hi all,

I have a question maybe you can help me.

My AC in my 2011 RX350 works and blows cold when driving, when I pull off the highway it starts to blow warm/hot and after a few minutes the cold air comes back.

I think my ac compressor is good since it blows ice cold.

What do you think this could be?

 

For starters please check the air flow .. anything blocking the air through the radiator ,, electrical fan kicking in when the A?C is on and also the cabin air filter.

Salim

 

checked the air flow. Nothing blocking the radiator ETC. Fans kicks on when AC is blowing cold.

 

does anyone think the AC Clutch could be bad? that it disengages? - Dealer trying to replace the compressor and clutch . but the compressor works fine. im thinking maybe the clutch is disengaging after running on the highway for some reason.

 

How many miles? If the clutch is starting to get old, it can disengage or slip when the wiring gets hot. The heat would increase resistance in the wiring and decrease the amperage to the clutch just a hair. Since the clutch was already worn, the decreased amperage was enough to make it not stick. I had this in my truck. In my case I had the option to adjust the clearance on my compressor clutch and that fixed it. Normally it would be an issue more on the highway, but your clutch might be getting hot due to less airflow coming in as you slow down, then it cooling down again once the fans kick on?

Easiest way to check is to see if the clutch is engaged well when it is blowing warm air.

Also, are you using Recirculation?

 

It could be this well known problem which has been covered in the following linked thread: https://www.clublexus.com/forums/rx-...-solution.html

Here is a paste of the opening post:

If you own a Lexux RX350 2010 or 2011 you may eventually run into this A/C problem outline in this Lexus technical bulletin. http://workshop-manuals.com/lexus/rx..._on/dtc_b1479/

This can also happen with 2011 Sienna and Highlander.

If you have this problem you'll notice the AC blows warm air and code B1479 may be set. The A/C blows warm air because the ECU/AC control computer won't engage the magnetic clutch on the A/C compressor because it sees a "faulty signal" from the compressor flow sensor.

Unfortunately this sensor is attached to the compressor (not sold separately) and Lexus's solution is to replace the compressor. The estimated parts+labour cost is around $2000.

I manage to study the problem a bit more and want to share my learning and solution. I found the flow sensor is still works but the voltage range it output had degraded over time. The A/C computer expects 3.8 to 4.2 V from the sensor when the A/C is off and below 3.8 V when the A/C is ON. (the lower the voltage the more refrigerant flows). On my Lexus the flow sensor output was 3.70 V when the AC was off, so the A/C computer panics and won't engage the system. Using Toyota/Lexus' techstream software I manually engaged the compressor and found that the A/C cools just fine. More importantly though, the AC flow sensor was working! (you can see the flow rate fluctuate)

Therefore the problem is that the sensor voltage had dropped below 3.8 V when the A/C button is off. All it needed was a little boost (in my case, from 3.7 V) so that it'll fall within the valid range. In my case, I want to multiply the 3.7 V by 1.07 = 3.96 V. To scale up the voltage I used a non-inverting operational amplifier. You can buy all the component from Amazon/digikey for less than 30 dollars. The output of the AC flow sensor is fed to the amplifier, and the output of the amplifier goes to the AC computer. Inserting this amplifier made the A/C system work again.

If anyone is interested in this solution please let me know. I can post more detail on the repair and the design of the non-inverting op/amp. Basic steps are outlined below

1. Get Techstream cable/software from Amazon/ebay and connect to your car to verify the problem (i.e. flow sensor output below 3.8 V and that the cooling system works when the compressor is manually engaged)
2. Remove the glovebox to gain access to the A/C computer (there are many steps involved, including removing the SRS knee airbag on the passenger side)
3. Design a non-inverting operational amplifier (this is easier than you think, check wikipedia article on operational amplifier)
4. Insert the amplifier between the compressor flow sensor and A/C computer. The amplifier power supply and ground should use the same ones as the flow sensor (pin 30 for power and pin13 for ground on the AC computer). The flow sensor voltage is fed to pin 7. you'll have to cut this line to insert the amplifier.
5. enjoy your A/C go spend $2000 somewhere else!

 

Warning .. getting too technical ..
Interesting solution, but do you really need a linear scaling factor? Why not just adding a low voltage bias?
Salim

 

evaporator dirty? Electrical fan cooling fan not running?