshield i
s cloudy and un-pressing it when it's clear.  But that does not keep the wi
ndshield damper open when defrost is off, which is what I'm trying to fix.

Also, although I have not verified this by checking under the hood, it seem
s to me that when I have defrost on, the compressor runs continuously becau
se it is ALWAYS blowing cold.  The AC button doesn't have any noticeable af
fect at all.  If I turn up the temperature knob, the air is heated before d
elivery but I presume that the compressor is still on.

Posted by Le Forgeron on April 15, 2015, 8:12 pm
 Le 15/04/2015 19:39, Bob Simon a écrit :

Of course you know how windshield demist (not defrost) is supposed to
work, don't you ?

For the innocents:

The air is cooled to have it condense any extra humidity, liquid water
is collected and throw away, then cooled air can be heated back a bit:
hotter air have a greater acceptance for water (heating cooled air makes
it dry if it cannot take back the water that went liquid during
cooling), so when meeting the windshield, the mist (condensed water on
glass) is transferred to the air (as invisible vapour), thus demisting
the windshield.

What you are asking (stop the compressor on demand, while demisting) is
a scientific non-sense. It can work a few moments, playing on the
inertia of the cold block, but on the long run it will just fail to
demist, as wet air to the windshield will not remove the current liquid
water (the white mist) from it, it might even provide more. (as the
windshield is exposed to external wind... it become the coolest place,
and that's where water in the inside air will condense).

If the outside of the car is a rainy tropical forest, you might consider
using closed-loop AC/demist instead of open-flow intake.
It might be wise considering too when inside a large mist like SF
sometime have. In the closed-loop situation, once demisted, the living
bodies inside the car are the only source of additional mist: you could
probably, once windshield is clear, just stay in AC mode (so no full
time compressor). The more passengers, the more humidity.

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Posted by Bob Simon on April 16, 2015, 5:55 pm
I disagree.  I expect that ambient air moving across the windshield will do some good for at least 15 seconds, which is my desired minimum on or off duration.

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