Manually control a/c compressor with defrost on

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.
Thanks!
Bob Simon
Reply to
Bob Simon
In article ,
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???
Reply to
Elmo P. Shagnasty
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 system. 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 is cloudy and un-pressing it when it's clear. But that does not keep the windshield 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 seems to me that when I have defrost on, the compressor runs continuously because it is ALWAYS blowing cold. The AC button doesn't have any noticeable affect at all. If I turn up the temperature knob, the air is heated before delivery but I presume that the compressor is still on.
Reply to
Bob Simon
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.
Reply to
Le Forgeron
Hi Bob,
I know how to find out. Toyota has a technical information service:
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With this service, you can register and pay for access to the shop manuals for your car. Download the schematics and parts of the shop manual that apply to your prius and the problem is solved.
You might also visit PriusChat.com, the Prius c forums. The moderators are very skilled at keeping the 'trash talkers' out. Best of all, you'll be working with folks who own your model car and walk out to the car to see what is there. Registration is free at PriusChat.com
GOOD LUCK! Bob Wilson
Reply to
bwilson4web
In article ,
Huh. Well, how about simply mechanically rigging the damper such that air always flows onto the windshield? Don't know if you can do that, but if so it may be simpler than messing with the electronics.
Well, by definition "defrost" means "remove moisture"--and the removal of moisture (not cooling) is why the air conditioning compressor is there in the first place. To defrost, one must first start with dry air.
To mitigate the cold effect that comes with removing moisture, one can add heat back to the dry air--which is what the system does.
I guess Toyota never considered that anyone would want air on the windshield for general purposes--that any air directed to the windshield is overall intended for the purpose of removing moisture from the windshield.
Ergo, I keep going back to the mechanical solution--can the door that diverts air to the windshield be rigged or removed such that air always goes to the windshield regardless? Would that solve the problem?
Reply to
Elmo P. Shagnasty
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.
Reply to
Bob Simon
for the duration
presume the best way
ase advise how
s for your car. Download the schematics and parts of the shop manual that a pply to your prius and the problem is solved.
e very skilled at keeping the 'trash talkers' out. Best of all, you'll be w orking with folks who own your model car and walk out to the car to see wha t is there. Registration is free at PriusChat.com
Bob, Thanks for the useful reference. Since this project will be a stretch for me, I'd like to like up some potential technical assistance. Are you aware of a newsgroup where Toyota electrical mods are discussed?
Reply to
Bob Simon
d for the duration
I presume the best way
lease advise how
als for your car. Download the schematics and parts of the shop manual that apply to your prius and the problem is solved.
are very skilled at keeping the 'trash talkers' out. Best of all, you'll be working with folks who own your model car and walk out to the car to see w hat is there. Registration is free at PriusChat.com
r me, I'd like to like up some potential technical assistance. Are you awa re of a newsgroup where Toyota electrical mods are discussed?
line up
Reply to
Bob Simon

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