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.