2015 RX 350 air conditioner seems to cool erratically. Occasionally on super hot days, it won't work at all. What's going on? If I take it to the dealer it may work. Can the dealer determine why it dosn't cool on some days. What controls the cooling is it a thermostat that may be failing on occasion.

 

https://us.lexusownersclub.com/forum...ioner-problem/

 

Higher the humidity the harder it takes to get the system to cool . test with an ac vent thermometer

 

I have an RX 400h 2015. My problem is that when the Air Conditioner is set to 70 degrees and the outside temperature is over 70 degrees, the Air Conditioner works as expected to cool the temperature. But when the outside temperature is 65 degrees, the Air Conditioner blows HOT air and the car becomes uncomfortably warm - certainly hotter than the 70 degrees I set it for.

Why is the Air Conditioner temperature dependent on the outside temperature? Shouldn't it be solely dependent on the interior temperature?

My previous RX 1999 and LS 400 had Air Conditioners which worked as expected to maintain interior temperature regardly of what the outside temperature is.

 

The car blows warmer air to make the interior temperature 70 degrees when it is colder than that outside. It does not blow 70 degree air.
The vehicle measures the outside and inside temperature and automatically uses heat or A/C to create the interior temperature you selected.
There is nothing wrong with your RX.
Home heating systems work the same way. Your home thermostat may be set to 70 degrees, but your furnace will blow 140 degree (I guess at the actual temp) air to heat the house. It would never get warm if the furnace blew 70 degree air.
Same with air conditioning. It will blow very cold air to make it 70 degrees in the Summer.

 

There is something wrong with the air conditioner the way it currently works.

I want the interior of the car to be 70 degrees at all times.
So when I set the air conditioner at 70 degrees I expect it to work to achieve a constant 70 degrees.
It should turn off and simply run the blower if the temperature is 70 degrees.
It should blow warm air if the interior is under 70 degrees.
And it should blow cool air if the temperature is over 70 degrees.

But it doesn't work this way.

If the interior temperature is 70 degrees, and the exterior temperature drops to below 70 degrees (e.g. 60 degrees), it blows warm air.
And it continues to blow warm air even if the interior temperature rises to 75+ degrees.
It does not stop blowing warm air even if the temperature is already at 70 degrees.
This causes the RX's interior temperature to go over 70 degrees.
So I have to constantly adjust/fidget with the temperature control to get the interior temperature the way I want: 70 degrees.
I often have to set the temperature to under 65 degrees in order to return the temperature back to 70 degrees
then I have to raise the temperature again to keep the temperature from becoming too cold.

In my previous RX 1999 and LS 400, when I set the INTERIOR temperature to 70 degrees
the air conditioner works as expected and maintains 70 degrees no matter the external temperature.
It shuts off the warm air if 70 degrees is reached to prevent the temperature from going over 70 degrees.
I don't have to constantly fidget with the temperature setting.

 

Then maybe your interior temperature sensor is bad. Time for an A/C shop or the dealer to inspect your HVAC system.

 

Its a design thing that I agree, isn't the best. Your old car may have had an HVAC setting based on simple math, when temp is low, heat, when temp is too high, cool. But Toyota went ahead and tried to add more intelligence to the "auto" setting. Not only is it based on outside temp, but it is also based on how much light is entering the vehicle. They've tried to do this to anticipate what passengers feel. For example, a really sunny day at 65F feels a lot different in a car then 65F in the middle of the night. The system tries to compensate for this difference, not always doing the best job. The last two vehicles I've had, a Camry and the RX have the same system, and I have to fiddle with mine as well based on the conditions. It sort of works well sometimes, but other times not so much. In mine, for example, in summer during the day if its 85F outside and I set the auto to 73F it will be a January blizzard inside the vehicle with fans on high blowing a lot of cold air. Booo, or should I say Brrrr. I usually have to set the A/C temp to 3-5 degrees below outside temp to keep from turning into ice cube. If its 85F then that means setting A/C to 80F to be comfortable. In the winter when I drive a lot more when its dark outside, the auto setting is a little better, I can generally leave the temp setting within a couple of degrees most of the time for comfort. I would be tempted to cover up the light sensor if the auto-headlights didn't depend on it as well.

 

Normally, the AC reacts on the air return temperature, not the outside air. Same way your home AC works. High possibility that your return air sensor is faulty. A car AC repair shop would be your best bet to get to the real root cause.

 

Here's an excerpt from a bit old now Toyota technical training manual on A/C systems, describing the various sensors that are used to calculate temperature output for automatic climate control operation. So inside temp, outside temp, and solar radiation are factored in using some sort of adjustment factor (probably fine turned for each vehicle type) for each. Elsewhere in the manual it also states that humidity sensors also play a role, but I think that is used mainly to control fan and A/C operation to clear fogging rather than actually determining temperature output as the below attempts to do.

 

Your AC system is not working properly, this is absolutely not normal.

First thing to check, is to make sure the passenger and the driver temp settings are the same (make sure the dual mode is off)

Let us know.

 

This is very interesting, I did not realize auto AC temperature control is so involved! But the OP's system is malfunctioning, I'm not sure how this will help them fix it.