Front Defrost doesn't work well, Bad Visibility, 2012 Rav4 limited - Page 2 - Toyota RAV4 Forums
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#11 (permalink) Old 01-02-2013, 06:29 PM
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Originally Posted by CanuckRAV4 View Post
Any merits?
Meritorious I would say.

I think we can agree that measuring the temperature in the box would be best to avoid icing. However outside air at 40F can pretty much only be 100% or a little more relative humidity. When the heater raises that to 60F or more, that air is pretty dry. Our RAV4s will not allow recirculate in the defroster positions.

Not running the compressor in Defrost settings when the coil temperature OR the outside temperature is low enough would make sense too.

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#12 (permalink) Old 01-02-2013, 07:45 PM
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If the outside temperatures are below freezing, that air will be extremely dry as virtually all of the moisture will have already condensed out - no need for the air conditioner to dehumidify that air, it's being done my mother nature already.

The volume inside the Rav is pretty small if you have wet stuff and one or more folks breathing in there. Warming the interior without exhausting all the extra moisture via turning off the recirculating function will quickly make a mini sauna inside!

Even when it's raining and warm outside, I find the defog always works better with recirculate turned off, with or without the air turned on.

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#13 (permalink) Old 01-03-2013, 08:22 AM
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I believe that the compressor is always running as it is a variable displacement type.

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#14 (permalink) Old 01-03-2013, 12:52 PM
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As you know I have paid very close attention to the behavior of my Rav while running a/c. In the morning when it's below freezing my RPMs do not jump from 700 to 900 when I turn on defrost. This has been tested when the engine was warm and driving 20 miles to work.

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#15 (permalink) Old 01-03-2013, 03:10 PM
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Originally Posted by Rickl View Post
The sensor is adjacent to the evaporator coil, which is behind the glove box. The sensor will shut off the compressor when evaporator temperature drops to about 4'C (39'F) approx. The reason for this is to prevent ice forming on the evaporator which would block air flow through it.

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The ambient temperature sensor is located at the front of the vehicle in the front part of the A/C condenser. It connects to the A/C Amplifier, which is the key part of the A/C control unit. This is separate from the evaporator temperature sensor, which is what you described, and is also connected to the A/C control unit.

This is according to the shop manual.
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#16 (permalink) Old 01-03-2013, 03:33 PM
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The ambient temperature sensor is located at the front of the vehicle in the front part of the A/C condenser. It connects to the A/C Amplifier, which is the key part of the A/C control unit. This is separate from the evaporator temperature sensor, which is what you described, and is also connected to the A/C control unit..
There is also an interior temperature sensor,and a solar intensity sensor, also feeding into the A/C control unit (found on the Limited models). These sensors allow feed-forward control of the HVAC system

Large differences between inside and exterior temperatures (compared to the set point) results in larger displacement of the "variable displacement" compressor, or more heat from the heater. When the solar sensor detects more sunshine it also increase cooling because it anticipates an increase in interior temperature even though the interior sensor has not yet detected a temperature rise.

The sensor on the evaporator coil is present on any vehicle with AC.

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#17 (permalink) Old 01-03-2013, 11:29 PM
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There is also an interior temperature sensor,and a solar intensity sensor, also feeding into the A/C control unit (found on the Limited models). These sensors allow feed-forward control of the HVAC system
Yes, but those are for hot weather use (at least when you actually want the A/C to engage)...and yes, I know those interior sensors will be used to engage the heater when needed on Limited models. In other words, they regulate the interior cabin temperature.

The discussion topic was what ambient external temperature inhibits the compressor from engaging when you want it to dehumidify the cabin in cold weather.

Last edited by keeton; 01-04-2013 at 07:19 AM.
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#18 (permalink) Old 01-04-2013, 09:18 AM
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The discussion topic was what ambient external temperature inhibits the compressor from engaging when you want it to dehumidify the cabin in cold weather.
OK, I had to reread this a couple of times to understand what you meant. I think your saying that in winter the compressor no longer cuts out when the evaporator is close to icing, but rather when the exterior temperature is close to freezing.

I looked over the manual and found that the ambient temperature sensor is connected to the dashboard display, and also to the A/C amplifier. I was surprised to find that both the automatic and manual systems have A/C amplifiers.

I cannot find any reference in the manual that the exterior sensor will shut off the compressor. If I look under the chart for compressor not operating, it lists a number of reasons including the evaporator sensor, but no mention of the exterior sensor. The only thing I can find of interest is that the diagnostic tool will show an open circuit of the exterior sensor when the temperature drops below -23.3C or -9.94F.

If you could refer me to the page that you found your information, I would appreciate it.

It seems to me that if the air blowing through the evaporator was above freezing, then there is still an opportunity to remove additional moisture.

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#19 (permalink) Old 01-05-2013, 05:02 PM
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If you could refer me to the page that you found your information, I would appreciate it.
You would make me look it up, wouldn't you.

After pouring through a zillion tedious .pdf files (most of which are merely remove/replace operations of the various A/C components), the closest thing I found was in a "body electrical" file that had a description of the A/C system and compressor operation, it stated that the solenoid that governs the variable displacement feature is governed by "various sensors".

However it does explicitly say in the owner's manual (Chapter 3-1, page 224 for my 2010 US model) that:
"When outside air temperature approaches 32F (0C)
The air conditioning system may not operate even when A/C is pressed."

Which of the "various sensors" that trigger that will remain a mystery.
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#20 (permalink) Old 01-05-2013, 05:26 PM
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Originally Posted by keeton View Post
You would make me look it up, wouldn't you.
It's only fair after all the digging I did after your last post.

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After pouring through a zillion tedious .pdf files (most of which are merely remove/replace operations of the various A/C components),
Go to Junebugs profile page and you will find a link to download those zillion files already compiled into a single file.

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file that had a description of the A/C system and compressor operation, it stated that the solenoid that governs the variable displacement feature is governed by "various sensors".
Yes, I mentioned that the system can look at inside, outside, and desired temperatures, and depending on how far apart they are it determines whether the compressor needs to produce maximum cooling, or a more gentle cooling. Sometimes the vehicle can be cooled with outside air alone and the compressor is not required.

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Originally Posted by keeton View Post
However it does explicitly say in the owner's manual (Chapter 3-1, page 224 for my 2010 US model) that:
"When outside air temperature approaches 32F (0C)
The air conditioning system may not operate even when A/C is pressed."

Which of the "various sensors" that trigger that will remain a mystery.
I think the statement about shutting down at 32'F is just a generalization to make it simple for owners to understand. If the heater was not preheating the incoming air, then the evaporator sensor probably would shut the compressor off at 32'F. But the air blowing through the evaporator is not always the same as the ambient temperature.

Cars used the evaporator sensor decades before computer control became common. It is the most reliable sensor to avoid icing, and it still allows the compressor to run when exterior temperatures are below freezing. Why would they trust this important task to something that could malfunction?

Rick L Ontario Canada
2008 Silver RAV4 Limited V6 4WD
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