Function
The Climate Control Module (CCM) uses a thermistor to determine the temperature of the inside of the car. When the temperature increases, the resistance of the thermistor decreases and as it gets colder, the resistance increases. The CCM measures the change of resistance to determine cabin temperature and automatically adjusts the blend door, blower speed and other items according to the temperature you have set with the knobs. If you have the temp set to 70F after the car has sat in the sun for a while, the car will try to get the cabin temp down to as close to 70F as it can using the data from the cabin temp sensor. As I mentioned, if this sensor (thermistor) is broken, it substitutes a value of 25C and the climate control doesn't do a good job of managing the temperature.
Measurement
I used what is called a Resistance Decade Box to determine what resistances correspond to what temperature using VIDA software. The resistance decade box has a number of switches on it that you can flip to add any number of values together for a specific resistance value. I managed to get the leads from the old temp sensor to poke through the CCM slots so I could hook up the decade box using alligator clips. The temperature range that the CCM can measure according to VIDA is -40C to 85C. Unfortunately VIDA does not have any information on resistance values which is why I needed to do this test. I started at -40C and kept punching in resistance values until I got the correct resistance corresponding to the temperature I want to get. Here is the following table I made showing resistance for each temperature in C going up by ten with the exception of 25C (which is the base value of the resistor and VIDA as it is considered "room temperature") and 85C as it is the end of the CCM measurement range:
Degrees C-----Resistance (Ohms)
-40 -------------- 190k
-30 -------------- 114k
-20 -------------- 69k
-10 -------------- 43k
0 ----------------- 28k
10 --------------- 17.5k
20 --------------- 12k
25 --------------- 10k
30 --------------- 8.2k
40 --------------- 5.7k
50 --------------- 4.1k
60 --------------- 2.95k
70 --------------- 2.24k
80 --------------- 1.6k
85 --------------- 1.4k
The yellow highlight indicates roughly the range that you can set the temperature dials to.
- Exposed leads, coating stripped off
- Resistance Decade Box
- Hooked up to old thermistor leads using alligator clips
- Live temp readout using VIDA
Replacement
DISCONNECT BATTERY BEFORE REMOVING CCM IF YOU DO NOT HAVE VIDA OR A HIGH END SCAN TOOL, FAILURE TO DO SO MAY RESULT IN SRS AIRBAG CODE THAT WILL NEED TO BE CLEARED
After gathering the data on resistance values, I found a similar glass bead NTC thermistor with values similar to what I measured to be as accurate as reasonably possible. I bought PN B57550G1103F000 thermistor made by TGK from DigiKey. This is a 10 kilo Ohm 32mW thermistor. It ended up being quite a bit smaller than the original thermistor so in hindsight I would recommend getting a higher mW rated thermistor, maybe 50mW or higher which is also available on DigiKey and other sites.
Here are photos of the replacement process. Read photo captions for step by step.
- Removing pen holder trim piece using bone tool
- Remove both T25 screws. Clean up grime while you're in there too
- Slide CCM out from dash using bone tool and remove connectors
- Back of CCM
- Back of CCM. This would also be a good time to replace the backlighting bulbs if some of yours are burned out
- Remove 2 Phillips screws holding the cabin temp sensor fan in place and pull fan assembly straight out. Clean the dust out of this fan as well
- Remove all the other silver Phillips screws. There are plastic tabs on the top and bottom of the back cover holding it in place. Carefully unclip these with a small screwdriver and remove the cover to expose the main board
- Carefully lift the board directly up from the CCM face
- Front of main board
- Broken thermistor on its own separate PCB attached to an air duct
- Remove thermistor PCB from air duct and remove air duct from main board. You could remove thermistor PCB from main board by desoldering the 2 wires if you wanted to, I didn't do that though
- Melt the solder joints and remove remnants of the old thermistor. Tin the leads of the new thermistor (that is, melt some solder onto the leads of the new thermistor) and solder new thermistor in place. Leave a little slack for placement into the air duct
- Place air duct back into main board and place thermistor PCB onto air duct
And that's it! Just reverse the steps and reassemble everything and you will have a better functioning climate control!
If you want a video tutorial, this one from MattDIY working on an XC90 is very similar.
