This is my first post here, I hope it saves some of you some money. I happen to have a background in chemical engineering and familiarity with refrigeration systems, but none of that is necessary here to fix this issue.

This specifically addresses an issue with the evaporator cooler thermistor, which controls the A/C compressor cycle (referred to by SM as COOLER THERMISTOR NO. 1). There are a LOT of other sensors (solar, ambient, cabin, etc.) but they are not used when the temperature is set to "LO" and the system is not in automatic mode. The only inputs that should affect it (per FM) are these:

- A/C button on dash
- evaporator temperature sensor
- liquid line high pressure sensor
- to prevent refrigerant release through vent valve, mounted
- Mounted by the bumper and easily accessible with the hood open
- Can be tested by jumping
- Full throttle from ECM

Here is how you know if this is sensor is the problem:

- The compressor engages and air is cooler but cycles back and forth between "cool" and not very cold at all
- Compressor cycles at idle frequently even though air is not cold
- Refrigerant charge is correct (or you are confident it is close enough - contrary to what you will find there is an acceptable range)
- I'm going to double down on this - if you are knowledgeable and reasonably certain of the charge by looking at the sight glass and gauges that the charge is correct, then that's NOT your problem
- *MOST IMPORTANTLY* - listen for a sighing sound coming from under the glove box if you listen closely as the expansion valve repeatedly becomes starved of liquid input due to the compressor cycling off before cooling temperatures are reached (while you are sighing and sweating a well). This is a good sign that the A/C controller (I am NOT calling it an amplifier) is receiving incorrect temperature input from the evap coil temperature sensor. There are other reasons this one can be checked with a multimeter, continue reading

Here are the specifics for my vehicle:
2007 4th Gen 4Runner
255k
V8 4WD Limited 2UZ-FE

The evaporator temperature sensor is located inside the evaporator housing in the middle of the dash - you cannot access it without removing the entire dash. It has a long wire and then sticks directly into the fins of the evaporator - there's no doing surgery by removing the glove box, it's all or nothing. Here are the steps to diagnose

DIAGNOSIS
----------------
- Remove glove box and gain access to the wire harness
- Disconnect the wiring harness and
- jumper the side to the controller with a 2k ohm resistor
- connect multimeter to measure resistance in the suspected faulty evap temp sensor thermocouple
- thermometer in vent directly to right of steering wheel on drivers side
- Start the car and run the a/c with the blower on low
- Note the resistance as the temperature drops past ~37 F (if it doesn't - stop, this probably isn't your problem, otherwise this is the number you need below in the "resistance is too..." calculations)

>>>>>>>>>> diagram of T/R for thermistor

One thing to note here - this an NTC (negative temperature coefficient) thermistor, so the resistance goes DOWN as temperature goes UP (this is in contrast to most conductors like copper). At 37F the resistance I got was 5.25kΩ - but it should be closer to 4.25kΩ. I know this because I purchased a replacement thermistor (wrong part for a Lexus, the one I needed wasn't available) and checked it's resistance at that temperature. In addition, I used a potentiometer to check what resistance the controller cycles the A/C off - 3.9kΩ.

If you get infinite or zero resistance from the thermistor, unfortunately you have to take apart the dash and evaporator to replace it. However, if the reading is high or low, and the "sort of cool" temperature has been pretty consistent for a while indicating that the thermistor is functional - it
is precise but inaccurate) you can adjust it with some resistors, up or down.

If the resistance is too HIGH (>4.25 at 37F):
- Use resistors wired in PARALLEL to reduce the total resistance according to this:

x = 1 / ((1/a) - (1/b))

If you do python:
def DecreaseResistance(desired, now):
new_resistors_add = (1 / ((1/desired) - (1/now)))
if new_resistors_add < 0:
raise ValueError("Numbers switched?")
return (1 / ((1/desired) - (1/now)))

If the resistance is too low (<4.25 at 37F):
- Use resistors wired in SERIES
x = b - a

where x is the necessary resistance to be added, a is the desired resistance (4.25 kΩ) and b is the resistance measured above (mine was 5.25 kΩ). In my case I wanted to reduce the resistance by 1 kΩ, so I added a 20 kΩ resistor in parallel which made the evap thermistor be 4.25 kΩ at 37F.


The wiring can be done however you want, I personally preferred to cut the side of the thermistor rather than the non-replaceable wiring going the other way. But the latter is easier to access.

I'll figure out how to post the pictures.

Pics

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Man! For a first post, this is pretty incredible. Thanks for posting the diagram as well for the resistance and temperatures. I'll have to give this a shot with the multimeter and see what comes up!

Great, let me know how it goes. Based on all the searching I have done it seems like a LOT of people have been having this problem, so I'm a little surprised that there hasn't been more responses. From what I can tell a lot of these vehicles were either (a) spec'ed and assembled with the wrong evap thermistor or (b) the A/C controller evap thermistor setpoint is wrong. Ironically the exterior temperature thermistor has always been dead on.

It's possible, although I feel like it might be more on the possibility (B) side. I ended up actually replacing my thermistor with a new OEM one, since mine is the earlier thermistor that doesn't clip into the evap core fins, it just rides on the "carrier" that clips onto the side of the evap core housing. I even played around a bit on the positioning of the sensor on the carrier but it made little difference. Only time it ever gets freezing cold is when I jump those two wires.

Mine also makes the same noise with the expansion valve, the "sighing" when the sensor is plugged in.

You are dead on the problem. Whether the problem is A or B, the fix is the same. Put a 20kohm resistor on that thermistor clip. The measurements aren't that important, it's the same incorrect Toyota part inaccurate by the same resistance differential all of them are. If you are worried about cutting wires you can stick the resistor prongs in the wiring harness and tape it temporarily (make sure it has contact). The only reason to not leave the circuit jumpered is to avoid icing up, especially in cold weather with defrost on.

It doesn't really matter how much contact the thermocouple has with the evap coil surface, they are very sensitive. If it's in the evap housing and in front of the air discharge it's fine.

I am pretty much 100% certain this will fix your problem based on what you've said.

https://www.amazon.com/EDGELEC-Resis.../dp/B07HDGCGB3

Hey man! Incredibly late reply, but I ended up testing your theory pretty significantly with my AC system in my '03. I started using a 20k ohm resistor on mine, and it was good for an extra 5 degrees pretty much all the time.

Turns out, my compressor starting going bad due to a bad clutch (it was the original compressor, 21 years old). So I just had that swapped out, and I took the resistor out beforehand. When I got it back, I was cycling between 49-56 degrees at idle (70 degrees outside temp). With the resistor back in place, I'm down to 44-50 degrees at idle. This is pretty much dead on with my fiancee's Ford Escape which has great AC (by our standards at least).

So in my case, it definitely made a difference. And so far, I haven't seen any temps low enough to cause a freezing situation inside the evaporator. That 20k ohm seemed spot on. We get a lot of humidity near the coast so I'll do some further testing as summer rolls in.